Mateja Ogrič
email: matejaogrič@gmail.com

Mathieu Dellinger

Katherine E Grant

Valier Galy

Xin Gu

Susan L Brantley

Robert G Hilton

LOW RATES OF Rock organic carbon oxidation and ANTHROPOGENIC CYCLING OF rhenium IN A SLOWLY DENUDING LANDSCAPE

Keywords: chemical weathering, rock organic carbon, oxidation, rhenium, low denudation, anthropogenic impacts

The oxidation of petrogenic organic carbon (OCpetro) is a source of carbon dioxide to the atmosphere over geological timescales. The rates of OCpetro oxidation in locations that experience low rates of denudation remain poorly constrained, despite these landscapes dominating Earth's continental surface area. Here, we track OCpetro oxidation using radiocarbon and the trace element rhenium (Re) in the deep weathering profiles, soils and stream waters of the Susquehanna Shale Hills Critical Zone Observatory (PA, USA). In a ridge-top borehole, radiocarbon measurements reveal the presence of a broad OCpetro weathering front, with a first order assessment of ~40% loss occurring over ~6 m. However, the low OCpetro concentration (<0.05 wt.%) and inputs of radiocarbon throughout the deepest parts of the profile complicate the assessment of OCpetro loss. The OCpetro weathering front coincides with a zone of Re depletion (~90% loss), and we estimate that >80% of Re in the rock is associated with OCpetro, based on Re/Na and Re/S ratios. Using estimates of long-term denudation This article is protected by

Introduction

The transfer of carbon between biogeochemical reservoirs is one of the main drivers of climate change over geological timescales [START_REF] Berner | A new model for atmospheric oxygen over Phanerozoic time[END_REF][START_REF] Derry | Neogene growth of the sedimentary organic carbon reservoir[END_REF]. Over long time periods, carbon dioxide (CO2) emissions through volcanism, carbonate weathering by sulfuric acid, and oxidation of rock-derived organic carbon (petrogenic, OCpetro) are countered by CO2 sequestration through silicate weathering, carbonate formation, and burial of biospheric organic carbon (OCbio) [START_REF] Hilton | Mountains, erosion and the carbon cycle[END_REF]. The CO2 emissions from OCpetro oxidation [START_REF] Petsch | Weathering of Organic Carbon[END_REF][START_REF] Gu | How Particle Size Influences Oxidation of Ancient Organic Matter during Weathering of Black Shale[END_REF] and carbonate weathering by sulfuric acid [START_REF] Torres | Glacial weathering, sulfide oxidation, and global carbon cycle feedbacks[END_REF][START_REF] Burke | Sulfur isotopes in rivers: Insights into global weathering budgets, pyrite oxidation, and the modern sulfur cycle[END_REF] may be of similar magnitude to the release of CO2 from volcanic degassing (79±9 MtC yr -1 , Plank and Manning, 2019), yet the global rates of OCpetro oxidation remain poorly constrained [START_REF] Petsch | Weathering of Organic Carbon[END_REF].

The oxidative weathering of OCpetro has been assessed in river catchments using the trace element rhenium (Re) [START_REF] Dalai | Dissolved rhenium in the Yamuna river system and the Ganga in the Himalaya: role of black shale weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere[END_REF][START_REF] Hilton | Geological respiration of a mountain belt revealed by the trace element rhenium[END_REF][START_REF] Hilton | Concentration-Discharge Relationships of Dissolved Rhenium in Alpine Catchments Reveal Its Use as a Tracer of Oxidative Weathering[END_REF][START_REF] Horan | Mountain glaciation drives rapid oxidation of rock-bound organic carbon[END_REF][START_REF] Horan | Carbon dioxide emissions by rock organic carbon oxidation and the net geochemical carbon budget of the Mackenzie River Basin[END_REF]. Rhenium is enriched in sediments deposited in reducing conditions, and preserved in sedimentary rocks [START_REF] Morford | Rhenium geochemical cycling: Insights from continental margins[END_REF], especially in organic carbon-rich black shales [START_REF] Colodner | The geochemical cycle of rhenium: a reconnaissance[END_REF], due to a close association of Re with organic matter in rocks [START_REF] Selby | Re-Os geochronology of organic rich sediments: an evaluation of organic matter analysis methods[END_REF]. Coupled loss of Re and OCpetro has been observed during oxidative weathering [START_REF] Jaffe | Mobility of rhenium, platinum group elements and organic carbon during black shale weathering[END_REF][START_REF] Hilton | Geological respiration of a mountain belt revealed by the trace element rhenium[END_REF], with the mobilization of Re in the dissolved load over the large pH range (5.5 to 9.5) and in oxygenated waters (Eh>0 V), typical of most river waters [START_REF] Colodner | The geochemical cycle of rhenium: a reconnaissance[END_REF]. As such, dissolved Re flux has been used as a proxy to estimate catchment-wide OCpetro oxidation rates in highly erosive catchments of the Himalaya [START_REF] Dalai | Dissolved rhenium in the Yamuna river system and the Ganga in the Himalaya: role of black shale weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere[END_REF], Taiwan [START_REF] Hilton | Geological respiration of a mountain belt revealed by the trace element rhenium[END_REF] and New Zealand [START_REF] Horan | Mountain glaciation drives rapid oxidation of rock-bound organic carbon[END_REF] and in moderate erosive setting of the Mackenzie River basin [START_REF] Horan | Carbon dioxide emissions by rock organic carbon oxidation and the net geochemical carbon budget of the Mackenzie River Basin[END_REF], the European Alps [START_REF] Hilton | Concentration-Discharge Relationships of Dissolved Rhenium in Alpine Catchments Reveal Its Use as a Tracer of Oxidative Weathering[END_REF] and Pacific Northwest of the USA [START_REF] Ghazi | Concentration-runoff relationships of contrasting small mountainous rivers in the Pacific Northwest, USA: Insights into the weathering of rhenium relative to other weathering products[END_REF].

Applying the Re proxy to settings with moderate to high erosion rates has been important for establishing the role of OCpetro supply in driving weathering fluxes [START_REF] Hilton | Geological respiration of a mountain belt revealed by the trace element rhenium[END_REF][START_REF] Horan | Mountain glaciation drives rapid oxidation of rock-bound organic carbon[END_REF][START_REF] Ghazi | Concentration-runoff relationships of contrasting small mountainous rivers in the Pacific Northwest, USA: Insights into the weathering of rhenium relative to other weathering products[END_REF]. However, there are reasons why lower erosion rate settings deserve focus. First, low denudation rates (<0.5 mm yr -1 ) are common over the majority of Earth's continents, and these settings contribute to ~60% of global chemical denudation [START_REF] Larsen | The contribution of mountains to global denudation[END_REF]. Second, low erosion rates can lead to the development of thick weathering profiles, helping to reveal reaction fronts of OCpetro and sulfide oxidation [START_REF] Petsch | A field study of the chemical weathering of ancient sedimentary organic matter[END_REF][START_REF] Wildman | The weathering of sedimentary organic matter as a control on atmospheric O2: I. Analysis of a black shale[END_REF]Brantley et al., 2013a;[START_REF] Gu | How Particle Size Influences Oxidation of Ancient Organic Matter during Weathering of Black Shale[END_REF] and track the mobility of Re during weathering [START_REF] Jaffe | Mobility of rhenium, platinum group elements and organic carbon during black shale weathering[END_REF]. Finally, settings with lower chemical denudation could be more sensitive to Re inputs from anthropogenic sources [START_REF] Miller | Re-assessing the surface cycling of molybdenum and rhenium[END_REF][START_REF] Rahaman | Rhenium in Indian rivers: Sources, fluxes, and contribution to oceanic budget[END_REF], helping us understand the modern-day perturbation of the Re cycle [START_REF] Sen | Anthropogenic Disturbance of Element Cycles at the Earth's Surface[END_REF]. This is because Re is enriched in fossil fuels [START_REF] Bertine | Fossil Fuel Combustion and the Major Sedimentary Cycle[END_REF][START_REF] Selby | Re-Os elemental and isotopic systematics in crude oils[END_REF], volatile at temperatures >270 °C [START_REF] Colodner | Rhenium in the Black Sea: comparison with molybdenum and uranium[END_REF], and can be deposited from ash fallout after coal combustion. Anthropogenic Re inputs have been invoked to explain high dissolved Re concentrations in large European rivers (Danube, Rhine), the Yangtze and the Mississippi rivers [START_REF] Miller | Re-assessing the surface cycling of molybdenum and rhenium[END_REF] alongside lake and marine records from North America [START_REF] Chappaz | Sequestration mechanisms and anthropogenic inputs of rhenium in sediments from Eastern Canada lakes[END_REF][START_REF] Prouty | Deep-sea coral record of human impact on watershed quality in the Mississippi River Basin[END_REF].

In this study we explore OCpetro weathering and the trace element Re in deep shale weathering profiles (~30 m) in an OC-poor and low denudation rate setting to provide estimates of long-term OCpetro oxidation rates. Having done so, we provide new constraints on anthropogenic Re perturbation in the near surface environment on a catchment scale.

