Cavailhes Thibault
email: thibault.cavailhes@u-bordeaux.frthibault.cavailhes@u-bordeaux.fr

Gillet Hervé

Guiastrennec-Faugas Léa

Thierry Mulder

Hanquiez Vincent

Thibault Cavailhes

The abyssal giant sinkholes of the Blake Bahama Escarpment: evidence of focused deep-ocean carbonate dissolution

Keywords: Carbonates, Sinkholes, Escarpments, Tectonics

This study reports the discovery of abyssal giant depressions located at the toe of the Bahamian carbonate platform, along the Blake Bahama structurally-controlled Escarpment (BBE) that exhibits up to 4 km of submarine elevation above the San Salvador Abyssal Plain (SSAP). Analysis of seismic reflection and bathymetric data collected during the CARAMBAR 2 cruise revealed the presence of 29 submarine depressions; their water depths range from 4584 m to 4967 m whereas their negative reliefs are elliptical in shape, range in diameter from 255 m to 1819 m, and in depth from 30 m to 185 m. The depression alignment trends are parallel to the BBE as well as to structural lineaments of the area, exclusively between 2200 and 5000 m from its toe, and overlies a buried carbonate bench in which a high-amplitude seismic anomaly has been detected. The depression density interestingly increases where the recognized structural lineaments intersect the BBE. Based on their physical attributes (i.e. location, jagged morphologies, water depths), we interpret these depressions as collapse sinkholes rather than pockmarks or plunge pools. The aforementioned observations suggest an atypical relationship between the spatial occurrence of the giant abyssal sinkholes, the carbonate platform tectonic structures, the buried carbonate bench that underlies the hemipelagites in the SSAP and the geomorphology of the area. According to the wider literature that reports fluid seepages along submarine carbonate escarpments, we propose that the ground water entrance during low sea-level stands, the dissolution of evaporites by meteoric water, the platform-scale thermal convection and the seawater entrance at the platform edge most probably collectively act in concert to favor the circulation of brines and therefore the corrosion within the Bahamian carbonate platform. These mechanisms are particularly efficient along the structural heterogeneities (e.g. the Sunniland Fracture Zone, SFZ) which act as fluid conduits localizing the dissolution and control the physiography of the area by maintaining the location of the sedimentary pathways. The dense fluids would migrate along the faults towards the BBE free edge and are subsequently trapped into the buried carbonate bench that laterally disappears below the low-permeability deep-sea hemipelagites of the SSAP. In consequence, the trapped corrosive fluids dissolve the carbonates preferentially along the tectonic structures such as the SFZ. They are this way at the origin of the BBE curvature and generate collapse-structures in the overlying fine-grained deposits finally resulting in the formation of giant abyssal sinkholes. This structurally-directed process of dissolution seems efficient to provide a brines density head to move out down to >4.5 km of water depth and is believed to have played a major role in the BBE 5-6 km erosional retreat.

Introduction

Localizing, describing and understanding the zones of the upper crust where the fluids enter, flow, are stored and leak, are critical to discuss and quantify the geo-bio-chemical fluxes between the lithosphere, the hydrosphere and the atmosphere of the Earth (e.g. [START_REF] Dickens | Rethinking the global carbon cycle with a large, dynamic and microbially mediated gas hydrate capacitor[END_REF]. Fluid flow and dynamics have been extensively studied at convergent and divergent continental margins whereas transform margins that constitute 16 % of the world continental margins (Mercier de Lépiner et al., 2016) have almost never been investigated in detail. Focusing on fluid circulation in carbonate margins allows scientists to interestingly trace and discuss the supply, the storage and the release of chemical elements implied in carbonate platform diagenetic reactions [START_REF] Staudigel | An abrupt Middle-Miocene increase in fluid-flow into the Leeward Margin Great Bahama Bank, constraints from δ 44 Ca and Δ 47 values[END_REF] as well as document their geomorphologic consequences, leading to surprising submarine karstic landscapes (e.g. [START_REF] Land | Submarine karst belt rimming the continental slope in the Straits of Florida[END_REF].

