Marco Rivera
email: mrivera@igp.gob.pe

Pablo Samaniego

Jessica Vela

Jean-Luc Le Pennec

Hervé Guillou

Jean-Louis Paquette

Céline C Liorzou

The eruptive chronology of the Yucamane-Calientes compound volcano: A potentially active edifice of the Central Andes (southern Peru)

Keywords: Calientes, Yucamane, Central Andes, Holocene, eruptive chronology, volcanic 52 hazards 53 54

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Introduction 55

The Central Volcanic Zone (CVZ) of the Andes extends from 16°S (Southern Peru) to

The eruptive chronology of Yucamane-Calientes compound volcano (YCCV)

The following description of the volcanic units of Calientes and Yucamane edifices 195 (Fig. 2) is based on our morphologic, stratigraphic, and geochronological data. The older volcanic units of the YCCV rest on top of lavas from Yucamane Chico 199 volcano (6025 m asl), which is located immediately north of the YCCV and is an old and 200 highly eroded edifice displaying a similar degree of erosion as the Barroso Group volcanoes 201 (i.e. Nazaparco). Yucamane Chico is elongated in the WNW-ESE direction, with a steep-202 sided southern flank (Fig. 3). Its lava flows have a porphyritic texture with a mineral 203 assemblage of plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides, and minor amphibole

Calientes III

291

The Calientes III unit predominantly comprises lavas of the middle and upper part of 292 the edifice between 4200 and 5374 m asl and represents the main cone-building stage of 293 Calientes. The unit consists of a ~1150-1200 m-thick succession of andesitic and dacitic 294 (60.2-65.1 wt.% SiO 2 ), blocky lava flows. These lavas rest discordantly on remnants of the 295 Calientes I lavas and the Callazas ignimbrite (Figs. 2,3), and extend as far as 8 km northwest 296 and 5 km southwest from the summit. The lavas are strongly affected by glacial erosion, 297 suggesting an age older than the LGM. This observation is confirmed by a K-Ar age of 126 ± 298 3 ka (YU-12-31, Table 1) and an 40 Ar/ 39 Ar age on plagioclase of 95.6 ± 16.7 ka 299 Table 2) obtained for two different lava flows above the Callazas ignimbrite deposits. As for 300 Callazas ignimbrite, a large amount of the 40 Ar/ 39 Ar dated feldspars are considered antecrysts, 301 only the three youngest and less radiogenic crystals were used to calculate the 40 Ar/ 39 Ar age 302 of this lava flow.

303

We include in this unit a succession of at least three block-and-ash-flow deposits 304 cropping out on the western side of the Salado river valley, 6-8 km from Yucamane's summit grey, structureless, unconsolidated, and all have gradational contacts between them. The 307 deposits contain 30-40 vol.% grey juvenile blocks of dacitic composition (63.0-65.9 wt.% 308 SiO 2 ) and 10-15 vol.% accidental lithic fragments in an ash-rich matrix. A juvenile block 309 from this block-and-ash-flow deposit was dated on plagioclase by 40 Ar/ 39 Ar at 133.4 ± 13.5 ka 310 (YU-12-68B, Table 2). The 40 Ar/ 39 Ar experiment evidences that even in juvenile blocks older 311 crystals can be trapped. Indeed, only 8 of the 19 dated populations of feldspars are juvenile, 312 the others are xenocrystic, 313 Lastly, despite not being directly related with this eruptive stage, we include in this 314 unit a young lava dome (~900 m diameter and ~300 m high) mapped in the upper southern 315 flank of the Mio-Pliocene Yucamane Chico edifice. A lava sample from this dome yielded a 316 K-Ar age of 102 ± 6 ka (YU-12-40, Fig. 2; Table 1). The dome's position and age suggest that 317 different eruptive vents were active during the Calientes III stage. Calientes are cut by a SE-facing, relatively small (1 km-wide) amphitheater probably formed 323 by glacial erosion. The younger dome complex is situated SE of the amphitheater (Figs. 2,3) 324 and is composed of at least three coalescent lava domes emplaced during a likely long-lasting 325 extrusive event. They extend approximately over a diameter of 1.2 km and height of 600 m.

326

These domes consist of porphyritic dacites (63-64 wt.% SiO 2 ). The fresh morphology and the 327 lack of glacial erosion of these domes suggest that they were emplaced during the Holocene.

328

A lava sample from the dome complex, which comes from the SW upper flank of Calientes 329 yielded a sub-zero K-Ar age of 3 ± 3 ka (YU-14-16, Table 1), supporting a Holocene age.

352

In the proximal areas, less than 8 km south and southwest of Yucamane's crater (i.e.

353

Quebrada Honda and Curancurane ravines, Fig. 6), the deposit's thickness ranges between 1 354 and 5 m, and the juvenile blocks have a maximum diameter of 40 cm. In Quebrada Honda, the 355 deposit overlies an unconsolidated andesitic block-and-ash-flow deposit whereas at Quebrada 356 Curancurane, the deposit overlies the Callazas ignimbrite deposit (Fig. 6). In medial zones, for 357 example near the town of Yucamane, 9 km south of Yucamane's crater, the deposit overlies 358 the Salado block-and-ash-flow deposits, and underlies the Campanani block-and-ash-flow 359 deposits (see below). No paleosol or erosive contact has been observed at that place. In distal 360 areas more than 12 km to the west and southwest of Yucamane's summit (i.e. Pampa Coirire, 361 and the town of San Pedro, Fig. 2), the deposit has a thickness between 30 and 80 cm. In such 362 areas, the deposit includes dense porphyritic juvenile blocks with a maximum diameter of 12 363 cm. The presence of the deposit in such distal sites implies that the pyroclastic density current 3).

367

We interpret this layer as a lateral blast deposit based on its sedimentological 368 characteristics, abundance of dense juvenile dacitic fragments, lobe-shaped distribution in a 369 narrow sector to the south of YCCV, and progressive thickness decrease without any 370 topographic control. We suggest that the deposit was emplaced by such a dilute pyroclastic 371 density current when a growing lava dome collapsed and subsequently exploded (i.e.,

Debris-avalanche deposit.

The deposit crops out between 3 and 14 km south and 375 southeast from Yucamane's crater, near the town of Santa Cruz and in the Mal Paso and 376 Pampa Pajonal sectors (Fig. 3). In the latter site, the deposit forms hummocky topography.

377

The debris-avalanche deposit is a structureless 40-60 m-thick unit overlying the Callazas 378 ignimbrite. In most locations, the debris-avalanche deposit displays a block-rich facies 379 composed of heterogeneous, dense, sub-angular lava blocks (58-62 wt.% SiO 2 ) that are tens 380 of centimeters to meters in diameter. Some blocks have typical jigsaw-like fractures. Locally, 381 however, the deposit displays a matrix-rich facies containing small fragments of porphyritic 382 lava blocks dispersed in a sandy matrix.

383

At Quebrada Curancurane, 6 km west of Yucamane, the debris-avalanche deposit is 384 over 8 m thick (Fig. 6). The deposit contains dense lava blocks of heterogeneous 385 compositions and up to 3 m diameter within a matrix composed of sand and gravel. Most 386 blocks have characteristic jigsaw fractures. At this site, the debris-avalanche deposit directly 387 overlies the blast deposit. Such observation, together with the petrographic and geochemical 388 similarities between the debris avalanche blocks and those of the subsequent block-and-ash-389 flow deposits, support a "comagmatic" origin for both deposits and implies that the debris-390 avalanche deposit originated from the collapse of a voluminous dome complex. 391 392 5.7.3. Campanani block-and-ash-flow deposit. The unit crops out in almost all ravines on 393 the south and southwestern flanks of Yucamane (Fig. 7). In the lower part of Quebrada 394 Campanani, near the town of Yucamane Pampa (9 km southwest of Yucamane's crater) and 395 in Quebrada Honda (9 km south of Yucamane's crater), the deposit is 1-8 m thick, and 396 composed of an ash-rich matrix with angular, andesitic blocks (62-63 wt.% SiO 2 ) that are tens 397 of centimeters to a meter in diameter. The deposit is grey, structureless, slightly consolidated, 398 and overlies the blast deposit (Fig. 7). The contact between the blast and the Campanani 399 deposits is gradational. 400 401 5.7.4. Honda block-and-ash flow deposit. In the Quebrada Honda, 9 km south of 402 Yucamane's crater another block-and-ash-flow deposit crops out, which has an average 403 thickness of 10-12 m (Fig. 6). This reddish grey deposit is structureless and slightly 404 consolidated. It overlies a thin (20-40 cm thick) unconsolidated ash-flow deposit that covers 405 the Campanani block-and-ash-flow deposit. In the middle and upper parts of the sequence, the 406 deposit is slightly reversely graded. The unit's matrix of coarse ash and lapilli composes 60-407 70 vol.% of the deposit with dense juvenile blocks (up to 1 m in diameter) forming the remaining 30-40 vol.%. The juvenile blocks are andesitic in composition (61-62 wt.% SiO 2 ).

409

Charcoal from the base of the deposit at a site 8.5 km west of Yucamane's crater yielded an 410 uncallibrated age of 29,200 +170/-160 BP (sample YU-14-11, Table 3). The deposit also 411 crops out in other ravines at the foot of Yucamane's south flank, such as Quebrada 412 Campanani where it is at the top of the pyroclastic succession. The succession is covered by at 413 least six lahar deposits with thicknesses of 10-15 m. Due to the estimated volume of this 414 succession of block-and-ash-flow deposits (0.08-0.11 km 3 ), we infer that a significant dome-415 forming phase followed the Yucamane sector collapse. Table 1). A third sample collected from a lava flow on the lower 425 southeast flank yielded a K-Ar age of 3 ± 2 ka (YU-12-16, Table 1). This latter lava flow is 426 overlain by a younger pumice lapilli tephra fall deposit from Yucamane (see below). Such 427 lava flow activity is in sharp contrast with the volcano's previous dome-collapse activity. Quebrada Gentilcahua (Fig. 2), the deposits overlie older lava flows of Yucamane's cone as 435 well as the Campanani block-and-ash-flow deposit. To the south, in Quebrada Condorcahua 436 the sequence comprises at least three scoria-flow units with gradational contacts between 437 them that are covered by a 1.5-2.0 m-thick pumice-rich lahar deposit (Figs. 7,8). This 438 sequence of scoria-flow deposits was emplaced during a period of large explosive eruptions at 439 Yucamane, which probably occurred in the late Pleistocene or early Holocene (see below). in thickness, the deposits have a dispersion axis to the southeast (Vela, 2015). These tephra-444 fall deposits overly moraines, which are probably associated with the LGM.