Methodology and methods

The study is set in the Shale Hills catchment of the Susquehanna Shale Hills Critical Zone Observatory (SSH-CZO). There, a low denudation rate results in deep weathering profiles [START_REF] Jin | Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory[END_REF][START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]Gu et al., 2020a). We quantify OCpetro weathering using a combination of new radiocarbon and Re measurements, with previously published S and major element concentration data [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF], applied to samples from subsurface boreholes, surface soils, rainwater and stream water.

Study site

Shale Hills is a small (0.08 km 2 ), first order catchment located in the Appalachia of central Pennsylvania (Figure 1a). The catchment is almost exclusively underlain by Rose Hill Formation of Silurian age, with low total organic carbon (TOC) concentrations of <0.05 wt.% [START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF]. Shale beds that are interlayered with more limestone-and sand-rich units have been observed towards the catchment outlet [START_REF] Jin | Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]. The site has gentle slopes, forest cover and very low long-term denudation rates of 0.010 -0.025 mm yr -1 based on measurements on meteoric beryllium-10 ( 10 Be; [START_REF] West | Regolith production and transport at the Susquehanna Shale Hills Critical Zone Observatory, Part 2: Insights from meteoric 10[END_REF] and uranium isotope disequilibrium [START_REF] Ma | Regolith production rates calculated with uranium-series isotopes at Susquehanna/Shale Hills Critical Zone Observatory[END_REF].

The climate is humid-continental, with a mean annual temperature of 10 °C and mean annual precipitation of 1070 mm [START_REF] Jin | Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory[END_REF]NADP, 2020).

From the early 1800s, the Shale Hills catchment and wider region was logged for charcoal production to support the local iron industry (Herndon et al., 2011;[START_REF] Kraepiel | Natural and anthropogenic processes contributing to metal enrichment in surface soils of central Pennsylvania[END_REF]. Today, iron ore smelting and the majority of steel production in the vicinity of Shale Hills has largely stopped. However, several studies report Mn and Pb enrichments in Shale Hills soils derived from coal burning and iron smelting inputs (Herndon and Brantley, 2011;Herndon et al., 2011;[START_REF] Ma | Quantifying an early signature of the industrial revolution from lead concentrations and isotopes in soils of Pennsylvania[END_REF]. This anthropogenic activity has resulted in acidic (pH ~5.4) and SO4 2-enriched local precipitation (Jin et al., 2011). A steadily increasing trend in the rain water pH is observed since the early 2000s (NADP, 2020).

Extensive prior research at the SSH-CZO provides insights on carbonate and silicate weathering [START_REF] Jin | Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory[END_REF][START_REF] Jin | Opening the "Black Box": Water Chemistry Reveals Hydrological Controls on Weathering in the Susquehanna Shale Hills Critical Zone Observatory[END_REF][START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF][START_REF] Shaughnessy | Machine Learning Deciphers CO2[END_REF], oxidative weathering of pyrite [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF], water-rock interactions (Herndon et al., 2015a;[START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF], porosity generation and chemical weathering (Gu et al., 2020a(Gu et al., , 2020b)), and accumulation of metals in top soils (Herndon and Brantley, 2011;Herndon et al., 2011;[START_REF] Ma | Quantifying an early signature of the industrial revolution from lead concentrations and isotopes in soils of Pennsylvania[END_REF][START_REF] Kraepiel | Natural and anthropogenic processes contributing to metal enrichment in surface soils of central Pennsylvania[END_REF]. In terms of physical rock degradation, four weathering layers on the Rose Hill shale have been identified: i) soil (handaugerable material), ii) highly fractured saprock, iii) less fractured saprock and iv) the largely unaltered protolith [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]Gu et al., 2020a). The term saprock describes fractured bedrock that can reach close to the catchment surface. Previous work attempted a first order estimate of OCpetro oxidation, assuming a complete loss of OC from deep samples to the surface [START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF]. The authors noted considerable uncertainty due to the low OC concentrations and inputs of carbon from the biosphere.

Materials

Samples were selected from boreholes to assess deeply weathered materials nearer the surface and less weathered samples at depth, using published sulfur concentrations [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]Gu et al., 2020a) to guide where oxidative weathering fronts were located. We focused on two boreholes located in the valley ('CZMW2' and 'CZMW10'; Figure 1b). Borehole CMZW2 (sample number; n=11) covers depths between 0.5 m and 16.0 m and documents prominent weathering fronts [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF]. Deeper samples (7.6 m to 34.9 m) from borehole CZMW10 (n=5) were selected, because they show a wider zone of alteration below the valley (Gu et al., 2020a). The boreholes CZMW2 and CZMW10 were drilled less than 5 m distance from one another, and are considered here as a continuous profile. A third borehole CZMW8, located on the South ridge of the catchment (Figure 1b), has a deeper weathering front, and 8 samples were selected from 0.2 m to 30.6 m depth. Finally, to examine Re in soils, 4 samples from the upper 50 cm deep soil profile previously collected on the south ridge of the catchment (Figure 1b) by Herndon et al. (2011) were selected.

Details of soil and borehole sample and data collection methods are provided in previous work (Herndon et al., 2011;[START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]. In summary, the solid samples were air dried, crushed and analyzed for major elements by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and total sulfur (S) by LECO Sulfur Coulometer and LECO Carbon Sulfur Analyzer at Pennsylvania State University. Concentration data have been previously published for boreholes in [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF], [START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF] and Gu et al. (2020a).

In this study, 11 stream water samples from the catchment outlet gauging station were collected between June 2018 and June 2019. In August 2018, groundwater samples were collected from wells CZMW2, CMZW8, CZMW6, and wells number 9, 11, and 12 (Figure 1b). Water samples were collected in clean Nalgene bottles, pre-rinsed with the sample water and immediately filtered through 0.22 μm Millipore filters and transferred into acid-cleaned Nalgene bottles [START_REF] Hilton | Geological respiration of a mountain belt revealed by the trace element rhenium[END_REF][START_REF] Horan | Mountain glaciation drives rapid oxidation of rock-bound organic carbon[END_REF]. All samples used for trace elements and major cations analyses were acidified to pH~2 with HNO3. An un-acidified aliquot was collected in bottles pre-cleaned with demineralized water and filtered stream water for anion analysis, and stored at 4 °C in the dark. Archived precipitation samples (n=4) collected and stored by similar methods from 2013 (July and October) and 2014 (February and October) were analyzed.

Geochemical analysis

Geochemical analysis of the solid samples from the boreholes included measurements of total organic carbon concentration (TOC), stable OC isotope ratios (reported as δ 13 COC relative to VPDB), radiocarbon measurements ( 14 C) of OC, and Re concentrations. Water samples (stream water, groundwater, precipitation) collected for this study were measured for major ion concentrations and trace element Re.

For TOC and δ 13 COC measurements, carbonate was removed by weighing ~40 mg of a dry sample into a pre-combusted (450 °C for 4 hours) silver capsule and adding 2 drops of 1M HCl, followed by drying of the sample in an oven at 60 °C. To ensure the complete removal of carbonates, each sample was rinsed with 1M HCl three times. The samples were measured on a Sercon ANCA-EA system linked to a Sercon 2022 isotope ratio mass spectrometer (Elemtex, Stable Isotope Analytical Laboratory, Cornwall, UK). Corrections for the procedure and instrumental blanks were applied. The method was validated by measuring the soil standard material NCS-DC73319 for TOC concentration and two international certified synthetic standards, IAEA 600 (caffeine) and IAEA CH3 (cellulose), for δ 13 COC values. The standard materials were treated in the same way as the samples. The precision for δ 13 COC values was 0.20 ‰ and 0.15 ‰ for IAEA 600 (n=6) and IAEA CH3 (n=6), respectively.

Radiocarbon measurements were made on nine rock samples from the CZMW8 ridgetop borehole. Crushed samples were acid leached (1M HCl) in pre-combusted glassware to remove carbonate minerals. Samples were centrifuged, rinsed with deionized water, and dried in a 60 °C oven at Durham University, Department of Geography. De-carbonated samples were sent to the National Ocean Sciences Acceleration Mass Spectrometry Facility (NOSAMS) at Woods Hole Oceanographic Institution (WHOI) for 14 C analysis. A sample aliquot was combusted with CuO and Ag at 850 °C in an evacuated quartz tube. CO2 was graphitized and measured for radiocarbon following standard procedures via Accelerated Mass Spectroscopy (AMS; [START_REF] Mcnichol | POC -unique aspects in the preparation of oceanographic samples for 14C-AMS[END_REF]. An aliquot of the CO2 was used to measure δ 13 COC using a VG Prism dual-inlet IRMS. All measurements were corrected for a total procedural blank (combusted quartz sand put through the whole process) according to conventional radiocarbon reporting standards and reported as Fraction modern (F 14 C) as per [START_REF] Stuiver | [END_REF].

Rhenium concentrations in the borehole solids were measured following complete acid digestion (HF+HNO3, followed by aqua regia) and anion exchange column chemistry as per methods of [START_REF] Dellinger | Measurements of rhenium isotopic composition in low-abundance samples[END_REF]. The Re solutions were measured on a Thermo Scientific Neptune Plus at Durham University, Department of Earth Sciences. Standard reference materials were digested in each batch to validate the method; a marine sediment (MAG-1), basalt (BHVO-2, BCR-2), and serpentinite (UB-N), which returned values for this study in close agreement; <15% with previously published data [START_REF] Meisel | Reference materials for geochemical PGE analysis: new analytical data for Ru, Rh, Pd, Os, Ir, Pt and Re by isotope dilution ICP-MS in 11 geological reference materials[END_REF][START_REF] Jochum | Reference Values Following ISO Guidelines for Frequently Requested Rock Reference Materials[END_REF] and <10% with long-term in-house reproducibility [START_REF] Dellinger | Measurements of rhenium isotopic composition in low-abundance samples[END_REF].