Since the 1970s, our 4D-conception of the upper crust fluid circulation has been significantly improved by the description and the analysis of offshore fluid circulation and related fluid-escape features distribution and geomorphologies (e.g. King and Mac Lean, 1970;[START_REF] Hornbach | Three-dimensional structure of fluid conduits sustaining an active deep marine cold seep[END_REF]. Most of the marine scientific community attention has been focused on pockmarks whereas offshore sinkholes appear to have been poorly investigated or even misinterpreted. Understanding the differences between these two types of offshore structures appears necessary at this point. In terms of implications, these offshore fluid-related collapse/expulsion features have major impacts on seabed stability, implying potential hazards to offshore infrastructures as well as to coastal infrastructures by changing the failure rock mechanics that potentially leads to geomorphologic evolution, submarine escarpment destabilization and possibly to tsunamis (e.g. Orange et al., 2002 and herein references). Secondly, their distribution affines our knowledge of the fresh/saline water distribution, storage and flows in coastal and marine systems where the freshwater supply to cities remains problematic (Garven and Freeze, 1984;[START_REF] Post | Offshore fresh groundwater reserves as a global phenomenon[END_REF]. Thirdly, subseafloor fluid-circulation markers and associated fluid-escape feature distribution patterns have also helped the oil and gas exploration geologists, which have used their presence to detect offshore seepages related to underlying petroleum systems since the 1930s [START_REF] Abrams | Best practices for detecting, identifying and characterizing nearsurface migration of hydrocarbons within marine sediments[END_REF]. Lastly, long-term seeping fluids through the seafloor have been identified as the primary control for chemosynthetic benthic ecosystems that currently localize a poorly understood submarine high biomass and productivity (e.g. pockmarks: [START_REF] Dando | Ecology of a North Sea pockmark with an active methane seep[END_REF][START_REF] Ondréas | ROV study of a giant pockmark on the Gabon continental margin[END_REF]sinkholes: Biddanda et al., 2006;Mitchel-Tapping et al., 1999). King and Mac Lean (1970, p. 3141) first defined as pockmarks the structures discovered off Nova Scotia as follows "Pockmarks are cone-shaped depressions… possibly formed by ascending gas or subsurface water leakage from underlying … sediments". These crater-like features or submarine depressions commonly occur in fine-grained sediments, have circular to ellipsoidal depression shapes and can reach up to 4000 m in diameter (e.g. [START_REF] Cole | Giant irregular pockmark craters in the Palaeogene of the Outer Moray Firth Basin, UK North Sea[END_REF][START_REF] Michaud | Fields of multi-kilometer scale sub-circular depressions in the Carnegie Ridge sedimentary blanket: Effect of underwater carbonate dissolution[END_REF]. They have been observed in all ranges of water depth, from shallow-water environments (e.g. ~ -15/40 m; [START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF] to deep-sea water (~ -3160 m; [START_REF] Marcon | Fluid-flow regimes and growth of a giant pockmark[END_REF] and are considered as giant pockmarks in the case where their diameter is greater than 250 m (Foland et al., 2007;[START_REF] Pilcher | Mega-pockmarks and linear pockmark trains on the West African continental margin[END_REF]. They have been recognized in different settings such as open oceanic environment (e.g. [START_REF] Michaud | Fields of multi-kilometer scale sub-circular depressions in the Carnegie Ridge sedimentary blanket: Effect of underwater carbonate dissolution[END_REF], lakes (e.g. [START_REF] Chapron | Fluid-escape features as a precursor of a large sublacustrine sediment slide in Lake Le Bourget, NW Alps, France[END_REF], active margins (e.g. Salmi et al., J o u r n a l P r e -p r o o f Journal Pre-proof 2011), passive margins (e.g. [START_REF] Ondréas | ROV study of a giant pockmark on the Gabon continental margin[END_REF], volcanic ridges (e.g. [START_REF] Michaud | Fields of multi-kilometer scale sub-circular depressions in the Carnegie Ridge sedimentary blanket: Effect of underwater carbonate dissolution[END_REF], clastic environments (e.g. [START_REF] Sultan | Hydrate dissolution as a potential mechanism for pockmark formation in the Niger Delta[END_REF], carbonate environments (e.g. [START_REF] Backshall | Drowned dolinesthe blue holes of the Pompey Reefs, Great Barrier Reef[END_REF][START_REF] Betzler | Giant pockmarks in a carbonate platform (Maldives, Indian Ocean)[END_REF] and even proposed for explaining extraterrestrial surface circular morphologies (e.g. Mars: [START_REF] Komatsu | Roles of methane and carbon dioxide in geological processes on Mars[END_REF]. Rare outcropping paleo-pockmarks have been described in onshore studies such as the ones from Cantabria in Spain or the southeast basin of France (e.g. [START_REF] Agirrezabala | Outcrop analogues of pockmarks and associated methane-seep carbonates. A case study from Lower Cretaceous (Albian) of the Basque-Cantabrian Basin, western Pyrenees[END_REF][START_REF] Gay | 3D morphology and timing of the giant fossil pockmark of Beauvoisin, SE Basin of Franc[END_REF].