446

Lower "Halloween" tephra-fall deposit. At Mal Paso hill (4 km from Yucamane's summit), 447 this deposit is 12-15 cm thick, whereas it is 8-10 cm thick in the Hueltajavira sector, which is 448 6-7 km from the crater (Figs. 8c,9). The deposit is slightly inversely graded and characterized 449 by a mix of oxidized scoria fragments with an intense orange patina and dark gray, fresh 450 basaltic-andesite (53.9 wt.% SiO 2 ) scoria. Both scoria types compose 60 vol.% of the deposit 451 with the remaining volume composed of dense, grey, aphanitic, and porphyritic andesitic 452 lithic fragments. At 4 km southeast of the crater, the scoria clasts are less than 4 cm in 453 diameter, while the lithic fragments are less than 3.5 cm. in medial to distal locations 8-9 km southeast of the crater (Fig. 11). The pumice fragments 484 are yellow grey in color and have an andesitic composition (61.1-62.7 wt.% SiO 2 ). The 485 pumice's maximum diameter decreases from 6.8 cm in the proximal zone to 2.5 cm in distal 486 areas. The lithics are composed of fresh (grey) and altered (brown and red) lava fragments 487 that decrease in size from 6 cm in the proximal zones to 1.5 cm in distal locations.

488

In the proximal and medial sectors, this deposit shows three distinct layers. The lower 489 layer is 6 cm thick in the proximal sector and <1 cm thick in the medial sector. It is composed 490 of dark ash dominated by fresh lava fragments. The lithics are 0.5-0.6 cm in diameter. The 3) yielded a 14 C age of 3,085 ± 35 501 BP. Given that no paleosol exists below the tephra-fall deposit, we interpret this layer and the 502 overlying tephra-fall deposit as corresponding to the same eruptive event. We should stress 503 that a quite older 14 C age of 3,270 ± 50 BP was reported by Rivera and Mariño (2004) from a 504 similar deposit at the base of the pumice lapilli fall deposit.

505

An isopach map reconstructed from 52 thickness measurements (Fig. 11a) allowed 506 plotting thickness (T) versus the square root of the isopach area (A 0.5 ) (Fig. 11b). The 507 resulting diagram reveals a regular thinning that fits a single exponential segment (dashed line 508 in Fig. 11b). Using the Pyle (1989) model, which is based on the exponential decay 509 hypothesis, the bulk tephra volume is 0.007 km 3 . The Fierstein and Nathenson (1992) method 510 yielded similar results. By averaging the diameter of the five largest clasts at each outcrop 511 over the unit's distribution and plotting the lithic isopleths, we estimate the eruption's plume 512 characteristics using Carey and Sparks' (1986) model. Based on this approach, we estimate a 513 column height of 11-12 km above sea level. Given that we are considering neither proximal 514 PDC deposits nor distal fine ash fall deposits, our estimated bulk volume should be 515 considered as a minimum. Based on this assumption, and the estimated column height, this 516 event is classified as a small VEI 3 eruption. 3). Comparison between the calibrated ages obtained from 531 this sample and that obtained at the base of the pumice lapilli fall deposit suggests that these 532 deposits probably correspond to two different eruptive events. of these deposits, we suggest this eruption formed a 3-5 km-wide caldera, which is likely 599 concealed beneath Calientes and Yucamane. The bulk volume of the ignimbrite deposits 600 suggests that this eruption is at the limit between VEI 5 and 6. We suggest, however, that 601 because the volume of the potential corresponding tephra-fall deposit is not included that this 602 volume should be considered a minimum. As such, we suggest that this eruption was likely a 603 VEI of 6. As shown in Fig. 13, following the Callazas ignimbrite, the main cone-forming 604 stage of Calientes occurred between 130 and 100 ka (Calientes III stage). After a long time-605 gap, volcanic activity resumed in the Holocene (Calientes IV stage).

606

Yucamane was constructed on top of the remnants of Calientes' older units (Calientes 607 III stage) and developed since at least ~36 ka (i.e. the age of the Yucamane blast deposits).

608

This age is a minimum because we have no age determination for the older Yucamane lava 609 flows (Yucamane I stage). This edifice was very active between ~40 ka and the Holocene

646

In their regional isotopic study of CVZ magmatism, Mamani et al. (2010) (Barreiro and Clark, 1984). In general, regional 651 studies highlight the link between Pb isotope compositions of the upper crust and those of the 652 erupted CVZ magmas (James, 1982;Harmon et al., 1984;Aitcheson et al., 1995). Such Pb 653 isotope data further supports the inference from Sr and Nd isotopic systems that CVZ magmas 654 assimilated significant amounts of the regional metamorphic basement during their evolution.

655

We infer that the Yucamane-Calientes magmas evolved by AFC processes which 656 include significant fractionation of amphibole coupled with assimilation of upper crustal 657 material, following the model for Central Andean magmatism proposed by several authors 658 (e.g., Hildreth and Moorbath, 1988;Davidson et al., 1991;Delacour et al., 2007;Mamani et 659 al., 2010;Godoy et al., 2014;Samaniego et al., 2016;Blum-Oeste and Wörner, 2016;Rivera 660 et al., 2017). The depth of contamination, however, is still debated. Some suggest 661 contamination in the lowermost crust through MASH-type processes (Hildreth and Moorbath, 662 1988;Davidson et al., 1990;Delacour et al., 2007), whereas others propose that 663 contamination occurred during ascent and storage of the magma through the upper crust 664 (James, 1984;Davidson et al., 1990;Gerbe and Thouret, 2004;Rivera et al., 2017). The 665 models are not mutually exclusive, but a detailed and quantitative assessment of such 666 petrogenetic processes is beyond the scope of this work. Based on geologic and tephrochronologic data, the most probable eruptive scenario at 675 Yucamane would be low-to-moderate magnitude (VEI 1-2) vulcanian activity accompanied 676 by tephra emissions, similar to recent eruptions at Ubinas [START_REF] Lee | A redetermination of the isotopic abundances of atmospheric Ar[END_REF]( -2009( , 2013( -2015 CE) CE) and events including the emplacement of more voluminous tephra-fall deposits and triggering of 679 pyroclastic density currents around the cone, as occurred during the 3,000 BP, sub-Plinian 680 (VEI 3) eruption of Yucamane. The large magnitude and explosive dome-forming events that 681 impacted the old Yucamane and Calientes edifices were also considered in the volcanic 682 hazard map for the YCCV (Rivera et al., 2018). Secondary lahars may follow any eruptive 683 phases because of rainfall or melting of the snow/ice that usually caps Yucamane's summit 684 during the local winter (December-July). based on periodic, replicate determinations of international dating standards of known K-Ar age using the same procedure for the unknown samples to be measured, as described in [START_REF] Charbit | Cross calibration of K-Ar standard minerals using an unspiked Ar measurement technique[END_REF]. This allows the total 40 Ar content of the sample to be determined with a precision of about ± 0.2% (2). Standards used include LP-6 (127.8 ± 0.7 Ma, [START_REF] Odin | Numerical Dating in Stratigraphy[END_REF] and HD-B1 (24.21 ± 0.32 Ma, Fuhrmann et al., 1987;Hess and Lippolt, 1984;[START_REF] Hautmann | 40 Ar/ 39 Ar dating of central European K-Mn oxides, a chronological framework of supergene alteration processes during the Neogene[END_REF]. At the 95% confidence level, the values adopted here are consistent with those obtained for several 40 Ar/ 39 Ar standards through the intercalibration against biotite GA-1550 by [START_REF] Renne | Intercalibration of standards, absolute ages and uncertainties in 40 Ar/ 39 Ar dating[END_REF] and [START_REF] Spell | Characterization and calibration of 40 Ar/ 39 Ar dating standards[END_REF]. Uncertainties for the K and Ar data are 1 analytical only, and consist of propagated and quadratically averaged experimental uncertainties arising from the K, 40 Ar (total), and 40 Ar* determinations. Uncertainties on the ages are given at 2.

Ar-39 Ar geochronology

Samples were crushed and sieved to the 400 µm-250 µm fraction size. After ultrasonic cleaning in acetic acid (1N) for 45 minutes at a temperature of 60°C, feldspars were concentrated through several density separations using Lithium Meta Tungstate (LMT) heavy liquid. Pristine feldspars crystals ranging from 250 μm up to 400 μm were handpicked under a binocular microscope and then slightly leached for 5 min in a 7% HF acid solution in order to remove adhered groundmass. A total of about 50 grains were finally handpicked and available for 40 Ar-39 Ar laser total fusion experiments. Feldspars were irradiated in the CLICIT facility at the Oregon State University TRIGA reactor for 120 minutes (Irr#CO001) or 60 minutes (Irr#CO003).

After irradiation, crystals were transferred into a copper sample holder and then loaded into a differential vacuum Cleartran window. Prior to the fusion, all crystals were degassed at 4% of the laser power to remove potential atmospheric component that could be found at the surface and/or in the feldspar fractures. For each experiment, a crystal was molten using a focused 25 W CO2 laser (Synrad) at about 10% of the full laser power. Extracted gas was purified by two GP-50 getter pumps one operating at 250°C, the second being at room temperature. The purified gas is then measured using a high-sensitivity noble gas GV5400 instrument operated in ion counting mode.

Neutron fluence (J) was monitored by co-irradiation of ACs-2 crystals [START_REF] Nomade | Alder creek sanidine (ACs-2): a quaternary 40 Ar/ 39 Ar dating standard tied to the Cobb mountain geomagnetic event[END_REF] placed in three pits encircling the sample. 2 ACs-2 crystal of each pit were analyzed and the J value was the arithmetic average of these 6 single crystal laser fusion analyses. It is reported in Supplementary Tables 2 along with the correction factors for interfering neutron reactions. Corresponding J values were calculated using an age of 1.1891 ± 0.0008 Ma [START_REF] Niespolo | Intercalibration and age of the Alder Creek sanidine 40 Ar/ 39 Ar standard[END_REF]. The total decay constants of Renne et al (2010[START_REF] Renne | Joint determination of 40 K decay constants and 40 Ar*/ 40 K for the Fish Canyon sanidine standard, and improved accuracy for 40 Ar/ 39 Ar geochronology[END_REF] and the 40 Ar/ 36 Ar atmospheric ratio at 298.56 [START_REF] Lee | A redetermination of the isotopic abundances of atmospheric Ar[END_REF] were used for age calculations. The precision and accuracy of the mass discrimination correction was monitored by periodical measurements of air argon. This monitoring is performed using a dedicated air-calibration system featuring a 6 liters tank filled with purified atmospheric argon. This tank is connected to the mass spectrometer vacuum line via two pneumatically-actuated air pipettes of approximately 0.1 and 1.0 cc. This system allows for a 1 cc and a 0.1 cc atmospheric aliquots to be delivered into the mass spectrometer and permits a careful monitoring of the mass discrimination over a wide dynamic range with a precision better than 0.15% (2σ; standard deviation for multiples of experiments) for any given bean size measured.