Major ions (Cl -, SO4 2-, Na + , K + , Ca 2+ , Mg 2+ ) in water samples were determined by ion chromatography (Dionex ICS 3000, Durham University, Department of Geography), validated by a certified river water reference standard material (Sangamon-03). The difference between our measurements and certified reference material values for all major elements was <5% (n=3). The analytical precision determined by running replicates on selected samples was <3% (n=5) for all reported major ions.

Concentrations of dissolved Re were determined by a direct calibration method [START_REF] Horan | The oxidative weathering of organic matter and its carbon dioxide emissions: Insight from the trace elements rhenium and molybdenum[END_REF] on Agilent Technologies 7900 inductively coupled plasma mass spectrometer (ICP-MS) at Durham University, Department of Geography. The detection limit defined as 3 standard deviations of the replicate measurements of a blank solution was 0.01 ppt for Re. Repeated measurements of the river water standard sample SLRS-5 for Re are 64.6±6.1 ppt (±1SD, n=6) and 10-and 100-times dilutions are 6.2±0.7 ppt (±1SD, n=6) and 0.63±0.1 ppt (±1SD, n=6), respectively. These are in agreement with the long-term in house reproducibility, that is SLRS-5=63.0±3.5 ppt (±1SD, n=24), SLRS-5×10=6.2±0.3 ppt (±1SD, n=21), and SLRS-5×100=0.61±0.10 ppt (±1SD, n=35), and with Re concentrations for SLRS-5 reported in literature: 66±12 ppt [START_REF] Yeghicheyan | A Compilation of Silicon, Rare Earth Element and Twenty-One other Trace Element Concentrations in the Natural River Water Reference Material SLRS-5[END_REF].

Calculation of mass transfer coefficient (τ)

In the borehole sediments, a mass transfer coefficient () was calculated to characterize the mobility of S, TOC, and Re during weathering. The coefficient  describes a gain or loss of an element relative to the parent material by applying Equation 1 [START_REF] Brimhall | Constitutive mass balance relations between chemical composition, volume, density, porosity, and strain in metasomatic hydrochemical systems: Results on weathering and pedogenesis[END_REF][START_REF] Anderson | Weathering profiles, mass-balance analysis, and rates of solute loss: Linkages between weathering and erosion in a small, steep catchment[END_REF].

Where C is the concentration of the mobile element (j) or immobile element (i) in the protolith (p) or soil/saprock (s). The approach assumes that composition of the protolith is relatively homogeneous, with mean composition and variability that can be defined, and that the species (i) is immobile through the weathering profile. A negative  value describes a relative depletion of an element compared to the immobile element in the parent material. We use Zr as the immobile element following the approach of [START_REF] Kraepiel | Natural and anthropogenic processes contributing to metal enrichment in surface soils of central Pennsylvania[END_REF].

Results

Here, a summary of previous results is provided before outlining new measurements of TOC, δ 13 COC, 14 C and Re concentrations (Figure 2). Mobility during oxidative weathering is assessed with mass transfer coefficients for S, TOC and Re (Figure 3). New measurements of major ion and Re concentrations in stream water, groundwater and precipitation are given to constrain Re cycling in the near surface environment.

Published sulfur concentrations in borehole materials

The depletion of S in the borehole samples provides insight on oxidative weathering of pyrite at Shale Hills because almost all sulfur in the parent rock is present in pyrite [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]Gu et al., 2020a). In the valley (CZMW2, CZMW10), the S concentrations are low from the surface through the weathered saprock to depths of ~6.5 m, with a mean S of 0.005±0.002 wt.% (n=17, ±1SD; [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF]Gu et al., 2020a). A wide (~15 m) pyrite depletion zone, with continuous increase in S concentration is observed between ~6.5 m and ~21.4 m, with a mean S of 0.08±0.03 wt.% (n=26, ±1SD). The highest S concentrations are measured bellow ~21.4 m, with a mean S of 0.15±0.04 wt.% (n=17, ±1SD) in the protolith. On the ridge, the weathered saprock extends deeper, to ~15.4 m and has mean S of 0.02±0.03 wt.% (n=10, ± 1SD; [START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]. Beneath the S-depleted layers, a sharp increase in S concentrations from below detection limit (~0.002) to 0.14 wt.% is observed between 15.4 m and 16.4 m depth. This increase is consistent with decrease of saprock porosity and fracture density [START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]Gu et al., 2020a).

TOC, δ 13 COC and 14 C in borehole materials

The TOC concentrations are highest in the surface soil, with 0.14 wt.% and 0.32 wt.% measured in the valley (down to ~1 m; CZMW2). Lower TOC concentrations were found in the upper part of the weathered saprock (down to ~6.5 m, i.e. above the pyrite depletion zone; CZMW2), with TOC values of 0.06 wt.% and 0.07 wt.%. At a depth of ~8.4 m the TOC concentrations are the lowest at 0.03 wt.% and they stay low until ~16 m depth, with a mean value of 0.04±0.01 wt.% (n=6, ±1SD, CZMW2). On the ridge, the TOC concentrations are also the highest in the surface soil at 0.41 wt.% (down to ~0.3 m; CZMW8). The TOC concentrations drop in the weathered saprock (~0.9 to ~12.4m) to span a range of values between 0.09 wt.% (the top most sample at 0.9m) to 0.05 wt.% at ~12.4m. At ~15.4 m depth the lowest TOC concentration is measured at 0.03 wt.%, which gradually increases in the protolith to a value of 0.06 wt.% at 30.5 m depth (CZMW8). The TOC values are similar to those

τ i,s = C j,s C i,p C j,p C i,s -1 (1)
reported by [START_REF] Jin | Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory[END_REF][START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF] sampled in borehole DC1, located on the North ridge of the Shale Hills catchment (Figure 1b).

The stable carbon isotopic composition of the organic carbon (δ 13 COC) varies by up to ~2 ‰, but does not show any systematic variation between the soil, saprock and protolith samples, in both boreholes CZMW2 (-26.3 ‰ to -24.4 ‰) and CZMW8 (-27.0 ‰ to -26.2 ‰). These values are similar to δ 13 COC values of soils (-25.8 ‰ to -25.3 ‰) and protolith samples (-27.1 ‰ to -25.9 ‰) previously published for Shale Hills in [START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF].

The radiocarbon activity, reported as Fraction modern (F 14 C), in the CZMW8 ridgetop borehole varies between 0.91±0.005 near the surface, to 0.24±0.003 in the saprock, at ~15.4 m depth. The high F 14 C measured in the top soil is consistent with organic matter dominated by the modern biosphere, resulting in a F 14 C value close to 1 (Trumbore, 2009). Within the saprock, lower F 14 C values are consistent with some input of OCpetro from the Silurian Rose Hill formation that was deposited ~443 to 416 million years ago (F 14 C value=0). However, the presence of 14 C, with F 14 C between 0.26±0.003 and 0.32±0.003 in the deep profile, suggests potential aging of biospheric OC and/or another source of 14 C-enriched C in the deepest samples, as discussed in Section 4.1.

Re concentration in borehole materials and surface soils

The protolith samples (below ~21.4 m) under the valley (CZMW2 and CZMW10) have the highest Re concentrations, ranging between 0.10 ppb and 0.42 ppb. These are similar to Re concentrations for the protolith under the ridge (below ~15.4 m in CZMW8) of 0.11 ppb to 0.23 ppb. The Re concentrations in the Shale Hills protolith are more than 100-times lower than those measured in black shale weathering profiles from the US [START_REF] Jaffe | Mobility of rhenium, platinum group elements and organic carbon during black shale weathering[END_REF] and Nepal (Pierson-Wickmann et al., 2002), but similar to those from river bed material in the western Southern Alps of New Zealand (mean Re = 0.12±0.02 ppb), which is also derived from an OC-poor (TOC ~0.13%) rock [START_REF] Horan | Mountain glaciation drives rapid oxidation of rock-bound organic carbon[END_REF].

In the valley, Re is depleted in the weathered saprock from ~0.9 m to ~6.5 m to between 0.02 ppb and 0.03 ppb. Similar Re concentrations are measured in saprock under the ridge. The pattern of Re depletion is spatially consistent with the S depletion [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]Gu et al., 2020b) and is consistent with weathering profiles developed on black shales [START_REF] Jaffe | Mobility of rhenium, platinum group elements and organic carbon during black shale weathering[END_REF]Pierson-Wickmann et al., 2002). A slight increase in Re concentration towards the surface soil is measured in both boreholes to ~0.06 ppb. In the ridgetop soil profile (Figure 2f), the highest Re concentration of 0.12 ppb is measured in the top soil (2.5 cm), which decreases to 0.01 ppb at 42.5 cm depth.