Pockmarks

The fluid sources related to pockmarks commonly originate from shallow subsurface sediments (e.g. [START_REF] Baltzer | Geophysical exploration of an active pockmark field in the Bay of Concarneau, southern Brittany, and implications for resident suspension feeders[END_REF], dewatering and degassing of the water/gas-charged sediments [START_REF] Agirrezabala | Outcrop analogues of pockmarks and associated methane-seep carbonates. A case study from Lower Cretaceous (Albian) of the Basque-Cantabrian Basin, western Pyrenees[END_REF], buried salt diapirs (e.g. [START_REF] Taylor | Trapping and migration of methane associated with the gas hydrate stability zone at the Blake Ridge Diapir: new insights from Seismic data[END_REF][START_REF] Gay | 3D morphology and timing of the giant fossil pockmark of Beauvoisin, SE Basin of Franc[END_REF], drowned carbonate banks (e.g. [START_REF] Betzler | Giant pockmarks in a carbonate platform (Maldives, Indian Ocean)[END_REF], buried unconformities (e.g. [START_REF] King | Pockmarks on the Scotian Shelf[END_REF], buried sedimentary channels [START_REF] Pilcher | Mega-pockmarks and linear pockmark trains on the West African continental margin[END_REF][START_REF] Gay | Sinuous pockmark belt as indicator of a shallow buried turbiditic channel on the lower slope of the Congo Basin, West African Margin[END_REF]Ondreas et al., 2005), buried Mass Transport Deposits [START_REF] Bayon | Multi-disciplinary investigation of fluid seepage on an unstable margin: The case of the Central Nile deep sea fan[END_REF] and/or underlying petroleum systems [START_REF] Pilcher | Mega-pockmarks and linear pockmark trains on the West African continental margin[END_REF]. The nature of the involved-fluids can be of various origins such as biogenic gas (e.g. [START_REF] Cole | Giant irregular pockmark craters in the Palaeogene of the Outer Moray Firth Basin, UK North Sea[END_REF][START_REF] Marcon | Fluid-flow regimes and growth of a giant pockmark[END_REF] and thermogenic gas, gas hydrate destabilization [START_REF] Pilcher | Mega-pockmarks and linear pockmark trains on the West African continental margin[END_REF], hydrocarbon seeps and/or sand injectites [START_REF] Cole | Giant irregular pockmark craters in the Palaeogene of the Outer Moray Firth Basin, UK North Sea[END_REF], fresh water seepage [START_REF] Chapron | Fluid-escape features as a precursor of a large sublacustrine sediment slide in Lake Le Bourget, NW Alps, France[END_REF] as well as brines and sulfides (e.g. H 2 S; [START_REF] Etiope | Methane and hydrogen sulfide seepage in the northwest Peloponnesus petroliferous basin (Greece): Origin and Geohazard[END_REF].