U-Pb geochronology

Zircons were separated at the Laboratoire Magmas and Volcans (LMV Clermont-Ferrand, France) using standard techniques of crushing and sieving, followed by Wilfley table, magnetic separation and heavy liquids before handpicking under binocular microscope. They were mounted in epoxy disks and ground and polished at 0.25µm to expose crystal interiors. U-Th-Pb isotopic data on zircons were obtained by laser ablation inductively coupled plasma spectrometry (LA-ICP-MS) at the LMV and reported in Supplementary Material 3. The analyses involved the ablation of minerals with a Resonetics M-50 excimer laser system operating at a wavelength of 193 nm. Spot diameters of 60 µm were associated to repetition rates of 4 Hz and fluency of 3.0 J/cm2.

The ablated material was carried into helium and then mixed with nitrogen and argon before injection into the plasma source of a Thermo Element XR Sector Field ICP-MS.

The alignment of the instrument and mass calibration were performed before every analytical session using the NIST SRM 612 reference glass, by inspecting the signals of 238U, 232Th and 208Pb and by minimising the ThO+/Th+ ratio. The analytical method for isotope dating with laser ablation is basically similar to that reported in [START_REF] Paquette | Sensitivity Enhancement in LA-ICP-MS by N2 Addition to Carrier Gas: Application to Radiometric Dating of U-Th-Bearing Minerals[END_REF] and adapted to the analysis of Pliocene-Pleistocene zircons [START_REF] Paquette | LA-ICP-MS U/Pb zircon timescale constraints of the Pleistocene latest magmatic activity in the Sancy stratovolcano (French Massif Central)[END_REF]. The 235U signal is calculated from 238U on the basis of the ratio 238U/235U = 137.818 [START_REF] Hiess | 238U/235U systematics in terrestrial uranium-bearing minerals[END_REF]. Single analyses consisted of 30 seconds of background integration with laser off followed by 1 minute integration with the laser firing.

Data are corrected for U-Pb fractionation and for instrumental mass bias by standard bracketing with repeated measurements of GJ-1 zircon standard [START_REF] Jackson | The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U-Pb zircon geochronology[END_REF]. Repeated analyses of 91500 zircon reference material [START_REF] Wiedenbeck | Three natural zircon standards for U-Th-Pb, Lu-Hf, trace element and REE analyses[END_REF] during each analytical session and treated as unknown, independently control the reproducibility and accuracy of the corrections. Data reduction was carried out with the software package GLITTER® from Macquarie Research Ltd [START_REF] Van Achterbergh | Data reduction software for LA-ICP-MS[END_REF][START_REF] Jackson | The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U-Pb zircon geochronology[END_REF]. Common Pb and initial disequilibria caused by 230 Th /238 U and 231 Pa/ 235 U fractionation in the zircon/melt system were corrected according to [START_REF] Sakata | A new approach for constraining the magnitude of initial disequilibrium in Quaternary zircons by coupled uranium and thorium decay series dating[END_REF] and [START_REF] Sakata | A practical method for calculating the U-Pb age of Quaternary zircon: Corrections for common Pb and initial disequilibria[END_REF] method and related Microsoft Excel® spreadsheet. The concentrations in U-Th-Pb were calibrated relative to the certified contents of GJ-1 zircon [START_REF] Jackson | The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U-Pb zircon geochronology[END_REF] reference material. The available fractionation factor of Pa/U in a zircon-melt system of rhyolitic composition roughly shows agreement with a value of 2.9 ± 1.0 [START_REF] Sakata | A practical method for calculating the U-Pb age of Quaternary zircon: Corrections for common Pb and initial disequilibria[END_REF]. This value and related uncertainty is propagated into the calculations. Weighted mean 206 Pb/ 238 U ages were generated using Isoplot/Ex v. 2.49 software package by [START_REF] Ludwig | User's manual for Isoplot/Ex Version 2.49, a Geochronological Toolkit for Microsoft EXCEL[END_REF].

C geochronology

Before 14 C measurement, samples were examined under a binocular microscope with the aim of eliminating any potential contaminant such as sand and roots. Then, the samples were chemically pre-treated to remove contaminants, in order to isolate a suitable fraction for dating. The treatment (referred to as the "AAA" treatment) consists of the following steps: (1) acid (HCl) leaching to remove soil carbonate; (2) alkali (NaOH) leaching to remove humic acids; and, (3) acid (HCl) leaching to remove any "modern" CO 2 absorbed during the previous steps. Variable amounts of the dried sample were used to obtain, after combustion, a CO 2 volume equivalent to at least 1 mg of C. Then, CO 2 is reduced by H 2 in the presence of iron powder. 14 C activity is finally measured by mass spectrometry. The 14 C ages are calculated following the procedure of [START_REF] Mook | Reporting 14C activities and concentrations[END_REF] and corrected for mass dependent effects (isotope fractionation) using the measured 13 C/ 12 C ratio. This  13 C includes the fractionation which occurred both during sample preparation and measurement.

  of Calientes and a younger dome complex were constructed 321 during the last eruptive stage of Calientes. The viscous lavas, domes, and coulees of the upper

  322

  of Yucamane consist of a 300-m-thick succession of andesitic 333 and dacitic (58.5-64.7 wt.% SiO 2 ), blocky lava flows (Figs. 2, 3) that crop out on the southern 334 flank of the cone and discordantly overly the upper Miocene-Pleistocene products of 335 Yucamane Chico. Samples from Yucamane I stage lavas did not yield meaningful ages 336 because atmospheric contamination was too high to allow detection of 40 Ar*. Field 337 relationships, nevertheless, support an age younger than the Calientes III stage (i.e. < 100 ka).

  ash-flow deposits overlies the basal lava flows of the Yucamane cone. Such deposits reflect a 342 voluminous dome-forming eruptive stage after the Yucamane sector collapse. directed blast deposit. This unit forms a conspicuous layer that crops out to 345 the west and south of Yucamane volcano, covering a lobe-shaped area of ~130-135 km 2 (Figs. 346 6, 7). Based on the observed maximum thickness of this deposit, we infer a southwestern 347 dispersion axis. The deposit is grey in color, unconsolidated, and its thickness decreases at a 348 relatively constant rate. The deposit is well sorted, fines-depleted, and mainly composed of 349 dense, lapilli-size juvenile blocks (40-50 vol.%), and accidental lithic fragments (20-30 350 vol.%). The juvenile fragments are homogeneous porphyritic andesites (62.3-62.7 wt.% 351 SiO 2 ).

364

  would have crossed the deep Callazas valley. A 14 C date from charcoal collected from the 365 deposit 11 km west from Yucamane's crater yielded an uncalibrated age of 36,450 ± 260 BP 366 (sample YU-13-20, Table

  Cone-forming lava flows. These lavas form Yucamane's upper cone between 4300 419 and 5508 m asl (Figs. 2, 3). The overlapping lava flows generally have slopes up to 30-40° 420 and maximum run-out distances of 4 to 5 km from Yucamane's crater. The lavas have 421 basaltic-andesite and andesite compositions (55.2-63.6 wt.% SiO 2 ). A lava sample from the 422 lower western flank was K-Ar dated at 23 ± 1 ka and a second sample from one of the 423 uppermost lava flows yielded a K-Ar age of 7 ± 1 ka (samples YU-12-61 and YU-12-27 424 respectively, Fig. 2 and

  Scoria-flow deposits. The deposits crop out on the lower southwestern and southern 430 flanks of Yucamane (Figs. 8, 9). They have a thickness of 10 m and are structureless, non-431 consolidated, and have a gray-brown, sandy matrix. The deposits have 20-30 vol.% of blocks 432 and bombs (up to 60 cm in diameter) and 10-20 vol.% of dense lithic fragments. Scoria blocks 433 and bombs are basaltic-andesite to andesite in composition (56.7-58.4 wt.% SiO 2 ). At 434

  tephra-fall deposit. At Mal Paso hill, the deposit is 16-18 cm thick (Figs. 8d, 9) and 456 crops out above the Lower Halloween deposit. It also crops out in Quebrada Honda, 9 km 457 south of the volcano, intercalated into the middle of the sequence of lahar deposits covering 458 the Honda block-and-ash-flow deposits. The Parda deposit is slightly inversely graded at its 459 base and slightly normally graded at the top. The proportion of both scoria and lithic 460 fragments is ~50 vol.% each. The andesite (56.6-57.4 wt.% SiO 2 ) scoria fragments are grey 461 with a light-to-medium brown patina and <3 cm in diameter at 4 km from the crater. At the 462 base and top of the deposit, grey, altered lithic lava fragments of sub-centimeter size are " tephra-fall deposit. In the area around Mal Paso hill, the deposit has a 466 thickness of 12-22 cm (Figs. 8b, 9) and crops out above the Parda and Lower Halloween 467 tephra-fall deposits. The deposit is composed of basaltic-andesitic to andesitic (55.5-56.4 468 wt.% SiO 2 ), slightly vesiculated scoria (60-70 vol.%) and dense lithic (30-40 vol.%) 469 fragments. It is characterized by the presence of altered scoria fragments with a characteristic 470 orange patina, especially in the lower and middle parts of the deposit. At its base, the deposit 471 is slightly inversely graded, while its top is slightly normally graded. At the layer's top, 472 abundant grey, lithic lava fragments of centimeter to millimeter scale are present. At 4 km 473 from the crater, the maximum size of the scoria measured is 4 cm and the maximum size of 474 the lithic fragments measured is 3 cm. 5.8.4. The deposits of the more recent explosive eruptions. On the lower eastern flank of 477 Yucamane two conspicuous deposits crop out, and are related to Yucamane's more recent 478 deposit. On the plateau located between the cone and the Salado river 481 (Figs. 2, 10) a tephra-fall deposit composed of pumice and lithic lapilli crops out. The 482 deposit's thickness decreases from >50 cm in proximal areas 2-3 km from the crater to <5 cm 483

  491lower layer is inversely graded and well sorted. The middle layer is up to 28 cm thick in the 492 proximal sector and gradually decreases to 6 cm in distal zones. The pumice lapilli are 493 angular to subangular and highly vesiculated. The layer's lithics are dark grey, non-altered, 494 aphanitic lava fragments. Some hydrothermally-altered lava fragments with a brown to red or 495 yellow color are also present. The layer is inversely graded and well sorted. The upper layer 496 is up to 21 cm thick in the proximal zone and gradually decreases to 10 cm thick. 497 Hydrothermally altered lithics are present in a lesser proportion than in the middle layer. The 498 upper layer is structureless and well sorted. The deposit only crops out sporadically in areas 499 >10 km from the volcano. A charcoal fragment collected in a fine-grained deposit at the base 500 of the pumice lapilli fall deposit (sample YU-13-15, Table

  deposits. At the northeast foot of Yucamane (Pampa Cambaya, 6 km from 519 the summit), a 5-7 m-thick pyroclastic-flow deposit crops out (Figs. 10, 11), which covers an 520 area of 4-5 km 2 and has a bulk volume of about 0.009 km 3 . A similar, though thinner (<1-2 521 m) deposit occurs on the southeast flank of Yucamane. These deposits are composed of three 522 different subunits that overlie the previously described pumice lapilli fall deposit. The lower 523 layer has a red-yellow color and is composed of centimeter size, slightly altered pumice (15-524 20 vol.%) and lithics (20-25 vol.%) in a friable, fine ash matrix (50-55 vol.%). The middle 525 layer is grey and composed of large pumices up to 50 cm in diameter. The uppermost layer is 526 ash-rich with disseminated pumices up to 20-30 cm in diameter. Pumice bombs with light 527 grey (63.0-64.3 wt.% SiO 2 ) and dark grey (61.5 wt.% SiO 2 ) bands are present in all of these 528 layers. 529 Charcoal from the base of this pyroclastic-flow deposit yielded a 14 C age of 2,990 ± 35 530 BP (sample YU-13-06A, Table

  bySiebert et al. (2010) suggest that Yucamane 536 experienced repeated eruptions in1780, 1787, 1802, 1862, and 1902 CE. Samaniego et al.