Mass transfer coefficient

To assess the loss or gain of elements, the composition of the parent material must be characterized. At SSH-CZO, this is not straightforward, due to potential heterogeneity of the parent material over the deep weathering zone (Brantley et al., 2013b;Gu et al., 2020b) and appearance of deep fractures that sometimes show localized zones of weathering depletion (Gu et al., 2020a). Previous studies have used an average over several samples to characterize the parent material, and the standard deviation to asses potential variability on tau values [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]Gu et al., 2020b). Due to the limited number of Re and 14 C measurements in the protolith (Figure 2b and e), we here use the deepest sample and assume that the variability in the protolith for those elements is similar to that measured for S (relative standard deviation).

The calculated tau values for S (Figure 3a andd), reveal nearly complete loss of S from the top 6.5 m under the valley (Zr,S=-0.95±0.08) and from the top 15.4 m under the ridge (Zr,S=-0.89±0.19), agreeing with previous results [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]Gu et al., 2020a). The tau values for Re are less negative than S, but still show substantial depletion in the valley (Zr,Re=-0.84 to -0.93; Figure 3f) and under the ridge (Zr,Re=-0.77 to -0.92; Figure 3c). Due to a lower number of samples analyzed for Re, the comparison between S and Re is not straightforward. Nonetheless, depletion of Re and S is coherent at 15.4 m and 6.5 m depth under the ridge and valley, respectively (Figure 3).

There is TOC accumulation in the top meter of the profile under the valley (Zr,TOC=+5.9 and +2.0; Figure 3e) and the ridge (Zr,TOC=+3.6; Figure 3b). For the ridge profile, TOC shows a decrease in tau between 21.4 and 15.4 m, with a minimum value of Zr,TOC=-0.55 (Figure 3b), tracking Re loss. A less pronounced depletion zone is observed in the borehole under the valley, with the deepest sample being the most depleted showing a Zr,TOC of -0.41.

Stream water chemistry

The stream water is dominated by Ca 2+ , with a mean value of 205±83 μmol l -1 (all stream waters: n=11, ± 1SD, unless otherwise stated). Mg 2+ , Na + , and SO4 2-concentrations are lower, with a mean value of 88±21 μmol l -1 , 28±5.6 μmol l -1 and 80±6.6 μmol l -1 , respectively. These measurements are consistent with data collected at SSH-CZO between 2006 and 2011 (Herndon et al., 2015a). Dissolved Re concentrations show less variation than the major ions, with values between 3.8 to 5.5 pmol l -1 , and a mean of 4.3±0.6 pmol l -1 , which is towards the lower end of published Re values for river water [START_REF] Miller | Re-assessing the surface cycling of molybdenum and rhenium[END_REF]. During the sampling period (2018-2019), average daily water discharge was 2.27 l s -1 , which is slightly higher than the long term mean (2008-2015) of 1.07 l s -1 [START_REF] Shi | SSHCZO --Streamflow / Discharge --Discharge Reanalysis --Shale Hills[END_REF].

Groundwater and precipitation chemistry

In agreement with previous work [START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF], groundwater is more concentrated in Na + , Ca 2+ , and Mg 2+ ions compared to stream water (Table 2). In contrast, the mean SO4 2-concentration in groundwater of 76±30 μmol l -1 , is similar to values measured in the stream. Rhenium concentrations in groundwater samples show a larger range, from 1.3 pmol l -1 to 10.0 pmol l -1 . The highest Re concentrations were measured in two wells located along the stream channel: 6.3 pmol l -1 and 10.0 pmol l -1 for CZMW2 and Well 11 (Figure 1b). In these wells, water table was close to the land surface, indicating a groundwater contact with surface soils.

The Re concentration in precipitation ranged between 0.43 pmol l -1 and 2.95 pmol l -1 , with a mean value of 1.27 ± 1.14 pmol l -1 (n=4; ±1SD) close to those measured in stream water. This is in contrast to very low dissolved Re concentrations of <0.2 pmol l -1 in rain from New Zealand [START_REF] Horan | Mountain glaciation drives rapid oxidation of rock-bound organic carbon[END_REF], the European Alps [START_REF] Hilton | Concentration-Discharge Relationships of Dissolved Rhenium in Alpine Catchments Reveal Its Use as a Tracer of Oxidative Weathering[END_REF] and the coastal range California [START_REF] Ghazi | Concentration-runoff relationships of contrasting small mountainous rivers in the Pacific Northwest, USA: Insights into the weathering of rhenium relative to other weathering products[END_REF], but is similar to the values reported for rainwater from Falmouth, Massachusetts, on the East coast of the US, which reach 5.9 pmol l -1 [START_REF] Miller | Re-assessing the surface cycling of molybdenum and rhenium[END_REF]. Discussion

The borehole materials from Shale Hills display notable Re loss, with a depletion front which broadly coincides with that of pyrite (S) loss. Assessing the location of the OCpetro reaction front using only %TOC is challenging, with clear evidence for modern OC inputs in the near surface samples. Here we use radiocarbon and attempt to quantify OCpetro content with an aim to determine the long-term Re and OCpetro oxidation rates. Finally, we discuss the enrichment of Re in the near surface soils, stream water and precipitation, in the context of anthropogenic inputs of Re in a low denudation setting.

Evidence for OCpetro loss during weathering

The borehole profiles of TOC and Zr,TOC contain information on OCpetro loss, and show a depletion of TOC coincident with the Re and S reaction fronts (Figure 3). However, the TOC concentration in the Rose Hill shale is low (Table 1, [START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF], and C inputs from organic matter of the modern biosphere or other non-petrogenic sources need to be assessed carefully. The δ 13 COC values are not helpful to distinguish source, showing no systematic variation between the soil, saprock, and protolith (Figure 2d).

Instead, we use the radiocarbon activity (F 14 C) of the bulk organic matter in the ridgetop borehole (CZMW8) (Figure 2e). However, we find no fully 14 C-depleted sample, contrasting with other sedimentary rock weathering profiles where OC content of the rock is >1% [START_REF] Petsch | Weathering of Organic Carbon[END_REF]Hilton et al., 2021). This requires a source of OC at depth that contains radiocarbon. The most 14 C-enriched source in the catchment is recent organic matter formed by photosynthesis (F 14 C ~1). The sample at 0.23 m depth, with a F 14 C=0.91, could be mostly explained by input from modern OCbio. To explain the input of 14 C over the deep weathering profile studied here, we discuss three possibilities: i) aging of plant derived particulate matter, ii) supply and aging of dissolved OC (DOC), and/or iii) chemotrophic fixation of DIC at depth.

The aging of plant-derived organic matter in mineral soil horizons lowers F 14 C (e.g., F 14 C value of 0.7 corresponds to 2,865 radiocarbon years). This could fully explain the surface most sample ( 14 C=0.91) and partly explain the 14 C-depletion of the sample at 0.88 m depth (F 14 C=0.47). Large particles of OC from the biosphere may be transported deeper into the profile during drilling process. However, other pathways, such as percolating dissolved OC (DOC) downwards from the surface via flow pathways through the saprock [START_REF] Marin-Spiotta | Carbon delivery to deep mineral horizons in Hawaiian rain forest soils[END_REF], or colloidal transport of OC [START_REF] Yan | Mobile Colloidal Organic Carbon: An Underestimated Carbon Pool in Global Carbon Cycles[END_REF][START_REF] Kim | Particle fluxes in groundwater change subsurface shale rock chemistry over geologic time[END_REF]) are also possible. A final mechanism to deliver 14 C-rich OC to depth could be via the dissolved inorganic carbon (DIC) pool via chemotrophic production of organic matter. The DIC pool may have F 14 C between 0.5 and 1.0 reflecting acid hydrolysis of carbonate and silicate minerals from a mixture of atmospheric CO2 and carbonate mineral weathering [START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF]. This could be incorporated into microbial biomass under water-saturated conditions (i.e. low O2) via SO4 2- reduction [START_REF] Schwab | 14C-Free Carbon Is a Major Contributor to Cellular Biomass in Geochemically Distinct Groundwater of Shallow Sedimentary Bedrock Aquifers[END_REF]. Distinguishing these pathways is not possible here, but remains an important avenue for future research if these processes can store significant amounts of recent carbon at depth in weathered rocks.

To quantify OCpetro using 14 C we consider all of these possible pathways for 14 C addition. We assume that the bulk OC pool is a mixture between OCpetro (F 14 C=0) and an 14 C-enriched pool [START_REF] Blair | The persistence of memory: the fate of ancient sedimentary organic carbon in a modern sedimentary system[END_REF][START_REF] Galy | Recycling of Graphite During Himalayan Erosion: A Geological Stabilization of Carbon in the Crust[END_REF][START_REF] Hemingway | Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils[END_REF]. In the near surface, we assume that the F 14 C values of the enriched pool vary between 1.1 (potential bomb 14 C inputs) to 0.9 (potential aged particulate or DOC). Deeper in the profile, we consider C with F 14 C values between 0.9 (potential aged DOC) to 0.5 (chemotrophic C input from DIC). As per [START_REF] Hilton | Concentration-Discharge Relationships of Dissolved Rhenium in Alpine Catchments Reveal Its Use as a Tracer of Oxidative Weathering[END_REF] this range is used for each sample with a Monte Carlo simulation (1,000-times per sample) to return modelled OCpetro concentration values, reported as the 50% (median), +2σ and -2σ, together including 95% of the predicted OCpetro values (Table 1, Figure 4a).