Submarine sinkholes

Karstic landscape geomorphologies also commonly exhibit sub circular depressions called sinkholes or dolines (e.g. [START_REF] Sauro | Dolines and sinkholes: aspects of evolution and problems of classification[END_REF][START_REF] Gutiérrez | A genetic classification of sinkholes illustrated from evaporite paleokarst exposures in Spain[END_REF]Yechieli et al., 2016;[START_REF] Parise | Sinkholes[END_REF]. Sinkholes are typical surface expressions of karst landscapes on Earth and potentially on extraterrestrial bodies (e.g. Titan's sinkholes; [START_REF] Cornet | Dissolution on Titan and on Earth: Toward the age of Titan's karstic landscapes[END_REF]. These either onshore or offshore striking features originate from underlying subsurface dissolution cavities (dissolved origin) in either evaporites, carbonates or quartzites, leading to surface collapse (e.g. [START_REF] Gutiérrez | A review on natural and human-induced geohazards and impacts in Karst[END_REF][START_REF] Sauro | Genesis of giant sinkholes and caves in the quartz sandstone of Sarisariñama tepui, Venezuela[END_REF]. A necessary condition for forming large sinkholes seems to be a large amount of fluid flow through the system, which allows and maintains the effective dissolution of the soluble host rock [START_REF] Hiller | The creation of collapse dolines: A 3D modeling approach[END_REF]. Submarine sinkholes and karstic landscapes are surprisingly absent from extensive reviews (e.g. [START_REF] Klimchouk | morphogenesis of hypogenic caves[END_REF][START_REF] Gutiérrez | A review on natural and human-induced geohazards and impacts in Karst[END_REF][START_REF] Parise | Sinkholes[END_REF]. Submarine sinkholes mostly occur in carbonates and have been interpreted as: (i) subaerial dolines (meteoric water-related) that have been drowned during sea level rise (e.g. [START_REF] Backshall | Drowned dolinesthe blue holes of the Pompey Reefs, Great Barrier Reef[END_REF][START_REF] Rosenberg | Erosion of the Florida and Yucatan carbonate margins of the gulf of Mexico: Subaqueous or subaerial?[END_REF], (ii) Dissolution derived crater-like features that originate from an underlying thermal convection of volcanic or basement-hydrothermal systems (e.g. Yubkho and Lygina, 2013; ~ -2300 m; [START_REF] Michaud | A honeycomb seafloor morphology in carbonate sediment of the Carnegie Ridge (offshore Ecuador): Formation and potential geodynamic significance[END_REF]; (iii) Submarine dolines related to the subsurface mixing of fresh and/or saline waters that dissolve limestones in coastal zones (e.g. ~ -450 m, [START_REF] Land | Submarine karst belt rimming the continental slope in the Straits of Florida[END_REF]; ~ -575 m, [START_REF] Land | Genesis of a submarine sinkhole without subaerial exposure: Straits of Florida[END_REF].