  , however, demonstrate that the 1780-87 and 1802 CE eruptions correspond to the 538 neighboring Tutupaca volcano, which experienced a sector collapse and large explosive event 539 that was radiocarbon dated at 218 ± 14 BP. The historical chronicles of Zamacola (1888) and 540 Valdivia (1847) corroborate that Tutupaca erupted between 1780 and 1802 CE. We found no 541 chronological or historical data supporting a possible eruption at Yucamane in 1862 and 1902 543 Yucamane only has low-intensity, but pervasive fumarolic activity in the summit crater. The 544 only other geothermal activity in the region is at Calientes hot springs, located 7porphyritic andesites (~40 % of the dataset) and dacites (~60 %) 550 with a mineral assemblage of plagioclase, amphibole, biotite, orthopyroxene, clinopyroxene, 551 and magnetite in an intersertal or intergranular groundmass. Plagioclase (<7 mm) is the most 552 abundant mineral, followed by amphibole (<2 mm), and pyroxene (<1 mm). Biotite (<1 mm) 553 is also present in dacitic samples from Calientes III and IV stages. Phenocrysts compose 554 about 15-20 vol.% of the rocks. Agglomerates of orthopyroxene, clinopyroxene, plagioclase, 555 and Fe-Ti oxides are also present. 556 Yucamane lavas are basaltic-andesites (~10 % of the dataset), andesites (~30 %), and 557 dacites (~60 %), with porphyritic textures and intersertal or intergranular groundmasses. 558 Basaltic-andesites are only present in the Yucamane III stage and have a phenocryst 559 assemblage of plagioclase (<2 mm), clinopyroxene (<1.5 mm), orthopyroxene (<600 m), 560 amphibole (<500 m), and magnetite (<200 m), with rare olivine (<300 m). Agglomerates 561 of plagioclase, clinopyroxene, amphibole, and magnetite are present in some samples. 562 Andesite and dacite samples have porphyritic textures with an intergranular, intersertal, or 563 glassy groundmass, and have a mineral assemblage of plagioclase (<2 mm), amphibole (<3 564 mm), biotite (<3 mm), clinopyroxene (<600 m), and magnetite (<300 m). Olivine (<300 565 m) and orthopyroxene (<1.0 mm) are present in some andesite samples. samples define a single high-K magmatic trend, from 569 andesites to dacites (60.1-67.7 wt.% SiO 2 ) for Calientes and from basaltic-andesites to dacites 570 (53.4-66.9 wt.% SiO 2 , Table 4) for Yucamane. In general, major oxides (except Na 2 O and 571 K 2 O) are negatively correlated with silica content. As is common in most arc magma series, 572 the light-ion lithophile elements (LILE, e.g. K, Rb, Th) and the light rare earth elements 573 (LREE, e.g. La, Ce, Nd) show positive correlations with silica content (Fig. 12), although the 574 LREE have scattered trends. In contrast, Sr, Y, the medium rare earth elements (MREE, e.g. 575 Sm, Eu, Gd), the heavy rare earth elements (HREE, e.g. Dy, Er, Yb) and the transition metals (e.g. Cr, Ni, V) are all reversely correlated with silica. Some high-field-strength elements 577 (HFSE, e.g. Zr) are also reversely correlated with silica. 578 The variation diagrams clearly show a single magmatic trend for samples from both 579 Calientes and Yucamane. Slight variations, however, are present in the major and trace 580 elements' behavior through time. For instance, Calientes volcano erupted dominantly 581 andesitic and dacitic magmas with rare eruptions involving silica-rich magmas such as the 582 Callazas ignimbrite (Calientes II stage). In contrast, Yucamane has a different pattern that 583 includes a progressive decrease in silica content through time, with Yucamane's Holocene 584 eruption products being among the most mafic examples from the entire Yucamane-Calientes 585 of two adjacent edifices 589 On the basis of the stratigraphic, geochronological and geochemical data, we propose 590 that Yucamane-Calientes represents a single compound volcano with two, adjacent and 591 partially contemporaneous eruption centers (Fig. 13). Calientes developed from ~500 ka to 592 the late Holocene (~3 ka) and was characterized by dome-forming activity with a large 593 explosive eruption at ~160-190 ka. The large volume (8-12 km 3 ) and distribution of the 594 ignimbrite deposits, and the presence of a proximal lag breccia facies in the Río Callazas 595 valley point to a source related with the YCCV. Such an interpretation is consistent with the 596 petrology, which shows that the dacitic pumices of the Callazas ignimbrite form the high-597 silica end-member of the Yucamane-Calientes magmatic series. Due to the significant volume 598

610(

  Yucamane II and III stages). During at least the late Holocene, both eruptive centers were 611 active. Both edifices, however, produced distinct magmatic products during the Holocene: 612 dominantly dacites for Calientes' recent domes, and basaltic-andesites to andesites for 613 Yucamane, suggesting a complex shallow magmatic plumbing system. Other examples for 614 adjacent and contemporaneously eruptive volcanoes in the Andes include the Mojanda-Fuya 615 Fuya volcanic complex (Robin et al., 2009), which shows contrasting eruptive dynamics and 616 magma chemistry during their Late Pleistocene history. Another example is the twin-peaked 617 Ilinizas volcanoes in the Ecuadorian segment of the Northern Volcanic Zone (Hidalgo et al., 618 2007). In the Peruvian volcanic arc, the Ampato-Sabancaya compound volcano (Samaniego et 619 al., 2016) also has a progressive shift of the eruptive activity from Ampato to Sabancaya with 620 a transitional period at the Pleistocene-Holocene transition during which both volcanoes were of the major and trace elements shows that the magmatic 625 evolution of the YCCV is controlled by processes that include fractional crystallization and 626 assimilation of crustal materials. This point is clearly shown by the well-defined trends for 627 most major and trace elements as well as the homogeneous spider diagrams for samples from 628 both Yucamane and Calientes edifices (Fig. 14a). To test these hypotheses, a diagram based 629 on the geochemical behavior of one of the most incompatible elements (e.g. Rb) versus a 630 compatible element (e.g. Ni) is commonly used to discriminate fractional crystallization (and 631 other differentiation processes) from source-related partial melting (Janoušek et al., 2015). In 632 a log (compatible element) versus log (incompatible element) plot, differentiated liquids 633 produced by partial melting show a sub-horizontal trend, whereas fractional crystallization 634 would rise to a sub-vertical trend. Plotting log (Ni) versus log (Rb) yields clearly defined sub-635 vertical trends (Fig. 14b), suggesting that fractional crystallization is the key process driving 636 magma differentiation at YCCV. Our mineralogical and geochemical data from Yucamane-637 Calientes highlights the prominent role played by fractional crystallization processes as 638 expressed by: (1) the presence of zoned plagioclase and clinopyroxene phenocrysts; (2) the 639 marked correlation between highly incompatible elements; and (3) the depletion of 640 compatible elements along differentiation trends in the magma. For instance, a decrease in Ni 641 and Cr with increasing silica suggests olivine and/or clinopyroxene fractionation, whereas a 642 decrease in Sr and Eu concentrations suggests plagioclase crystallization. The depletion in strongly suggests a key role of amphibole fractionation in the arc crust (Davidson et al., 645 2007).

  phase of Calientes occurred during the Holocene and was 670 characterized by dome-growth and subsequent collapse. Yucamane also experienced at least 671 4-5 explosive events during late Holocene times. These eruptions had a local impact and were 672 characterized by tephra-fall deposits that were usually dispersed to the ESE and frequent 673 pyroclastic density currents that swept down the cone, reaching 4-5 km from the summit.

  674

Fig. 2 .Fig. 3 . 937 Fig. 5 .

 239375 Figures

Fig. 6 .Fig. 7 .Fig. 8 .

 678 Fig. 6. (a) Sequence of blast, debris-avalanche, and block-and-ash-flow deposits exposed on

Fig. 9 . 970 Fig. 12 .Fig. 13 .

 99701213 Fig. 9. Stratigraphic sections of the pyroclastic-flow and tephra-fall deposits associated with

Fig. 14 .

 14 Fig. 14. (a) Multi-element diagrams normalized to primitive mantle (Sun and McDonough,

  994

  published 647 some isotopic data for Yucamane samples. These data reveal high 87 Sr/ 86 Sr (0.7064-0.7068) 648 and low 143 Nd/ 144 Nd ratios (0.51226-0.51233). They further report Pb isotope ratios 649 ( 206 Pb/ 204 Pb 18.066-18.220; 207 Pb/ 204 Pb 15.584-15.643, and 208 Pb/ 204 Pb 38.474-38.685)

	650	similar to those of the metamorphic basement

Table 1 .

 1 K-Ar ages for rocks from the Yucamane-Calientes compound volcano.

	986
	987

Table 2 .

 2 40 Ar/ 39 Ar ages for rocks from the Yucamane-Calientes compound volcano. The ages 988 considered in the text are the weighted mean ages (in bold).

	989
	990

Table 3 .

 3 14 C data for YCCV measured at the Center for Isotope Research, Groningen

	991	University (The Netherlands).
	992
	993

Table 4 .

 4 Selected geochemical analyses representative of the main volcanic units of YCCV.

(Figs. 2, 6) and overlying the Callazas ignimbrite deposit. Each layer is 12-18 m-thick, light

SoufrièreHills 1997; Voight et al., 2002; Belousov et al., 2007).

Elsevier, Amsterdam, 779 pp.

evaluación preliminar de las amenazas volcánicas. Bol. Soc. Geol. Perú 98, 7-27.