The least well constrained OCpetro contents are from the most surface sample, with modelled values between 0.057% and 0.001%. These correspond to a Zr,OCpetro value varying between +0.46 and -0.97 (Figure 4b). The uncertainty comes from the modest 14 C-depletion, which could be entirely explained by biospheric OC aging (i.e., negligible OCpetro inputs), and because the OCpetro contents are low compared to the %TOC values (Figure 2c). It is likely that the approach here over-estimates OCpetro in the upper most sample. With increasing depth OCpetro concentration is better constrained. Between 21.5 m and 15.4 m there is a decrease in modelled OCpetro, from 0.04% to 0.02% (Figure 4a). The corresponding Zr,OCpetro decreases from +0.22 to -0.36 (Figure 4b).

Despite the very low OCpetro contents of the protolith at Shale Hills, the challenges of applying 14 C as a proxy for OCpetro input and limited number of samples, there is evidence for an OCpetro reaction front. Admittedly, there may be variability in the OCpetro content of the bedrock, but this variation would have to be coincident with the depths at which S and Re are depleted. Future work could unpack OC inputs and C cycle processes using the ramped pyrolysis oxidation (RPO) coupled to radiocarbon technique, to isolate OC with different thermal reactivity [START_REF] Hemingway | Assessing the Blank Carbon Contribution, Isotope Mass Balance, and Kinetic Isotope Fractionation of the Ramped Pyrolysis/Oxidation Instrument at NOSAMS: Radiocarbon[END_REF][START_REF] Grant | Thermal oxidation of carbon in organic matter rich volcanic soils: insights into SOC age differentiation and mineral stabilization[END_REF]. Here, we turn to the loss of Re as an alternative approach to constrain oxidative weathering processes and fluxes at Shale Hills.

Source of Re in borehole sediments

The application of Re as a proxy for OCpetro oxidation rates is built on an assumption that OCpetro represents a major source of Re in the weathered lithology [START_REF] Dalai | Dissolved rhenium in the Yamuna river system and the Ganga in the Himalaya: role of black shale weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere[END_REF][START_REF] Hilton | Geological respiration of a mountain belt revealed by the trace element rhenium[END_REF].

Here, we use the ratio of Re to Na and S to quantify the role of sulfide-and silicate-derived Re (as per [START_REF] Horan | Carbon dioxide emissions by rock organic carbon oxidation and the net geochemical carbon budget of the Mackenzie River Basin[END_REF][START_REF] Hilton | Concentration-Discharge Relationships of Dissolved Rhenium in Alpine Catchments Reveal Its Use as a Tracer of Oxidative Weathering[END_REF]. In general, Re concentrations of silicate and sulfide minerals are low, resulting in low Re/Na ratios for silicates, and low Re/S ratios for sulfides (Peucker-Ehrenbrink and Hannigan, 2000; [START_REF] Dalai | Dissolved rhenium in the Yamuna river system and the Ganga in the Himalaya: role of black shale weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere[END_REF][START_REF] Miller | Re-assessing the surface cycling of molybdenum and rhenium[END_REF]. In contrast, Re enrichment associated with OC can increase Re/Na and Re/S ratios [START_REF] Das | Disproportionately high rates of sulfide oxidation from mountainous river basins of Taiwan orogeny: Sulfur isotope evidence[END_REF]. [START_REF] Horan | Carbon dioxide emissions by rock organic carbon oxidation and the net geochemical carbon budget of the Mackenzie River Basin[END_REF] proposes a Re/S ratio of 6.0×10 -4 pmol/μmol; however, recent work showed that Re/S ratio can be as low as 9.1x10 -5 in a sedimentary, shale environment [START_REF] Ogrič | Chemical weathering of sedimentary rocks as a source of carbon dioxide to the atmosphere: Durham Univeristy[END_REF]. The end-member composition for silicates (Re/Na) is between 2.4×10 -4 and 2.2×10 -3 pmol/μmol [START_REF] Dalai | Dissolved rhenium in the Yamuna river system and the Ganga in the Himalaya: role of black shale weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere[END_REF].

The three-component mixing domain suggests that in the deeper samples, Re is largely associated with OCpetro (Figure 5), having a range of relative inputs between 77% and 93%, and a mean value of 83±6% (±1SD n=13). Moving towards the surface, an increase in Re/S and decrease in Re/Na ratios are seen in weathered samples (Figure 5). The increase in Re/S ratio though weathering implies preferential loss of S relative to Re. The decrease in Re/Na ratio suggests that Re associated with OCpetro is lost during weathering, consistent with evidence for OCpetro loss (Figure 4). These observations suggest that, despite the low %TOC of the rocks, the majority of Re is associated with OCpetro at depth, and that Re loss results in a residual that becomes more dominated by silicatederived Re. Rhenium isotope measurements from a black shale weathering profile were explained invoking similar behavior [START_REF] Miller | Theoretical modeling of rhenium isotope fractionation, natural variations across a black shale weathering profile, and potential as a paleoredox proxy[END_REF].

Loss of Re, S and OCpetro through weathering

To provide further insights on the mobility of Re, S and OCpetro during weathering we discuss the mass transfer profiles in the ridgetop borehole (CZMW8) (Figure 3, Figure 4). While direct comparison is challenging because of a lower number of samples analyzed for Re and OCpetro compared to S, we observe a broadly coincidental loss of S, OCpetro and Re at 15.4 m. The pyrite oxidation (S loss) front is sharp, with an abrupt change in Zr,S from -0.99 to +0.20 over a vertical distance of ~1 m (Figure 3a). The depletion of OCpetro across the weathering front appears more gradual (Figure 4b), albeit with lower sample resolution, with Zr,OCpetro values from -0.36 to +0.22 over a longer vertical distance of ~6 m. The number of samples for Re at this depth make the comparison harder, but the Re loss profile shows a gradual increase over ~6 m from -0.93 to -0.53 (Figure 3c), similar to the OCpetro profile.

A decoupling of S loss from OCpetro loss has been reported in other shale weathering profiles [START_REF] Jaffe | Mobility of rhenium, platinum group elements and organic carbon during black shale weathering[END_REF][START_REF] Gu | How Particle Size Influences Oxidation of Ancient Organic Matter during Weathering of Black Shale[END_REF], with the S oxidation front being deeper than the coupled Re and OCpetro oxidation. The deeper S front than OCpetro oxidation front presumably reflects the faster reaction kinetics of pyrite [START_REF] Petsch | A field study of the chemical weathering of ancient sedimentary organic matter[END_REF][START_REF] Gu | How Particle Size Influences Oxidation of Ancient Organic Matter during Weathering of Black Shale[END_REF]. However, this assumes that O2 in air and water has access to phases in the subsurface. This may not be the case at Shale Hills, where the generation of porosity and permeability is inherently linked to chemical weathering processes themselves (Gu et al., 2020a(Gu et al., , 2020b)). At SSH-CZO, previous work has suggested the oxidation of pyrite minerals plays a crucial role in subsequent oxidative weathering and acid hydrolysis reactions [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF]Gu et al., 2020a). This is because pyrite oxidation produces sulfuric acid, an acid that can dissolve both carbonates and clay minerals, resulting in increased pore space in the weathered matrix (Gu et al., 2020b). After pyrite is completely exhausted from the saprock, oxygen can more effectively enter through the new porosity, reacting with the remaining rock matrix [START_REF] Jin | Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory[END_REF][START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF]Gu et al., 2020a). This may explain the link between the depths of S, OCpetro and Re depletion.

The observed lower degree of OCpetro loss compared to the Re and S loss in the surface samples could come about by incomplete OCpetro oxidation. The slight metamorphic alteration of the Rose Hill shale (Gu et al., 2020b) could have thermally matured the organic matter in the rock, making the OC in the rock more resistant to weathering [START_REF] Petsch | Weathering of Organic Carbon[END_REF]. Alternatively, association of OCpetro or its weathered products with clay minerals may protect them from further oxidation [START_REF] Hemingway | Mineral protection regulates long-term global preservation of natural organic carbon[END_REF][START_REF] Gu | How Particle Size Influences Oxidation of Ancient Organic Matter during Weathering of Black Shale[END_REF].

Long-term OCpetro oxidation rates

Depletion profiles of Re (Figure 3c) and OCpetro (Figure 4b) in the borehole samples can be used to quantify OCpetro oxidation rate that occurred over the timescales of the weathering profile formation, and by inference, estimate the release of CO2 from sedimentary rock weathering. We find evidence for OCpetro loss at a weathering front, which could correspond to a CO2 release over the timescales of weathering front formation, quantified by:

where ω is denudation rate (10-25 m Myr -1 ; [START_REF] West | Regolith production and transport at the Susquehanna Shale Hills Critical Zone Observatory, Part 2: Insights from meteoric 10[END_REF], [OCpetro]p is the concentration below the reaction front (0.04±2σ%) and [OCpetro]s is the concentration above the weathering front (0.02±2σ %). Density of the protolith samples (ρP) is 2.7 g cm -3 (Gu et al., 2020b). Using these values, the OCpetro weathering at Shale Hills is estimated between 2.0×10 -3 and 1.7×10 -2 tC km -2 yr -1 , the range reflecting the uncertainty on the quantification of OCpetro (Section 4.1) and denudation rate at this site.