Inactive pockmarks and most of the submarine sinkholes are difficult to discriminate because none of them expels fluids at the time of data acquisition. In a few cases, several relatively hot water seeps have been observed in sinkholes (e.g. groundwater; [START_REF] Biddanda | Exploration of a Submerged Sinkhole Ecosystem in Lake Huron[END_REF]saline waters;[START_REF] Mitchell-Tapping | Mud Hole: A Unique Warm-Water Submarine Spring, Located Offshore Southwestern Florida[END_REF]. However, in terms of formation mechanisms, it is generally admitted that pockmarks are preferentially physically excavated by focused fluid expulsion in comparison to sinkholes that originate from dissolution and physical collapse, this latter process leading to jagged morphologies [START_REF] Hovland | The significance of pockmarks to understanding fluid flow processes and geohazards[END_REF][START_REF] Judd | Seabed fluid flow: the impact of geology, biology and the marine environment[END_REF]. Detailed studies of pockmarks did not reveal any systematic ejecta rim around the depression despite the physical excavation (e.g. [START_REF] Pilcher | Mega-pockmarks and linear pockmark trains on the West African continental margin[END_REF]; it seems that the venting of fluids preferentially removes the fine-grained sediments by suspending them into the water column [START_REF] Webb | Ecology and geology of pockmarks[END_REF]. The fluid movements giving birth to J o u r n a l P r e -p r o o f pockmarks or sinkholes can be favored along structural surfaces or rocky basements (e.g. [START_REF] Shaw | Marine Geology of Saint George's Bay, Newfoundland, as interpreted from multibeam bathymetry and back-scatter data[END_REF][START_REF] Etiope | Methane and hydrogen sulfide seepage in the northwest Peloponnesus petroliferous basin (Greece): Origin and Geohazard[END_REF], through fractures in the sediments (e.g. [START_REF] Marcon | Fluid-flow regimes and growth of a giant pockmark[END_REF] and through fault-zones of different scales (e.g. pockmarks: [START_REF] Sultan | Hydrate dissolution as a potential mechanism for pockmark formation in the Niger Delta[END_REF][START_REF] Micallef | Fluid flow systems of the Malta Plateau, Central Mediterranean Sea[END_REF]sinkholes: Mitchell-Tapping et al., 1999). In this paper, (i) we aim to address the type (sinkholes or pockmarks), the distribution, the morphometry and the different mechanisms at the origin of the abyssal giant depressions alignment located at the toe of the Blake Bahama Escarpment (BEE; Fig. 1). In doing so, we use the bathymetric/seismic data in order to (ii) document and discuss the interactions between the km-scale structural features of the area, the carbonate platform-scale circulation, the circular submarine depressions and the current-day geomorphology. To our knowledge, pockmarks and sinkholes have never been described on modern sea floor below the Carbonate Compensation Depth (CCD; ~ -4000 m), in abyssal carbonate contexts (> 3000 m of water depth), at the toe of a structurally-controlled mega-carbonate escarpment such as the BBE (> 4 km-high). In addition, we have most probably localized the geomorphologic depressions at the toe of the Bahamian platform where 'brines' would seep, as suggested in the Paull and Neumann's model (1987; the figure 2 of their publication).

Scientific context and geological setting

Tectonic setting and platform establishment

The Bahamas are located along a transform continental margin and are underlain by a stretched pre-Triassic continental crust [START_REF] Mullins | Origin of the northwestern Bahama Platform: Review and Reinterpretation[END_REF][START_REF] Mercier De Lépinay | Transform continental margins -Part 2: A worldwide review[END_REF] where a large carbonate platform developed in the Late Jurassic and Early Cretaceous in response to high carbonate production rates [START_REF] Freeman-Lynde | Marine geology of the Bahama Escarpment[END_REF][START_REF] Carlo | Characterization of rock-magnetic signature during subaerial exposure in Platform Carbonates from new providence[END_REF]. The clockwise rotation of the North American continental structural block caused a complex transtensional ocean-continent transition where N-S striking normal basement faults (e.g. Blake Plateau) have coexisted with E-W striking pre-Jurassic basement Atlantic Ocean fracture zones (e.g. Blake Spur FZ, Great Abaco FZ; [START_REF] Sheridan | Conceptual Model for the Block-fault origin of the North American Atlantic Continental Margin Geoscyncline[END_REF]. Since the early Cenozoic, active faulting occurred in response to the slight tectonic interactions between the southern Cuban orogeny and the Mesozoic-aged tectonic structures [START_REF] Masaferro | Episodic folding inferred from syntectonic carbonate sedimentation: the Santaren anticline, Bahamas foreland[END_REF][START_REF] Kindler | Discovery of Miocene to early Pleistocene deposits on Mayaguana, Bahamas: Evidence for recent active tectonism on the North American Margin[END_REF]. Post-Oligocene reactivation of vertical basement faults have been reported by [START_REF] Mullins | Walkers Cay Fault, Bahamas: Evidence for Cenozoic faulting[END_REF] in Walker's Cay (northern Bahamas), as well as a Neogene to present-day folding in the Santaren anticline [START_REF] Masaferro | Episodic folding inferred from syntectonic carbonate sedimentation: the Santaren anticline, Bahamas foreland[END_REF] and during the Quaternary tectonic tilting, using the Sunniland Fracture Zone along the Bahama escarpment [START_REF] Kindler | Discovery of Miocene to early Pleistocene deposits on Mayaguana, Bahamas: Evidence for recent active tectonism on the North American Margin[END_REF]Fig. 1a).