Scientific Drilling Project (phase 2), Hilo, Hawaii. Geochemistry, Geophysics,

Sylvestre, F., Khodri, M. (eds). Past climate variability in South America and

Acknowledgements 710

This work is part of a Peruvian-French cooperation program carried out between the

 Sabancaya (1990-1998, 2015-2018 CE). This scenario could evolve to higher magnitude SiO 2 ), while Yucamane's rocks have a slightly larger compositional range from basaltic-703 andesites to dacites (53.4-66.9 wt.% SiO 2 ). The rocks have a similar mineral assemblage of 704 plagioclase, amphibole, biotite, ortho-and clino-pyroxene, Fe-Ti oxides, and scarce olivine.

705 Such characteristics suggest that both Calientes and Yucamane belong to a common 706 magmatic system. On the basis of the geologic, geochronological, and petrologic data, we 707 consider Yucamane-Calientes as a potentially active volcanic center. Cobeñas, G., Thouret, J.-C., Bonadonna, C., Boivin, P., 2012. The c. 2030 Sun, S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: 879 Implications for mantle composition and processes. Magmatism in the Ocean Basin, Geol. 880 Soc. Am. Bull. Spec. Pap. 42, Thouret, J.C., Finizola, A., Fornari, M., Suni, J., Legeley-Padovani, A., Frechen, M., 2001. 882 Geology of El Misti volcano nearby the city of Arequipa, Peru. Geol. Soc. Am. Bull. 113,

Supplementary material 1

The eruptive chronology of the Yucamane-Calientes compound volcano: a potentially active edifice of the Central Andes (Southern Peru)

Description of methods

Major and trace element whole-rock data

Major and trace element were determined at the Institut Universitaire Européen de la Mer (Brest, France), following the analytical procedure of [START_REF] Cotten | Origin of anomalous rare-earth element and Yttrium enrichments in subaerial exposed basalts: evidence from French Polynesia[END_REF].

Typically 250 mg of rock power were dissolved in closed screw-top teflon vessels (Savillex) at about 90°C for one day using 3 ml of concentrated HF, and 1 ml of concentrated HNO 3 . Next, 96 ml of H 3 B0 3 aqueous solution (20 g/L H 3 B0 3 ) were added to neutralize the excess HF. All reagents used are analytical grade. Elements were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES) using a Horiba Jobin Yvon® Ultima 2 spectrometer. The boron included in the solution was used as an internal standard. Calibrations were made using international standard, ACE, ME, WSE, JB2. For major elements, relative standard deviation is ≤1% for SiO 2 and ≤2% for the other major elements, for trace elements standard deviation is ≤5%.

K-Ar geochronology

After macroscopic and microscopic inspections, groundmass from fresh samples was prepared following methods detailed in [START_REF] Guillou | Dating of the upper Pleistocene -Holocene volcanic activity of La Palma using the Unspiked K-Ar Technique[END_REF]. The groundmass is assumed to have formed shortly after eruption and is unlikely to contain any significant excess argon. All the samples were crushed and sieved to 0.250-0.125 mm size fractions and ultrasonically washed in acetic acid (1N) during 45 minutes at a temperature of 60°C, to remove any secondary mineral phases that might be present in minute amounts. Phenocrysts which are potential carriers of extraneous 40 Ar (including excess and inherited components) were filtered out using magnetic, gravimetric, and visual hand picking separation.

The isotopic composition and abundance of Ar were determined using an unspiked technique described by [START_REF] Charbit | Cross calibration of K-Ar standard minerals using an unspiked Ar measurement technique[END_REF]. In the unspiked technique, argon extracted from the sample is measured in sequence with purified aliquots of atmospheric argon at comparable working gas pressure in the mass-spectrometer, to suppress mass discrimination effects between the atmospheric reference and the unknown. This approach allows young samples to be dated reliably, as demonstrated via direct comparisons with the 40 Ar/ 39 Ar dating method [START_REF] Guillou | On the age of the Laschamp geomagnetic event[END_REF][START_REF] Guillou | Effectiveness of combined Unspiked K-Ar and 40 Ar/ 39 Ar dating methods in the 14 C age range[END_REF][START_REF] Singer | Multiple Brunhes Chron excursions recorded in the West Eifel (Germany) volcanics: Support for long-held mantle control over the non-axial dipole field[END_REF][START_REF] Singer | Refining the Quaternary Geomagnetic Instability Time Scale (GITS): Lava flow recordings of the Blake and Post-Blake excursions[END_REF]. Aliquots of groundmass were separated by means of densitometric and magnetic separations following the procedure described in [START_REF] Guillou | Dating of the upper Pleistocene -Holocene volcanic activity of La Palma using the Unspiked K-Ar Technique[END_REF]. Determination of K was carried out at the CRPG (Nancy, France) by atomic absorption (flame photometry) with a relative precision of 1%. Argon was extracted by radio frequency heating of 0.5 -2.0 g of sample, then transferred to an ultra-high-vacuum glass line and purified with titanium sponge and Zr-Ar getters. Isotopic analysis was performed on total 40 Ar contents ranging between 1.0 and 5.9 x 10 -11 moles using a 180°, 6 cm radius mass spectrometer with an accelerating potential of 620V. The mass spectrometer sensitivity is 5.2 x 10 -1 mol/A @ m/e = 40 with amplifier backgrounds of 5 x 10 -14 A @ m/e = 40 and 1.0 x 10 -15 A @ m/e = 36.

Beam sizes were measured simultaneously on a double Faraday collector in sets of 100 on-line acquisitions with a 1 s integration time, using two Keithley Model 6514 System Electrometers. A manometrically calibrated dose of atmospheric argon (cal. Dose) is used to convert beam intensities into atomic abundances. This dose, when comparable in pressure with the unknown, or a separate measurement of atmospheric argon (atm.


Stéphane Blain
email: stephane.blain@obs-banyuls.fr

Mathieu Rembauville
email: mathieu.rembauville@obs-banyuls.fr

Olivier Crispi
email: olivier.crispi@obs-banyuls.fr

Ingrid Obernosterer
email: ingrid.obernosterer@obs-banyuls.fr

Synchronized autonomous sampling reveals coupled pulses of biomass and export of morphologically different diatoms in the Southern Ocean

Keywords: carbon stock, carbon export, diatoms, southern ocean, aggregates, seasonal time series, autonomous sampling

The Southern Ocean hosts a large diversity of diatoms that play a major role in carbon fluxes.

How the seasonal dynamics in the abundance of specific taxa in surface waters are linked to their contribution to carbon export remains, however, poorly understood. We present here synchronized observations from autonomous samplers deployed in the mixed layer (42 m) and at depth (300m) during an entire productive season (October 2016 to March 2017) in the iron fertilized region of the central plateau of Kerguelen. Microscopic observations of surface water collected every 11 days revealed 30 different diatom taxa, each contributing to > 1% of total carbon biomass throughout the season. The synchronised sampling revealed a common pattern for diatom taxa belonging to 12 different genera, consisting, for a given taxon, in short pulses of abundance in surface waters followed by export. We explain these coupled dynamics by the formation of aggregates that are produced when a critical diatom cell abundance is reached. This control of the maximum abundance of a given diatom drives the seasonal change in the slope of the size-class distribution of the diatom community. It further constrains the total carbon diatom biomass in a narrow range of values due to the inverse relationship between total diatom abundance and their community-weighted mean biomass. This coupling let us conclude that aggregate formation, and the export to depth, occurs throughout the season for diatoms with different morphologies.

Introduction

During the past 250 million years diatoms have evolved a large diversity of forms and life styles [START_REF] Armbrust | The life of diatoms in the world's oceans[END_REF]. They are successful unicellular phototrophic organisms that are present in almost all aquatic environments. They are characterized by an ornamented siliceous cell wall with diverse forms that serve as morphological criteria for species classification, an approach that is today complemented by DNA sequencing [START_REF] Malviya | Insights into global diatom distribution and diversity in the world's ocean[END_REF]. From a biogeochemical perspective, diatoms play a central role in the carbon cycle [START_REF] Tréguer | Influence of diatom diversity on the ocean biological carbon pump[END_REF]). In the past they have contributed to the lowering of atmospheric CO2 thanks to their efficiency to massively bury carbon in the sediment [START_REF] Katz | Biological overprint of the geological carbon cycle[END_REF]) and thereby formed large reservoirs of fossil fuels. In the contemporary ocean they contribute to 40 % of global primary production [START_REF] Nelson | Production and dissolution of biogenic silica in the ocean: Revised global estimates, comparison with regional data and relationship to biogenic sedimentation[END_REF] and they sustain marine food webs.

The success of diatoms is attributed to several features. On the one side, genomic adaptations [START_REF] Armbrust | The genome of th diatom Thalassiosira pseudonana: ecoly, evolution and metabolism[END_REF][START_REF] Allen | Evolution and metabolic significance of the urea cycle in photosynthetic diatoms[END_REF]) allow diatoms to thrive in various environments. They form blooms that can result in large sinking fluxes but require substantial nutrient supply, as provided in well-mixed water columns. This occurs preferentially in regions where turbulent water columns dominate [START_REF] Margalef | Life-form of phytoplankton as survival alternatives in an unstable environment[END_REF]. On the other side, the siliceous armour provides an efficient protection against predators, and possibly also against parasites and pathogens [START_REF] Hamm | Armor: Why, When, and How[END_REF]. The dense frustule enhances cell sinking [START_REF] Smetacek | Role of sinking in diatom life-history cycles: ecological, evolutionary and geological significance[END_REF], necessitating elaborated strategies for maintaining the cells in the euphotic zone during the growth period [START_REF] Raven | The evolution of silicification in diatoms: inescapable sinking and sinking as escape?[END_REF].