𝐽 𝑂𝐶𝑝𝑒𝑡𝑟𝑜 = 𝜔 × ([𝑂𝐶𝑝𝑒𝑡𝑟𝑜] 𝑝 -[𝑂𝐶𝑝𝑒𝑡𝑟𝑜] 𝑠 ) 100 × 𝜌 𝑃 (2)
The trace element Re has been more precisely determined in these OC-poor samples. To apply the Re proxy to track OCpetro oxidation rates (JOCpetro-Re), the following Equation 3is used:

where FRe-OC is the fraction of Re derived from OCpetro=83±6% (Figure 5). The [Re/OC]petro and [Re]p are the ratio and Re concentration below the weathering front, 4.3×10 -7 g g -1 and 0.11 ppb, respectively and [Re]s is the Re concentration above the front (0.02 ppb). The long-term OCpetro emissions at Shale Hills determined by the Re proxy are estimated between 4.4×10 -3 and 1.3×10 -2 tC km -2 yr -1 , reflecting uncertainty on the denudation rate estimates and the [Re/OC]petro ratios of the protolith samples. In the protolith samples from the CZMW8 borehole, [Re/OC]petro varies between 3.1×10 -7 g g -1 and 8.1×10 -7 g g -1 . Globally, this ratio can vary in a range between 10 -7 g g -1 and 10 -8 g g -1 across different sedimentary environments [START_REF] Peucker-Ehrenbrink | Behavior of Re and Os during low-temperature alteration: Results from Himalayan soils and altered black shales[END_REF][START_REF] Jaffe | Mobility of rhenium, platinum group elements and organic carbon during black shale weathering[END_REF][START_REF] Ross | The importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs[END_REF]Sheen et al., 2018).

A previous estimate of CO2 release from OCpetro oxidation at Shale Hills by [START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF] assumed that all TOC in the bedrock was depleted by weathering during soil production (20 mM yr -1 , Ma et al., 2010). The corresponding CO2 release rate was 1.5×10 -2 tC km -2 yr -1 [START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF], which is broadly similar, given uncertainties, to our maximum estimates presented above, using OCpetro loss (1.7×10 -2 tC km -2 yr -1 ) and the Re proxy (1.3×10 -2 tC km -2 yr -1 ).

The OCpetro oxidation rate at Shale Hills is expected to be low for two reasons: i) the shale is OCpetrodepleted; and ii) denudation rates are extremely low. Numerical models of OCpetro oxidation, constrained by the reactivity of coal from abiotic laboratory incubations [START_REF] Chang | Coal weathering and the geochemical carbon cycle[END_REF], suggest that OCpetro oxidation is limited by supply of fresh, un-oxidized material [START_REF] Bolton | The Weathering of Sedimentary Organic Matter as a Control on Atmospheric O2: II. Theoretical Modeling[END_REF]. Dissolved Re fluxes from catchments in Taiwan and New Zealand confirm this, showing increases in erosion are linked to increases in OCpetro weathering [START_REF] Hilton | Geological respiration of a mountain belt revealed by the trace element rhenium[END_REF][START_REF] Horan | Mountain glaciation drives rapid oxidation of rock-bound organic carbon[END_REF]. OCpetro oxidation at Shale Hills is 2 to 3 orders of magnitude lower than OCpetro oxidation rates based on the dissolved Re proxy in high erosive catchments of the Himalaya [START_REF] Dalai | Dissolved rhenium in the Yamuna river system and the Ganga in the Himalaya: role of black shale weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere[END_REF], Taiwan [START_REF] Hilton | Geological respiration of a mountain belt revealed by the trace element rhenium[END_REF], New Zealand [START_REF] Horan | Mountain glaciation drives rapid oxidation of rock-bound organic carbon[END_REF], and the Mackenzie river system [START_REF] Horan | Carbon dioxide emissions by rock organic carbon oxidation and the net geochemical carbon budget of the Mackenzie River Basin[END_REF] (Figure 6).

The SSH-CZO OCpetro oxidation estimate reaffirms that supply of OCpetro to the weathering zone is a key driver of OCpetro oxidation on a global scale (Figure 6). The observed variability in Figure 6 may be associated with variable OCpetro concentrations and weathering intensity (χ), which can vary between different settings [START_REF] Hilton | Mountains, erosion and the carbon cycle[END_REF]. The weathering of OCpetro may be locally limited by temperature (Soulet et al., 2021), oxygen supply [START_REF] Bolton | The Weathering of Sedimentary Organic Matter as a Control on Atmospheric O2: II. Theoretical Modeling[END_REF], microbial activity [START_REF] Hemingway | Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils[END_REF], and/or the particle size of OCpetro [START_REF] Gu | How Particle Size Influences Oxidation of Ancient Organic Matter during Weathering of Black Shale[END_REF]. These processes could be tracked with time-series data for reaction products in the weathering zone and may be most important in erosive settings where sediment supply to the near surface environment is high.

A decoupling of stream water Re from deep weathering

𝐽 𝑂𝐶𝑝𝑒𝑡𝑟𝑜-𝑅𝑒 = 𝜔 × ([𝑅𝑒] 𝑝 -[𝑅𝑒] 𝑠 ) [ 𝑅𝑒 𝑂𝐶 ] 𝑝𝑒𝑡𝑟𝑜 × 𝐹 𝑅𝑒-𝑂𝐶 × 𝜌 𝑃 (3)
Stream water Re concentrations are high at SSH-CZO (Table 2), despite the very deep Re weathering front and relatively low Re concentrations of the protolith (Figure 3). Previous work has suggested that dissolved Re could be influenced by anthropogenic perturbation of the Re cycle [START_REF] Miller | Re-assessing the surface cycling of molybdenum and rhenium[END_REF][START_REF] Rahaman | Rhenium in Indian rivers: Sources, fluxes, and contribution to oceanic budget[END_REF]. Here we explore its role at Shale Hills, where denudation rates and OCpetro contents are low. We take advantage of our precise characterization of Re content in bedrock, soils and water to build a dissolved-particulate mass budget to quantify the Re contribution from weathering vs. anthropogenic inputs in the present day.

To assess the surface cycling of Re in this catchment, the present-day Re flux in stream water leaving the catchment (Restream, pmol m -2 yr -1 ) is estimated using the discharge-weighted average (Equation 4):

where Re (pmol l -1 ) is Re concentration measured in stream water over the sampling period, Qi (l s -1 ) is the instantaneous discharge measure at the time of sample collection (Table 2), and 𝑄 𝑎 ̅̅̅̅ is the mean annual stream discharge. Using the long term mean annual discharge (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) as 𝑄 𝑎 ̅̅̅̅ (1.07 l s -1 ) gives dissolved Re flux estimate of 2.1×10 3 pmol m -2 yr -1 . The 𝑄 𝑎 ̅̅̅̅ over our sampling period (2018-2019) was higher at 2.27 l s -1 , meaning the annual Re flux from June 2018 to June 2019 may have been closer to 4.4×10 3 pmol m -2 yr -1 .

We can compare this flux to an estimate of how much Re is produced by weathering of the protolith (ReCW; pmol m -2 yr -1 ), using Equation 5.

Here the mean Re concentration in the protolith ([𝑅𝑒] 𝑝 ̅̅̅̅̅̅̅ ) and the saprock ([𝑅𝑒] 𝑆 ̅̅̅̅̅̅̅ ) are used. Other terms are defined in Equation 3. The expected release of Re due to weathering of the protolith (ReCW) is between 0.02 and 0.05×10 3 pmol m -2 yr -1 , the range reflecting variability in the denudation rate estimates. This is <3% of the dissolved Re flux from the stream, suggesting a negligible contribution of dissolved Re from chemical weathering.

To explore this stark decoupling of weathering inputs and dissolved exports, we turn to the precipitation and surface soil samples. The area surrounding the Shale Hills is known for coal burning activities, with a number of historical iron furnaces, fueled by local Pennsylvania coal (Herndon et al., 2011;[START_REF] Kraepiel | Natural and anthropogenic processes contributing to metal enrichment in surface soils of central Pennsylvania[END_REF]. Furthermore, contemporary coal power plants are located upwind of the catchment [START_REF] Lima | High-resolution historical records from Pettaquamscutt River basin sediments: 2. Pb isotopes reveal a potential new stratigraphic marker[END_REF][START_REF] Ma | Quantifying an early signature of the industrial revolution from lead concentrations and isotopes in soils of Pennsylvania[END_REF]. During coal combustion (500-700 ºC), Re may be volatized and released to the atmosphere as Re2O7 [START_REF] Colodner | Rhenium in the Black Sea: comparison with molybdenum and uranium[END_REF]. It can then be deposited on the surrounding landscapes through either wet or dry deposition [START_REF] Novo | Plants to harvest rhenium: scientific and economic viability[END_REF]. The abundance of Re in coal fly ash, a by-product of coal burning, may be high, as suggested by high Re concentration (5.2 ppb) measured in a Coal Fly Ash standard reference material 1633a (product of Pennsylvania and West Virginia coals) for this study.