Carbonate platform circulation

Present-day carbonate platforms are commonly subject to dissolution, corrosion, dolomitization and dedolomitization by fresh and/or saline groundwater in the first kilometers deep (e.g. [START_REF] Sanford | Porosity development in coastal carbonate aquifers[END_REF][START_REF] Hughes | Three-dimensional flow in the Florida platform: Theoretical analysis of Kohout convection at its type locality[END_REF][START_REF] Wierzbicki | Burial dolomitization and dissolution of Upper Jurassic Abenaki platform carbonates, Deep Panuke reservoir, Nova Scotia, Canada[END_REF]. This dissolution process can lead to relatively deep submarine sinkhole formation that are heterogeneously distributed, down to ~ 575 m in water depth [START_REF] Land | Genesis of a submarine sinkhole without subaerial exposure: Straits of Florida[END_REF], i.e. deeper than the last glacial low stand of sea level (~ 120 m) and much shallower than the CCD (~ J o u r n a l P r e -p r o o f architecture and the scale of fluid circulations in carbonate platforms.

Fluid circulation into the Bahamian platform

The Bahama platform offers within a same succession many reservoir rocks with a wide range of porosities (thick reefal and shallow water platform limestones and dolomites), organic matter-rich source rocks (restricted marine limestones) as well as the presence of vertical and lateral seals (fine-grained slope carbonate, pelagites and contourites, thick restricted marine anhydrites and halites), all needed for petroleum systems [START_REF] Walles | Tectonic and diagenetically induced seal failure within the south-western Great Bahamas Bank[END_REF]. In addition, the Blake Bahama Escarpment (BBE) offers a 4 km-high and steepening with depth submarine topography that increases the vertical interacting section between groundwater and seawater, where the structural lineaments shape the submarine physiography. This provides an ideal case study to better constrain a structurally-controlled and dissolved carbonate system at the > km-scale. Indeed, water temperatures around faults into the Bahamian carbonate platform (depth: -3300 m to -3800 m) can be similar to water surface temperatures; these data demonstrate the direct and efficient hydrologic relations between surface and great depths (3500 m) [START_REF] Walles | Tectonic and diagenetically induced seal failure within the south-western Great Bahamas Bank[END_REF]. Drilling and core data show that hydrodynamic alteration processes in carbonates near major fault surfaces create buried dissolution conduits and caves (e.g. 8 meters high, porosity = 100%) at a depth of 4000 meters in the Great Bahama Bank [START_REF] Walles | Tectonic and diagenetically induced seal failure within the south-western Great Bahamas Bank[END_REF]. Solution collapse brecciation of dolomite sequences, in response to the dissolution of embedded anhydrites by corrosive fluids along faults, are suspected to cause those surprising reservoir properties within the platform.