The Southern Ocean hosts abundant and diverse communities of diatoms. Their ecology has been extensively described thanks to expeditions in different regions of the Antarctic circumpolar current [START_REF] Queguiner | Biogeochemical dynamics and the silicon cycle in the Atlantic sector of the Southern Ocean during austral spring 1992[END_REF][START_REF] Tremblay | Significance of the Polar Frontal Zone for large-sized diatoms and new production during summer in the Atlantic sector of the Southern Ocean[END_REF][START_REF] Leblanc | Fe and Zn effects on the Si cycle and diatom community structure in two contrasting high and low-silicate HNLC areas[END_REF]) and to observations realised in naturally iron fertilized regions [START_REF] Korb | Magnitude and maintenance of the phytoplankton bloom at South Georgia: a naturally iron-replete environment[END_REF][START_REF] Salter | Diatom resting spore ecology drives enhanced carbon export from a naturally iron-fertilized bloom in the Southern Ocean[END_REF][START_REF] Lasbleiz | Composition of diatom communities and their contribution to plankton biomass in the naturally iron-fertilized region of Kerguelen in the Southern Ocean G. King[END_REF] or during artificial fertilization experiments [START_REF] Gall | Phytoplankton processes. Part 2: Rates of primary production and factors controlling algal growth during the Southern Ocean Iron RElease Experiment (SOIREE)[END_REF][START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF]. Additional information was also gathered from investigations on the sinking en masse of the giant diatoms like Rhizosolenia spp. and Thalassiothrix spp. [START_REF] Kemp | Production of giant marine diatoms and their export at oceanic frontal zones: Implications for Si and C flux from stratified oceans[END_REF] or of empty cells of Corethron criophylum (now named Corethron pennatum (Grunow) Ostenfeld ) after entering the sexual stages [START_REF] Crawford | Spatial and temporal distribution of assemblages of the diatom Corethron criophilum in the Polar Frontal region of the South Atlantic[END_REF]. One of the main motivation to link diatom ecology and biogeochemistry lies in the critical role they play in the carbon cycle [START_REF] Tréguer | Influence of diatom diversity on the ocean biological carbon pump[END_REF]. Their role as vectors of carbon export in the water column and storage at the seafloor has been demonstrated using sediment traps [START_REF] Salter | Diatom resting spore ecology drives enhanced carbon export from a naturally iron-fertilized bloom in the Southern Ocean[END_REF][START_REF] Rigual-Hernández | Seasonal dynamics in diatom and particulate export fluxes to the deep sea in the Australian sector of the southern Antarctic Zone[END_REF]Rembauville et al. 2015a) and by the examination of sediments [START_REF] Allen | Submillennial variations in ocean conditions during deglaciation based on diatom assemblages from the southwest Atlantic: diatom record from the falkland through[END_REF][START_REF] Armand | Diatoms preserved in surface sediments of the northeastern Kerguelen Plateau[END_REF][START_REF] Rigual-Hernández | The fate of diatom valves in the Subantarctic and Polar Frontal Zones of the Southern Ocean: Sediment trap versus surface sediment assemblages[END_REF]. Considering all these observations, conceptual frameworks linking diatom ecology and the carbon cycle in the Southern Ocean were derived.

Different biogeochemical roles were attributed to different diatom taxa [START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF][START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF]. For example, fast growing taxa characterized by en masse sinking behaviour (e.g. Chaetoceros, Thalasssiosira) were described as carbon sinkers or type 1, and highly silicified slow growing species as silica sinkers or type 2. The dominance of one category over the other leads to a strong decoupling between the carbon and silicon cycles [START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF]. However, the coupling between the temporal dynamics of surface stocks and export at depth of diatoms at the taxon level is still poorly studied and understood, because it requires to collect samples evenly spaced in time during the entire growth season. This task is particularly challenging in the open ocean and even more in harsh environments like the Southern Ocean. In our study we have addressed this issue using synchronized autonomous samplers deployed for 4 months both within (42 m) and below (300 m) the surface mixed layer. Our samples were collected in the long lasting Southern Ocean blooms that are sustained by natural iron fertilization above the Kerguelen Plateau [START_REF] Blain | Effect of natural iron fertilisation on carbon sequestration in the Southern Ocean[END_REF]). Throughout the season, these blooms are dominated by a succession of different diatom taxa [START_REF] Lasbleiz | Composition of diatom communities and their contribution to plankton biomass in the naturally iron-fertilized region of Kerguelen in the Southern Ocean G. King[END_REF][START_REF] Rembauville | Plankton Assemblage Estimated with BGC-Argo Floats in the Southern Ocean: Implications for Seasonal Successions and Particle Export[END_REF]). This contrasts with spring to summer transitions in most oceanic regions where a succession of diatoms and non diatoms are observed [START_REF] Barber | A rising tide lifts all phytoplankton: Growth response of other phytoplankton taxa in diatom-dominated blooms: A RISING TIDE RAISES ALL PHYTOPLANKTON[END_REF]. The Kerguelen plateau waters therefore represent a unique environment for our investigations.

MATERIALS AND METHODS

Moorings.

Two moorings were deployed during the SOCLIM cruise (doi/10.17600/16003300), one for the sediment traps on 13 October 2016, and one in subsurface waters on 18 October 2016.

Both moorings were recovered on 7 April 2017. The sub surface mooring (50°37'135 S / 072°06'179 E, bottom depth 527m) was equipped with an aluminium framework located at 42 m below the surface that contained a remote access sampler (RAS Mac Lane) allowing the collection of 48 x 500 mL samples. We collected 12 samples of unfiltered seawater every 11th day, preserved either with glutaraldehyde or mercury chloride, and 18 filtered and unfiltered samples preserved with mercury chloride (Fig 1a). The sub surface framework was also equipped with a CTD sensor (SeabirdSBE 16) and a current meter (Aquadrop). Below the instrument package and down to 300 m, the mooring line was equipped with additional temperature (SBE 56) and temperature/salinity sensors (SBE 37) to monitor changes in the mixed layer depth. The second mooring (50°38'344 S/ 71°59'854E, bottom depth 527 m) was equipped with a Technicap PPS3 sediment trap (0.125 m 2 collecting area, 4.75 aspect ratio) located at 300 m below the surface. The 12 cups (250 ml) were filled with 5% formalin hypersaline solution buffered at pH=8 with sodium tetraborate. The collection time for each cup was 11 days. After recovery of the RAS mooring the samples were immediately transferred from the bag into 500 mL clean polycarbonate bottles and stored in the dark at room temperature until processing in the laboratory. For the sediment traps, 1 ml of the supernatant of the cups were replaced by fresh hypersaline formalin buffered (pH=8) solution before storage at room temperature. Upon recovery of the cups in the laboratory, the swimmers were removed and the samples split into eight aliquots using a Jencons peristaltic splitter [START_REF] Rembauville | Export fluxes in a naturally iron-fertilized area of the Southern Ocean -Part 1: Seasonal dynamics of particulate organic carbon export from a moored sediment trap[END_REF])

Microscopy and morphometry

Microscopic observations were conducted within four months after recovery of the moorings. For the identification of diatoms, counting and size measurements, we used the protocol described by [START_REF] Salter | Diatom resting spore ecology drives enhanced carbon export from a naturally iron-fertilized bloom in the Southern Ocean[END_REF]) and modified by Rembauville et al. 2015a that allows to separately consider full and empty cells. This issue is critical for our work which aims to reconstruct the carbon flux and stock attributed to diatoms. For the preparations of diatom counting (RAS or TRAP) the samples were processed as follows. For the RAS, depending on the diatom abundance, 20 mL, or 2 mL diluted with 18 mL of artificial seawater (S=34), were decanted in a Sedgewick Rafter counting chamber (Pyser SGE S52, 1mL chamber volume). For the trap samples, 2mL of one-eight aliquot was diluted with 18 mL of artificial seawater and decanted in a Sedgewick Rafter counting chamber. Diatoms were enumerated and identified under an inverted microscope with phase contrast (Olympus IX170) at magnification 400X. The morphometric measurements were done using high resolution images (Olympus DP71 camera) and Fiji image processing software. The biovolume was calculated from morphometric measurements [START_REF] Hillebrand | Biovolume calculation for pelagic and benthic microalgae[END_REF]). The carbon content was derived from biovolumes using allometric relationships reported in the literature [START_REF] Menden-Deuer | Carbon to volume relationships for dinoflagellates, diatoms, and other protist plankton[END_REF][START_REF] Cornet-Barthau | Biovolume and biomass estimates of key diatoms in the Southern Ocean[END_REF]) and taking into account specific relationships for spores (Rembauville et al. 2015a). The size-abundance distributions of the diatom community collected at a given time point was based on an octave (log2) scale of cell volume.

Diatom carbon stocks and export fluxes.

The carbon stock accounted for by diatoms was calculated by summing up the contributions of the different taxa. The integrated diatom carbon stock was calculated by multiplying the total carbon contained in full diatoms (POCdiat) by the mean mixed layer depth at the sampling time.

The export flux of diatoms was calculated using the equation:

𝐶𝑒𝑙𝑙 𝑓𝑙𝑢𝑥 = 𝑁 𝑑𝑖𝑎𝑡 × 𝑑 × 8 × 𝑉 𝑎𝑙𝑖𝑞𝑢𝑜𝑡 × 1 0.125 × 1 11 × 𝑘
Where Ndiat (cell m -2 d -1 ) is the number of cells counted in one chamber, d is the dilution factor, Valiquot is the volume of the aliquot, and k is the fraction of the chamber counted.

The diatom flux was then converted to POC flux for each taxon using allometric equations.

Biogeochemical analysis.

Dissolved major nutrients (NO3 -and Si(OH)4) in the RAS were measured in the 0.8 µm filtered (polycarbonate filters ) samples preserved with mercuric chloride using standard protocols that have been previously applied to samples of this site [START_REF] Blain | Distributions and stoichiometry of dissolved nitrogen and phosphorus in the iron-fertilized region near Kerguelen (Southern Ocean)[END_REF].

Community weighted mean (CWM)

.

CWM is defined using the equation

𝐶𝑊𝑀 = ∑ 𝑝 𝑖 × 𝐶 𝑑𝑖𝑎𝑡,𝑖 𝑁 𝑖=1
where for the taxa i, pi is its relative abundance and Cdiat,i is its carbon content.

To estimate the effect of the variability of the diatom composition on the CWM we simulated virtual diatom communities. We first randomly selected 14 taxa among the 30 taxa contributing to at least 1% of the biomass and then we allocated to these selected diatom taxa a random abundance ranging between 0 and their respective maximum abundance observed during the season. Finally Nt and CWM were calculated. The simulation was repeated 1000 times.

Calculation of the critical cell concentrations.

The critical cell concentration was calculated according to the equation of [START_REF] Jackson | A model of the formation of marine algal flocs by physical coagulation processes[END_REF] .

𝐶 𝐶𝑟 (𝑐𝑒𝑙𝑙 𝑚 -3 ) = 0.096×𝜇 𝛼×𝛾×𝐸𝑆𝑅 3 where ESR is the Equivalent Spherical Radius (m), µ is the net growth rate (s -1 ) , α is the stickiness and γ is the shear rate (s -1 ) estimated according to the formulation [START_REF] Mackenzie | Wind-based models for estimating the dissipation rates of turbulent energy in aquatic environments: empirical comparisons[END_REF].

𝛾 = √ 𝜀 𝜗 where 𝜀 = ( 𝜏 𝜌 𝑤 ) 3 2 ⁄ × 1 𝑘𝑍
τ is the mean wind stress of 0.22 Nm -2 for the period of interest obtained from the Japanese 55-yr reanalysis (JRA-55; available online at http://jra.kishou.go.jp). ν is seawater kinematic viscosity (10 -6 m 2 s -1 ), ρw is the seawater density in the mixed layer (1027 kg m -3 ). k is the von Kármán constant (0.41) and Z is the mixed layer depth (70 m).

Simulations represented in SI Fig. 3 were obtained using the equations above with different values of α (0.1, 0.25, 0.5 and 1) and µ (0.05 and 0.13 d -1 ).