The present-day atmospheric Re flux (ReAtm) can be estimated by multiplying the average Re concentration of precipitation samples (1.28±0.99 pmol l -1 ) with the mean annual precipitation for

𝑅𝑒 𝑠𝑡𝑟𝑒𝑎𝑚 = 𝑅𝑒 × 𝑄 𝑖 ̅̅̅̅̅̅̅̅̅̅̅ 𝑄 𝑖 ̅ × 𝑄 𝑎 ̅̅̅̅ (4)
𝑅𝑒 𝐶𝑊 = ([𝑅𝑒] 𝑝 ̅̅̅̅̅̅̅ -[𝑅𝑒] 𝑆 ̅̅̅̅̅̅̅ ) × 𝜌 𝑃 × 𝜔 (5)
Shale Hills, of 1070 mm yr -1 [START_REF] Jin | The CO2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory[END_REF]. A present-day atmospheric deposition of Re at Shale Hills is 1.4±1.1×10 3 pmol m -2 yr -1 (Figure 7). By itself, the modern Re input from precipitation is almost 20-times the chemical weathering supply of Re. Precipitation inputs can account for ~60% of the long term (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) Re flux estimate in this small catchment.

Additional insight comes from shallow groundwater samples, which have elevated Re concentrations (Table 2), suggesting inputs from surface soils, where rainfall events may leach previously deposited Re. These observations suggest that past deposition of Re on the landscape may have been higher, as recently observed for atmospheric deposition of sulfate at the study site [START_REF] Shaughnessy | Machine Learning Deciphers CO2[END_REF]. At the Shale Hills catchment, the surface soils appear to act as a temporary Re reservoir enriched in anthropogenic Re, suggesting that the present-day Re cycle in the dissolved load has been perturbed by recent human activities.

Rhenium deposited in the catchment through wet or dry atmospheric deposition, might be retained in the soil through either a pH-dependent control on the Re mobility, and/or as a solid particle form which subsequently undergoes weathering and oxidation. In terms of the pH control, the perrhenate ion, ReO4 -, can be mobilized at low pH of forest soils at the SSH-CZO (pH 4.0±0.1 on the ridge and 4.7±0.2 in the valley; [START_REF] Jin | Opening the "Black Box": Water Chemistry Reveals Hydrological Controls on Weathering in the Susquehanna Shale Hills Critical Zone Observatory[END_REF]Herndon et al., 2015b). A laboratory based study by [START_REF] Kim | Removal of dissolved rhenium by sorption onto organic polymers: study of rhenium as an analogue of radioactive technetium[END_REF] showed that more than 40% of ReO4 -could be reabsorbed onto organic molecules at pH ~4. Continuous acidic precipitation, with pH 4.2 -4.4 monitored since the 1980s (NADP, 2020), but may have started as early as the 1800s (Herndon et al., 2011;[START_REF] Kraepiel | Natural and anthropogenic processes contributing to metal enrichment in surface soils of central Pennsylvania[END_REF] may cause absorption of Re into the forest soils at SSH-CZO. With the increasing precipitation pH over recent years (pH~5.6; NADP Program Office, 2020), the reabsorbed Re could be leached into the surface waters, explaining the high Re concentrations in stream waters. While this mechanism is viable, we cannot rule out the role of Re-enriched particles deposited on the catchment, which subsequently release Re as they weather.

A deposition of Re on the catchment is consistent with the enrichment of Re concentration towards the top-most soil layer in a 50 cm deep soil profile (Figure 2f). Accumulation of several other metals in the top soil is also seen at the SSH-CZO, e.g. Mo and Mn (Herndon et al., 2011;[START_REF] Kraepiel | Natural and anthropogenic processes contributing to metal enrichment in surface soils of central Pennsylvania[END_REF],Pb [START_REF] Ma | Quantifying an early signature of the industrial revolution from lead concentrations and isotopes in soils of Pennsylvania[END_REF] and Hg [START_REF] Richardson | Mercury Sourcing and Sequestration in Weathering Profiles at Six Critical Zone Observatories[END_REF]. To further explore the behavior of Re in the surface soils we plot Re, Mo and Mn concentrations normalized to Al (Figure 7 b-d). Aluminum is not impacted by atmospheric deposition [START_REF] Kraepiel | Natural and anthropogenic processes contributing to metal enrichment in surface soils of central Pennsylvania[END_REF] and is immobile in the soil profile [START_REF] Kim | Particle fluxes in groundwater change subsurface shale rock chemistry over geologic time[END_REF]. All three elemental ratios are elevated in the upper 5 cm of the soil profile, which would imply present-day inferred inputs from anthropogenic activities, i.e., coal burning, gasoline combustion, and/or steel plants) (Herndon et al., 2011). Down to 25 cm depth, Re/Al and Mn/Al ratios decrease by a factor of ~4 and Mo/Al for a factor of ~1.5. Below 25 cm depth, Mn/Al and Mo/Al remain constant, but Re/Al ratio further decreases. These differences probably reflect the biogeochemical processes impacting the elements. For example, oxidized Mo is soluble (MnO4 -2 ) and mobile, like Re. However, previous studies have suggested adsorption of Mo to oxyhydroxides and/or organic compounds in soils at ~pH 4 [START_REF] King | Molybdenum isotope fractionation during adsorption to organic matter[END_REF]. In contrast, Mn is thought to be largely involved in bio-cycling (Herndon and Brantley, 2011;Herndon et al., 2015b) resulting in long residence times in the soil [START_REF] Kraepiel | Natural and anthropogenic processes contributing to metal enrichment in surface soils of central Pennsylvania[END_REF]. The precise nature of Re adsorption in soils could be explored in future studies using the Re isotopic system [START_REF] Miller | Theoretical modeling of rhenium isotope fractionation, natural variations across a black shale weathering profile, and potential as a paleoredox proxy[END_REF][START_REF] Dellinger | Measurements of rhenium isotopic composition in low-abundance samples[END_REF][START_REF] Dellinger | Fractionation of rhenium isotopes in the Mackenzie River basin during oxidative weathering[END_REF].

To explore the legacy of anthropogenic deposition of Re to the catchment soils we here provide an estimate of the time needed to deplete the accumulated Re in surface soils (turnover time). First, the total Re mass in soil per unit land surface area (pmol m -2 ) is summarized over depth intervals (Δd) as shown in Equation 6:

Where [Re]soil is Re concentration in soil at certain depth (Table 1) and ρsoil is the soil density at different intervals as reported in [START_REF] Lin | Soil moisture patterns in a forested catchment: A hydropedological perspective[END_REF]. The total Re mass in the upper 50 cm of a soil profile is 1.1×10 5 pmol m -2 . To explain the measured dissolved Re flux, soil leaching (Releach,soil=Restream-Reatm-Recw) would have to contribute 0.7×10 3 pmol m -2 yr -1 (Figure 7a) to the long term (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) dissolved Re flux. With this constraint, we can estimate the turnover time, assuming this leaching of Re is constant over time and acts to deplete the surface soil reservoir:

This returns an estimate of TRe of 160 years. This is a notable timeframe over which dissolved Re concentrations could be elevated.

This study shows that in locations where denudation rates are low and atmospheric metal deposition rates are high, the surface Re cycling is significantly perturbated by anthropogenic impacts. While our study covers a small catchment, these findings echo global assessments suggesting that the current cycling and usage of Re associated with human activities is equivalent to a Re flux of 0.15±0.01 Gg yr -1 , which is three times larger than the natural Re flux 0.05±0.03 Gg yr -1 [START_REF] Sen | Anthropogenic Disturbance of Element Cycles at the Earth's Surface[END_REF]. Our study calls for future work to better constrain modern and historic Re inputs to the surface of catchments, and highlights that the dissolved Re proxy for OCpetro oxidation [START_REF] Dalai | Dissolved rhenium in the Yamuna river system and the Ganga in the Himalaya: role of black shale weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere[END_REF][START_REF] Hilton | Geological respiration of a mountain belt revealed by the trace element rhenium[END_REF] may result in overestimation in low denudation settings.

Conclusions

This study combines measurements of radiocarbon in the organic carbon reservoir and trace element Re to examine oxidative weathering of OCpetro in a setting experiencing low denudation. In deep weathering profiles from the Susquehanna Shale Hills Critical Zone Observatory, we find evidence for OCpetro depletion across a broad, ~6 m wide weathering front in a borehole from the ridge (CZMW8), with a tau (Zr,OCpetro) value of around -0.4. However, the measurements of radiocarbon require a deep input of 14 C, which makes quantifying OCpetro loss challenging. We turn to the trace element Re, and use Re/Na and Re/S ratios to estimate that ~80% of Re is derived from OCpetro in this setting. The depletion profile of Re occurs over a broad reaction front (similar to OCpetro front), with Zr,Re between -0.77 and -0.93. In contrast, the pyrite weathering front (S loss) in the ridgetop borehole appears narrower (~1 m) at a similar depth of 15.4 m and the S is almost fully depleted (Zr,S=-0.99). We use the OCpetro depletion and the Re loss profiles to quantify long-term OCpetro oxidation rates, which are low (<1.7×10 -2 tC km -2 yr -1 ), but consistent with low denudation rates (0.01-0.025 mm yr -1 ), confirming the important role of mineral supply and erosion rate on OCpetro oxidation.