Fluid circulation within the Bahamas carbonate platform has operated at different times, involving a wide range of fluid volumes, natures (freshwater, seawater and brines) and mechanisms; this has therefore needed efficient and long-term drivers of circulations [START_REF] Melim | Subsurface geology of the Bahamas banks[END_REF]. The fluid circulation would be related to: (i) thermally driven circulation and density gradients between cold saline oceanic waters and ground waters of the carbonate platform that are warmed by the geothermal heat flux [START_REF] Kohout | Hydrogeology related to the geothermal conditions of the Floridan Plateau[END_REF][START_REF] Whitaker | Active circulation of saline ground waters in carbonate platforms: Evidence from the Great Bahama Bank[END_REF]. (ii) Lateral flow due to an across-the-bank head difference [START_REF] Whitaker | Circulation of saline groundwaters in carbonate platforms: A review and case study from the Bahamas[END_REF][START_REF] Whitaker | Dolomitization by near-normal seawater? Field evidence from the Bahamas[END_REF] and the reflux of mesosaline brines along the platform (salinity of 38-45 ‰) that also drive circulation [START_REF] Simms | Dolomitization by thermal convection in carbonate platforms: Abstract[END_REF][START_REF] Whitaker | Active circulation of saline ground waters in carbonate platforms: Evidence from the Great Bahama Bank[END_REF]. (iii) The topographically-driven meteoric groundwater is probably a key factor for efficiently driving fluid-circulation into the carbonate platform, in particular during low sea-level stands [START_REF] Wilson | Fresh and saline groundwater discharge to the ocean: A regional perspective[END_REF]. The re-flooding of the banks at the last-deglaciation also strengthened the thermal convection (Dawans and Sawart, 1988;[START_REF] Whitaker | Dolomitization by near-normal seawater? Field evidence from the Bahamas[END_REF]. (iv) Jointing optimizes the dissolution process by increasing surface areas exposed to corrosive waters [START_REF] Freeman-Lynde | Erosional modification of Bahama Escarpment[END_REF] and most probably enhances the existing circulation by supplying efficient, both porous and permeable km-scale structural-pathways for fluids. The structural heterogeneities of a platform are clearly the most efficient entities for fluid entrance, pathway, storage and seepage available spaces in carbonate rocks, allowing both episodic and persistent subsurface fluid-flows and overprinting an anisotropy of permeability in the faulted system i.e. the structural permeability [START_REF] Sibson | Structural permeability of fluid-driven fault-fracture meshes[END_REF]. At the cm-scale, pre-existing fractures and faults control the distribution, the orientation and the intensity of the carbonate dissolution by the circulating brines (defectdriven kinetic reactions); channelizing the shapes of dissolution targets of fractures and zones of structural weakness, preferentially along their strikes [START_REF] Gouze | X-ray tomography characterization of fracture surfaces during dissolution[END_REF][START_REF] Garcia-Rios | Influence of the flow rate on dissolution and precipitation features during percolation of CO2-rich sulfate solutions through fractured limestone samples[END_REF][START_REF] Privalov | Structural control of a dissolution network in a limestone reservoir forced by radial injection of CO2 saturated solution: Experimental results coupled with X-Ray computed tomography[END_REF].

J o u r n a l P r e -p r o o f

Journal Pre-proof

With the exception of onshore compressive settings (e.g.; [START_REF] Bruna | High permeability zones in folded and faulted silicified carbonate rocks: Implications for karstified carbonate reservoir[END_REF][START_REF] Pisani | Structurally controlled development of a sulfuric hypogene karst system in a fold-and-thrust belt (Majella Massif, Italy)[END_REF][START_REF] Pontes | Flow pathways in multiple direction fold hinges : implications for fractured and karstified carbonate reservoirs[END_REF], the structurally-controlled dissolution of carbonates, in particular in abyssal submarine settings, has not been described and is therefore poorly understood; this process remains to be documented at the km-scale, in particular for the Bahamian platform system.

Current-day physiography and escarpment basal erosion

The present-day steep morphology of the Blake Bahama Escarpment (BBE) has an erosional origin (~5-6 km of retreat, up to 4.2 km high), essentially developed during the Cretaceous (Freeman-Lynde and Ryan, 1985) and Tertiary times [START_REF] Schlager | Early Cretaceous platform re-entrant and escarpment erosion in the Bahamas[END_REF]. This probably still active erosional retreat overprinted a buried "benchshaped" geomorphology exhibiting a relict ~5-6 km-wide flat carbonate surface currently buried by Miocene to Quaternary sediments lying in the San Salvador Abyssal plain [START_REF] Schlager | Early Cretaceous platform re-entrant and escarpment erosion in the Bahamas[END_REF]. Similarly to the Blake Escarpment (10-15 km of retreat), an Oligocene unconformity (A u ) is present at the top of the Cretaceous carbonates of the buried-bench and predates the Miocene massive sequences of turbidites and the Quaternary hemipelagites [START_REF] Paull | Erosional origin of the Blake Escarpment: An alternative hypothesis[END_REF]. Contourites are present at the toe of the BBE at least since the Oligocene in response to abyssal currents activity [START_REF] Bliefnick | Deposition and provenance of Miocene intraclastic chalks, Blake-Bahama Basin, western North Atlantic[END_REF][START_REF] Mulder | Into the deep: A coarse-grained carbonate turbidite valley and canyon in ultra-deep carbonate setting[END_REF].