Environmental context and temporal phytoplankton bloom dynamics

The region of interest for our study is located south-east of Kerguelen island on the central part of the Kerguelen Plateau (Figure 1a). The sampling site is located in a zone that has been identified during previous studies in Jan-Feb 2005 [START_REF] Blain | Effect of natural iron fertilisation on carbon sequestration in the Southern Ocean[END_REF]) and Oct-Nov 2011 [START_REF] Blain | Distributions and stoichiometry of dissolved nitrogen and phosphorus in the iron-fertilized region near Kerguelen (Southern Ocean)[END_REF] as the core of the Kerguelen bloom. The ocean colour satellite images for the season 2016-2017 confirm that the large bloom, typical of this region, was also present during the time period considered here. The temporal variations of surface chlorophyll in the vicinity of the sampling site show the succession of two blooms peaking in November and January (Figure 1B). This seasonal pattern is consistent with the dual blooms as revealed from the climatology for this site (Figure 1B). Ocean colour images combined with backtrajectories derived from current speeds and directions at 42 m (SI Figure 1) show that the horizontal advection of surface water parcels that reached the sampling site during a time step of 11 days did not result in large variability of chlorophyll. The influence of spatial variability was therefore limited and we will interpret changes observed at the sampling site as mainly resulting from temporal variability.

The seasonal changes of the mixed layer depth (MLD) show that all the samples collected by the RAS were located in the mixed layer (Figure 2A). The variation of diatom carbon biomass integrated over the mixed layer, which accounted for the surface biomass [START_REF] Pellichero | Relaxation of Wind Stress Drives the Abrupt Onset of Biological Carbon Uptake in the Kerguelen Bloom: A Multisensor Approach[END_REF], presented similar temporal variations to chlorophyll concentrations (Figure 2C). The build-up of diatom biomass affected the concentrations of major nutrients in surface waters (Figure 2D). The concentrations of nitrate and silicic acid were high at the beginning of the season and decreased during the first bloom. This decrease was much larger for silicic acid than for nitrate. Silicic acid concentrations increased slightly after the first bloom before being further depleted after the second bloom.

Temporal dynamics of surface diatom communities

Main taxa contributing to carbon biomass

Throughout the season, the Kerguelen blooms were dominated by diatoms [START_REF] Lasbleiz | Composition of diatom communities and their contribution to plankton biomass in the naturally iron-fertilized region of Kerguelen in the Southern Ocean G. King[END_REF][START_REF] Rembauville | Plankton Assemblage Estimated with BGC-Argo Floats in the Southern Ocean: Implications for Seasonal Successions and Particle Export[END_REF], in contrast to blooms in temperate regions where diatoms are replaced by other phytoplankton during part of the productive season [START_REF] Barber | A rising tide lifts all phytoplankton: Growth response of other phytoplankton taxa in diatom-dominated blooms: A RISING TIDE RAISES ALL PHYTOPLANKTON[END_REF]. For the RAS time series, diatoms accounted for 89 ± 9 % of carbon phytoplankton biomass observed by microscopy supporting the idea of diatom dominance.

Because we wanted to explore the relationship between biogeochemistry (carbon biomass and export) and composition of the diatom community, we describe the latter using microscopic observations rather than DNA sequencing. In this context, the advantages of microscopic observations are that they allow the taxonomic identification, the determination of absolute counts of full cells (i.e; containing carbon) and quantitative estimates of the carbon content (Cdiat) derived from size measurements [START_REF] Hillebrand | Biovolume calculation for pelagic and benthic microalgae[END_REF][START_REF] Menden-Deuer | Carbon to volume relationships for dinoflagellates, diatoms, and other protist plankton[END_REF][START_REF] Cornet-Barthau | Biovolume and biomass estimates of key diatoms in the Southern Ocean[END_REF]). In the following, we consider only diatoms at the genus or species level (hereafter referred to as taxa) that contribute to > 1% of the total diatom C biomass (POCdiat). In the mixed layer, we identified a total of 30 different taxa during the entire season (Fig. 2 b). The number of taxa was relatively constant throughout the season (min =11, max =15, median=14), whereas the composition of the community varied dramatically (Fig. 2B). The first peak of Chlorophyll and POCdiat (Fig. 2c) was dominated by taxa with low Cdiat, typically Chaetoceros (Hyalochaete), Seasonal changes of abundance, size and carbon content.

The abundance of the main diatom taxa (30 in total) contributing to the total carbon biomass POCdiat varied by 3 orders of magnitude and their individual biovolumes varied by more than 3 orders of magnitude (from 112 µm 3 up to 152600 µm 3 ). To examine how these variations affect the total carbon biomass POCdiat, we characterized the diatom communities observed at 12 time points during the season in two different manners.

In the first approach, we used the diatom community size spectrum. The slope of the size spectra revealed marked changes throughout the season (Table 1 and SI Fig. 2).

At the beginning of the season, the slope was within the range [-0.8,-0 obtain the total carbon biomass POCdiat of the community. However, this approach could be biased if the contribution of diatom spores to a size class is non-negligible. Indeed, the volumetric carbon content of a spore is higher than that of a vegetative cell (Rembauville et al. 2015a).

For this reason we also characterized the diatom communities using a second approach and estimated the community weighted mean (CWM) (Fig. 3A). Obviously, the CWM and the total carbon biomass POCdiat are linked using the equation POCdiat=Nt x CWM where Nt is the total abundance of diatoms of the community. CWM and Nt show clear opposite seasonal trends, increasing and decreasing, respectively, throughout the season (Fig. 3A,B). These changes provided insight to the seasonal variations of the total diatom carbon biomass.

Coupling between diatom taxa in surface waters and their export

The carbon fluxes attributed to diatoms in each cup of the sediment trap are presented in The relative contributions of the different taxa to diatom carbon export are shown in Fig. 4B.

The first event was entirely dominated by the export of Chaetoceros and Thalassiosira resting spores while more diverse taxa were contributing to the second event. For the latter, when cups #9 and #10 are cumulated, the relative contributions of the different taxa were:

Chaetoceros resting spores (43%), Pseudo-nitzschia heimii (2%), Pseudo-nitzschia lineola (3%), Navicula directa (0.7%), Eucampia antarctica (20.4%), Corethron inerme (4.2%),

Odontella weisflogii spores (15.3%), Proboscia inermis (1.3%), Thalassiothrix antarctica

(3,6%), Membraneis challenger (5%), Rhizosolenia styliformis (1.5%), Asteromphalus hookeri(1.7%).

The synchronized sampling of diatom communities in the mixed layer and in the sediment traps (Fig. 2A) allowed a detailed investigation of the coupling between the dynamics of the abundance of individual diatom taxa in the mixed layer and of their export at 300 m. To compare both dynamics we calculated, for a given genus, its relative contribution at each time point to its total seasonal abundance and to its total seasonal export flux (Fig. 5).

It is evident that the maximum seasonal abundance of the different diatom genera in the mixed layer occurred during a short time period, closely followed by a carbon export event of the same genus. We observed that more than 44.2 % (for Thalassionema) and up to 100 % (for Proboscia) of the seasonal export attributable to a given genus took place during a relatively short time period, representing 17 % of the entire sampling period (132 days) (Table 2). To confirm the link between the dynamics of diatom surface abundance and export we estimated for each genus the correlation between the two time series with different time lags (SI Fig. 3). The correlation is significant for 10 of the genera, the exceptions being Fragilariopsis and Odontella. The time lag leading to the best correlation between both time series was 0 day for Membraneis, Thalassiosira, 11 days for Proboscia, Rhizosolenia, Eucampia, Thalassionema and Chaetoceros and 22 days for Corethron and Thalassiothrix.

DISCUSSION

Our study was designed to address the coupling between the seasonal dynamics of diverse diatoms in the surface layer and their export at depth. Our major aim was to refine previous conceptual views both on a temporal scale and on the taxon level, and thereby to make an important step forward. Thanks to the seasonal coverage with good temporal resolution, and to the synchronized observations between the surface layer and at depth we are able to reveal a common pattern for diatom species belonging to 10 different genera that are short pulses of surface abundance followed by export. Fragilariopsis kerguelensis and Thalassiothrix antarctica in the Antarctic zone [START_REF] Rigual-Hernández | Seasonal dynamics in diatom and particulate export fluxes to the deep sea in the Australian sector of the southern Antarctic Zone[END_REF]. However, seasonal changes of the surface diatom communities were not described in these previous studies, limiting further investigation of the origin of these diatom pulses for carbon export. In addition, empty and full diatom cells were not distinguished by the methodology applied [START_REF] Romero | Siliceous phytoplankton of the western equatorial Atlantic: sediment traps and surface sediments[END_REF].

Short-term artificial iron fertilization experiments in the Southern

Our Our simulation (Fig. 3D) shows that this unique constraint, that is size, produces an envelope of communities characterized by CWM and Nt consistent with the communities observed in situ.

The slopes of the size-class spectrum determined in our study at the beginning of the season were close to those reported for coastal eutrophic and upwelling ecosystems (around -0.8; [START_REF] Cermeño | Species richness and cell-size distribution: size structure of phytoplankton communities[END_REF], while slopes up to -1.3 are reported for subtropical regions [START_REF] Cermeño | Species richness and cell-size distribution: size structure of phytoplankton communities[END_REF]. A 10 year time series at a shelf station in a temperate region revealed little variation (-0.96), with only episodically low slopes (-0.6) during the relaxation of upwelling [START_REF] Huete-Ortega | General patterns in the size scaling of phytoplankton abundance in coastal waters during a 10-year time series[END_REF]. In comparison, the slopes obtained in the present study during the second part of the season (-0.29 to -0.25) are low.

Both, the changes of the slope of the size-class spectrum (Table 1) and of the CWM are explained by the replacement of small and abundant diatoms during the first part of the season by large and less abundant diatoms during summer.