We find a pronounced contrast between the deep weathering and surface cycle of Re in this catchment. The low oxidative weathering rates supply only small amounts of Re (<0.05×10

yr -1 ), and the saprock is Re depleted. In contrast, the long term annual dissolved Re export from the catchment is estimated at 2.1×10 3 pmol m -2 yr -1 , with precipitation, shallow groundwater, and surface soils all showing elevated Re concentrations. An anthropogenic Re input due to historical steel production and coal burning activities can explain these patterns. Rhenium may have accumulated in the surface soil and is now being slowly leached to the stream waters. At the current mobilization rate, the reservoir of anthropogenic Re in the surface soil could take centuries to deplete. This study highlights that anthropogenic inputs of Re can be a large component of the Re budget in settings with low denudation rates. It calls for renewed focus on the recent perturbation of the Re cycle and how this element could help elucidate the legacy of pollution impacts in catchment soils.

TABLE 1

Published sulfur, zirconium and titanium concentrations alongside new measurements of total organic carbon (TOC), stable carbon isotope of organic carbon (δ 13 COC), radiocarbon activity (F 14 C), and rhenium (Re) in the studied borehole profiles and the topsoil profile. Classification of material type is based on previous work by Sullivan et al., 2016 (1), [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF]( 2) Gu et al, 2020a (3) and Herndon et al., 2011 (4). Radiocarbon activities of the biospheric OC input used in the two end-member mixing model applied for determination of OCpetro concentrations are discussed in section 4.1 (F 14 C = 1.1 -0.9 for samples labelled * and F 14 C = 0.9 -0.5 for samples labelled **). This article is protected by copyright. All rights reserved. From the borehole CZMW8 two types of samples were collected: bulk core material homogenized over 30 cm depth (hexagons) and cm-sized rock fragments (triangles). Bar to the right side of plots (e) and (j) show approx. depths of changes in structure due to chemical and physical degradation of the protolith [START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]. H.F. stands for highly fractured saprock and L.F. stands for less fractured saprock. The yellow shading on all plots show depth intervals where pyrite depletion was detected [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]Gu et al., 2020a). The uppermost white band for each borehole represents the bulk oxidation zone because almost no pyrite remains unoxidized at shallower depths (lower yellow bands are fracture sets as described by (Gu et al., 2020a).

FIGURE 5

Source of rhenium at the study site using Re/Na versus Re/S ratio from the three boreholes studied here: CZMW2 (valley, squares), CZMW10 (valley, diamonds), CZMW8 (ridge, hexagons). The shaded areas represent the likely end-member composition, based on limited data available in the literature [START_REF] Dalai | Dissolved rhenium in the Yamuna river system and the Ganga in the Himalaya: role of black shale weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere[END_REF][START_REF] Horan | Carbon dioxide emissions by rock organic carbon oxidation and the net geochemical carbon budget of the Mackenzie River Basin[END_REF].

FIGURE 6

Global compilation of the control of erosion on OCpetro oxidation rates (tC km -2 yr -1 ) determined with the Re proxy. The suspended sediment yield is a proxy for erosion rate. For Shale Hills, the suspended sediment yield is estimated from the long-term denudation rate estimates (Ma et al., 2011). The stippled lines represent expected weathering rates for various initial organic carbon contents (as shown) and a weathering intensity (χ) of 50%.

FIGURE 1

 1 FIGURE 1 (a) The Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania. (b) The first order Shale Hills catchment and samples analyzed in this study: recovered borehole material (brown triangles) as cuttings (CZMW2, CZMW10) or core (CZMW8); a soil profile (brown square), groundwater samples (blue triangles), and time series stream water samples (June 2018 -June 2019; blue circles). Previously published results from DC1 borehole (white triangle; Jin et al., 2014) are used to help interpretation of the results.

FIGURE 2

 2 FIGURE 2Geochemical profiles for boreholes CZMW8 (southern ridge) and CZMW2, CZMW10 (valley). (a) Previously published measurements of sulfur (S) concentrations from boreholes CZMW2 and CZMW10 in[START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF] andGu et al. (2020a) and from the borehole CZMW8 in[START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF] andGu et al. (2020a). Samples were selected (black symbols in figures (a) and (g) and analyzed for rhenium (b) and (h), total organic carbon (c) and (i), stable carbon isotope of organic carbon (d) and (j), and radiocarbon (Fraction modern, e). From the borehole CZMW8 two types of samples were collected: bulk core material homogenized over 30 cm depth (hexagons) and cm-sized rock fragments (triangles). Bar to the right side of plots (e) and (j) show approx. depths of changes in structure due to chemical and physical degradation of the protolith[START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]. H.F. stands for highly fractured saprock and L.F. stands for less fractured saprock. The yellow shading on all plots show depth intervals where pyrite depletion was detected[START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF] Gu et al., 2020a). The uppermost white band for each borehole represents the bulk oxidation zone because almost no pyrite remains unoxidized at shallower depths (lower yellow bands are fracture sets as described by(Gu et al., 2020a).Figure (f) shows Re concentrations from a soil profile sampled next to CZMW8.

  FIGURE 2Geochemical profiles for boreholes CZMW8 (southern ridge) and CZMW2, CZMW10 (valley). (a) Previously published measurements of sulfur (S) concentrations from boreholes CZMW2 and CZMW10 in[START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF] andGu et al. (2020a) and from the borehole CZMW8 in[START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF] andGu et al. (2020a). Samples were selected (black symbols in figures (a) and (g) and analyzed for rhenium (b) and (h), total organic carbon (c) and (i), stable carbon isotope of organic carbon (d) and (j), and radiocarbon (Fraction modern, e). From the borehole CZMW8 two types of samples were collected: bulk core material homogenized over 30 cm depth (hexagons) and cm-sized rock fragments (triangles). Bar to the right side of plots (e) and (j) show approx. depths of changes in structure due to chemical and physical degradation of the protolith[START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF]. H.F. stands for highly fractured saprock and L.F. stands for less fractured saprock. The yellow shading on all plots show depth intervals where pyrite depletion was detected[START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF][START_REF] Sullivan | Oxidative dissolution under the channel leads geomorphological evolution at the Shale Hills catchment[END_REF] Gu et al., 2020a). The uppermost white band for each borehole represents the bulk oxidation zone because almost no pyrite remains unoxidized at shallower depths (lower yellow bands are fracture sets as described by(Gu et al., 2020a).Figure (f) shows Re concentrations from a soil profile sampled next to CZMW8.

FIGURE 3

 3 FIGURE 3Mass transfer coefficient profiles for boreholes CZMW8 (southern ridge) and CZMW2, CZMW10 (valley). Parent composition was determined from the concentration measured on the deepest protolith sample in the CZMW8 and CZMW10. Zirconium (Zr) was used as an immobile element. The error bars show variation of the parent material for S (33%) and Zr (18%). For TOC and Re it is assumed that the variability in the parent material is similar to the S (~30%). Shaded grey zone represents one SD of uncertainty due to variable parent composition within the catchment(a) Zr,S profile from the ridge (CZMW8). Data and interpretation is fromGu et al. (2020a). (b) Zr,TOC

FIGURE 7

 7 FIGURE 7 Reservoirs (pmol m -2 ) and fluxes (pmol m -2 yr -1 ) of rhenium (Re) at the Shale Hills catchment. The blue arrows represent Re input to the catchment through different geochemical processes. The purple arrow represents Re output from the catchment. Abbreviations: Reatm = Re derived through present day atmospheric deposition; Recw = Re derived through chemical weathering of the protolith; Releach,soil = Re leaching through surface soils; Restream = Re export from the catchment via stream discharge; mRe,soil = Re stock in the top 0.5 m of the surface soils; mRe,protolith = Re stock over 11 m of the protolith.

  𝑚 𝑅𝑒,𝑠𝑜𝑖𝑙 = ∑ ∆𝑑 × 𝜌 𝑠𝑜𝑖𝑙 × [𝑅𝑒] 𝑠𝑜𝑖𝑙(6)

	𝑇 𝑅𝑒 =	𝑚 𝑅𝑒,𝑠𝑜𝑖𝑙 𝑅𝑒 𝑙𝑒𝑎𝑐ℎ,𝑠𝑜𝑖𝑙

3

pmol m

-2

funded by the European Commission via a European Research Council Starting Grant "ROC-CO2" (project 678779) to RGH. Work at Shale Hills is facilitated by the Penn State College of Agricultural Sciences and Department of Ecosystem Science and Management as part of Penn State's Stone Valley Forest.

The SSH CZO was funded by National Science Foundation Grant no. EAR 13 31726, 13-31726 to SLB. We thank Jordon Hemingway, Jennifer Williams, and Brandon Forsythe for field work assistance and help with archived samples and databases. We also thank Amanda Hayton, Martin West and Eleanor Ross for assistance with laboratory work at Durham University.

profile from the ridge (CZMW8). (c) Zr,Re profile from the ridge (CZMW8). (d) Zr,S profile from the valley (CZMW2, CZMW10). Data and interpretation is from [START_REF] Brantley | Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface: Earth Surface Processes and Landforms[END_REF]Gu et al., 2020a). (e) Zr,TOC profile from the valley. (f) Zr,Re profile from the valley.