The paradigm that small phytoplankton dominate when resources are limited, as is typically the case in oligotrophic gyres, and that large cells thrive when sufficient amounts of resources are available, typical for coastal environments or upwelling regions, has been invoked to explain spatial differences in the slopes of the phytoplankton community size spectrum and can be explained by the size scaling of nutrient requirements and growth [START_REF] Irwin | Scaling-up from nutrient physiology to the size-structure of phytoplankton communities[END_REF]. Our observations that large cells dominate in summer when essential nutrients like silicic acid (Fig. 2D) and iron [START_REF] Blain | Distribution of dissolved iron during the natural ironfertilization experiment KEOPS (Kerguelen Plateau, Southern Ocean)[END_REF]) are at their lowest values seems at odds with this explanation. Therefore, another mechanism should control the size distribution of the diatom communities in this region. We propose the formation of diatom aggregates as an explanation. The first observations of diatom aggregates have been reported by [START_REF] Billett | Seasonal sedimentation of phytoplankton to the deep-sea benthos[END_REF][START_REF] Smetacek | Role of sinking in diatom life-history cycles: ecological, evolutionary and geological significance[END_REF]. Theoretical approaches have shown that the formation of aggregates occurs when a critical cell abundance is reached for given environmental and cell properties [START_REF] Jackson | A model of the formation of marine algal flocs by physical coagulation processes[END_REF]) and such predictions have been confirmed

in situ [START_REF] Boyd | Are mesoscale perturbation experiments in polar waters prone to physical artefacts? Evidence from algal aggregation modelling studies[END_REF]. The critical abundances calculated by Jackson's model were also consistent with POC and biovolumes determined at the Bermuda Atlantic time series and off southern California [START_REF] Jackson | A model of the formation of marine algal flocs by physical coagulation processes[END_REF]). In the Kerguelen bloom the formation of aggregates has been suggested to occur during the spring bloom based on in situ observations of changes in the size spectrum and modelling [START_REF] Jouandet | Rapid formation of large aggregates during the spring bloom of Kerguelen Island: observations and model comparisons[END_REF]. The critical abundance depends on both, the properties of diatoms (ESR, stickiness, and net growth rate) and the properties of the environment (shear rate). When applying our data to an aggregation model [START_REF] Jackson | A model of the formation of marine algal flocs by physical coagulation processes[END_REF]), it showed that the observed maximum abundances for individual taxa (Fig. 6), or for size classes (SI Fig. 4) was determined by the net-growth rate and the stickiness of individual diatom taxa.

If we look at the results of the model considering different stickiness values, a parameter that can vary largely depending on the taxa [START_REF] Kiørboe | Coagulation efficiency and aggregate formation in marine phytoplankton[END_REF]), the agreement between the model and the data is better with α=1 for small cells (µ=0.13 d -1 ). The maximum abundances are even below the critical abundances predicted by the model with this value of α. It is conceivable that the ESR of small cells is underestimated in the context of the formation of aggregates because it does not take into account the presence of spines, which likely increase the ESR. For the larger cell (or size classes), that were growing around µ=0.05 d -1 , the better fitting is obtained for values of α in the range 0.5-0.25 (Fig. 6 and SI Fig. 4). This suggests that the stickiness of these larger cells would be lower. These results obtained at the diatom taxon level extend the previous conclusion of Jackson and Kiørboe 2008 obtained for bulk POC that the formation of diatom aggregates is a major factor of control for the maximum diatom abundance and consequently for their export.

When aggregates formed in the mixed layer are exported below the mixed layer depth, pulsed export events are expected. This was obviously the case considering the pulsed export dynamics observed in the present study (Fig. 5). We note that a similar seasonal pattern of carbon diatom export was observed in 2011-2012 at the same site (Rembauville et al. 2015a) suggesting that there is little interannual variability in the shape and magnitude of the carbon fluxes transported by diatoms. However, in the present study the total seasonal flux of POC was roughly 10 times higher than in 2011-2012 suggesting a large interannual variability in the carbon export due to faecal pellets. This observation is puzzling and advocates for seasonal monitoring of the composition of zooplankton community composition. Such observations were technically not feasible during the present study, but the rapid ongoing technological developments of underwater video profilers integrated to profiling floats [START_REF] Claustre | Observing the Global Ocean with Biogeochemical-Argo[END_REF] or moorings provide promising perspectives for the near future.

Our sediment trap samples were collected using cups filled with hypersaline solution, and transported back to the laboratory for processing, including splitting. This protocol is not as well adapted to preserve aggregates as the collection and direct observation with gel traps [START_REF] Ebersbach | Sinking particle properties from polyacrylamide gels during KEOPS: zooplankton control of carbon export in an area of persistent natural iron inputs in the Southern Ocean[END_REF]. Considering the different estimates of

  .7]. By mid-December which coincided with the end of the declining phase of the first bloom, the slope increased rapidly up to values in the range [-0.2,-0.3], and these values were maintained until the last week of February. These latter slopes were less significant due to one or two outliers located in the middle of the size-class range that could indicate a bi-modal distribution rather than a linear one. At the end of February, the slope increased to a value of -0.51. If we consider that the biovolume scales with the diatom abundance, as shown by the size spectrum, and if the individual carbon content of diatoms scales with the biovolume as supported by previous studies (Menden-Deuer and Lessard 2000; Cornet-Barthau et al. 2007), it is in principle possible to sum up the contributions of the different size classes to

Fig. 4A .

 4A Fig. 4A. The first export event was mainly collected by cups #3, #4 and #5. The cumulated

  Ocean and more specifically the 35 days long EIFEX cruise[START_REF] Smetacek | Deep carbon export from a Southern Ocean iron-fertilized diatom bloom[END_REF]) revealed the critical role of the pulsed nature of POC export due to boom-and-burst diatoms[START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF] exemplified by Chaetoceros dichaeta. On a seasonal scale, the pulsed export of different diatom taxa was inferred from sediment trap deployments in the Australian sector of the Southern Ocean. The relative abundance of Pseudo-nitzschia and of the small Fragilariopsis spp. in sediment traps was closely related to POC pulses in the Polar Front and in the Sub Antarctic zones[START_REF] Rigual-Hernández | The fate of diatom valves in the Subantarctic and Polar Frontal Zones of the Southern Ocean: Sediment trap versus surface sediment assemblages[END_REF]. Similar observations were made for

  work provides evidences of the pulsed export of diatoms for additional diatom taxa, but it also allows us to go a step further and to address the question: what causes such a common pattern? To answer this question, we start with the examination of the seasonal changes of diatom abundance and size in the mixed layer. The maximum abundance reached during the season by diatoms belonging to 12 different genera was strongly related to size, characterized by the spherical equivalent radius (Fig 6 and SI Fig. 4).

  time lags we can also estimate the sinking velocity of aggregates. These are in the range 25 to 250 m d -1 and thus in good agreement with values determined in rolling tanks for aggregates formed with different Southern Ocean diatoms ([13-260 m d -1 ]; Laurenceau-Cornec et al. 2015). Collectively, our observations let us conclude that the formation of diatom aggregates occurs throughout the season, and is driven by a threshold in the abundance of a given diatom taxon. This leads to pulsed temporal dynamics in the abundance of diatoms with diverse morphology and their export to depth. The control of the maximum abundance of a given diatom by the formation of aggregates drives the seasonal change in the slope of the size-class distribution of the diatom community. It also constrains the total carbon diatom biomass in a narrow range of values due to the inverse relationship between Nt and CWM.

aFigure 1 :

 1 Figure 1: Chlorophyll seasonal changes and location of the mooring site. (A) Global view of

Figure 2 :

 2 Figure 2: Seasonal changes in the Kerguelen bloom. (A) The synchronized sampling strategy

Figure 3 :

 3 Figure 3: Seasonal changes of diatom abundance in relation to carbon biomass.

Figure 4 :

 4 Figure 4 : Carbon export fluxes by diatoms. A) Total diatom carbon flux in each cup B) relative

Figure 5 :

 5 Figure 5 : Temporal changes in the relative contribution to the cumulated seasonal

Figure 6 :

 6 Figure 6 : Comparison of the maximum seasonal abundances of the different diatom taxa

  Figure 1

  The second maximum of Chlorophyll a and POCdiat was dominated by taxa characterized by high Cdiat (Eucampia antarctica, Corethron inerme, Ondotella weissflogii, Thalasssiothrix antarctica) with the exception of the pennate Pseudo-nitzchia, a genus commonly identified in artificially iron fertilized blooms[START_REF] Marchetti | Comparative metatranscriptomics identifies molecular bases for the physiological responses of phytoplankton to varying iron availability[END_REF]. The iconic pennate species of the Southern Ocean Fragilariopsis kerguelensis had substantial contributions to POCdiat in our first sample (25 October). Its contribution continuously decreased as the first bloom developed, became negligible during the second bloom and started to recover in late summer.

Thalassiosira antarctica, including their resting spores, and other small centric diatoms.

Table 1 :

 1 Seasonal changes of diatom communities size class spectrum parameters where

	558
	559	biovolume were in µm 3 and abundance in cell L -1
		date	sample	slope	intercept	R_squared	p
		10/25/2016	1	-0.81	7.001	0.563	0.012
		11/06/2016	2	-0.82	7.683	0.529	0.017
		11/17/2016	3	-0.73	7.225	0.732	0.003
		11/28/2016	4	-0.71	7.082	0.745	0.002
		12/09/2016	5	-0.50	6.309	0.559	0.020
		12/20/2016	6	-0.48	5.801	0.270	0.123
		12/31/2016	7	-0.20	5.111	0.040	0.577
		01/11/2017	8	-0.25	5.413	0.120	0.360
		01/22/2017	9	-0.25	5.214	0.175	0.262
		02/02/2017	10	-0.22	5.067	0.361	0.115
		02/13/2017	11	-0.19	4.577	0.102	0.400
		02/24/2017	12	-0.51	5.731	0.400	0.090
	560

Table 2 : Diatom taxa, and biological and environmental parameters

 2

	Diatoms	% a	Morphology b	Spores	µ c	T max d	Si(OH4) e	NO3 e
					d -1		µM	µM
	Proboscia inermis	100	CR	N		7	3.18	22.1
	Membraneis challengeri	84.3	PRap	N		10	2.45	22.8
	Thalassiosira antarctica	78.7	CR	Y	0.17	2	17.9	25.4
	Thalassiothrix antarctica	77.2	PArap	N		7	3.18	22.1
	Rhizosolenia chunii	76.6	CR	N		7	3.18	22.1
	Pseudo-nitzschia spp.	68.1	PRap	N	0.067	7	3.18	22.1
	Eucampia antarctica	67.1	CMP	Y	0.072	7	3.18	22.1
	Corethron inerme	65.3	CR	N	0.046	7	3.18	22.1
	Odontella weisflogii	63.4	CMP	Y		7	3.18	22.1
	Chaetoceros (Hyalochaete)	59.6	CMP	Y	0.13	2	17.9	25.4
	Navicula directa	58.4	PRap	N		8	4.52	21.9
	Fragilariopsis kerguelensis	49.6	PRap	N	0.077	2	17.9	22.8
	Thalassionema nitzschioides	44.2	PRap	N	0.11	2	17.9	22.8

ACKNOWLEDGMENT

We thank the captains and the crew of the R/V Marion Dufresne for their support during the cruises. We thank E. de Saint Léger, F. Pérault and L. Scoarnec from DT-INSU, and people of IPEV (Institut Polaire Paul Emile Victor) for the technical support during preparation, deployment and recovery of moorings. We also thanks Tom Trull who inspired the use of RAS for our project and Diana Davis for her valuable advises in the preparation of this equipment. We thanks the associate editor S. Menden-Deuer and the 4 anonymous reviewers for their constructive comments. This work is part of the project SOCLIM supported by the Climate Initiative of the foundation BNP Paribas, the French research program LEFE-CYBER of INSU-CNRS, IPEV, Sorbonne Université, and the Flotte Océanographique Française. The authors declare no conflict of interest.