More generally, the basal erosion of carbonate escarpments such as the BBE, the Blake escarpment or the Florida escarpment appears to be related to several non-exclusive mechanisms: (i) the circulating saline waters and brines that seep from the base of the escarpment dissolve the carbonates and therefore destabilize the overlying slopes [START_REF] Walles | Tectonic and diagenetically induced seal failure within the south-western Great Bahamas Bank[END_REF][START_REF] Henderson | Fluid flow through carbonate platforms: constraints from 234 U/ 238 U and Clin Bahamas pore-waters[END_REF][START_REF] Paull | Continental margin brine seeps: Their geological consequences[END_REF], such as described along at least 10% of the Florida carbonate Escarpment base [START_REF] Paull | Morphology of Florida Escarpment chemosynthetic brine seep community sites: deep-tow, seabeam, and GLORIA surveys[END_REF][START_REF] Chanton | Control of pore-water chemistry at the base of the Florida escarpment by processes within the platform[END_REF]. These are made of 94% of seawater and 6% of dense brines [START_REF] Chanton | Control of pore-water chemistry at the base of the Florida escarpment by processes within the platform[END_REF] and can reach temperatures up to 115°C within the platform under Florida [START_REF] Paull | Continental margin brine seeps: Their geological consequences[END_REF]. The combination of hydrogen sulfides (HS -) and oxygenated seawater (from the platform edge) are suspected to favor acid (H + ) formation that corrodes the toes of a carbonate escarpment along open vertical fractures [START_REF] Hill | Sulfuric acid speleogenesis of Carlsbad Cavern and its relationship to hydrocarbons, Delaware Basin, New Mexico and Texas[END_REF][START_REF] D'angeli | Sulfuric acid caves of Italy: a review[END_REF]. The slope of the BBE escarpment steepens with depth and inner platform carbonate facies at its base strongly suggests that collapse dismantlement operates in response to brine seeps. (ii) Locally, the tectonic activity related to movements along fracture zones implies rock jointing that increases areas exposed to corrosive waters and slope destabilization [START_REF] Freeman-Lynde | Erosional modification of Bahama Escarpment[END_REF]. (iii) The dissolution by oceanic corrosive waters would also imply escarpment undercutting, leading to overlying slope destabilization [START_REF] Paull | Erosional origin of the Blake Escarpment: An alternative hypothesis[END_REF], in particular for the anhydrite layers dissolved by ambient waters along the Florida escarpment [START_REF] Paull | Seabed measurements of modern corrosion rates on the Florida escarpment[END_REF]. (iv) The removal of sediments and the lateral scarp retreat at the base of the slope by abyssal currents, suggested for the Blake escarpment and the BBE (Paul and Dillon, 1980;[START_REF] Schlager | Early Cretaceous platform re-entrant and escarpment erosion in the Bahamas[END_REF][START_REF] Freeman-Lynde | Erosional modification of Bahama Escarpment[END_REF][START_REF] Land | Abyssal erosion and scarp retreat: Deep Tow observations of the Blake Escarpment and Blake Spur[END_REF], have been excluded by [START_REF] Paull | Continental margin brine seeps: Their geological consequences[END_REF] for both Florida and Yucatan escarpments where abyssal currents are absent. In addition, (v) bioerosion has probably a very limited impact as argued in Freeman- [START_REF] Freeman-Lynde | Erosional modification of Bahama Escarpment[END_REF]. In contrast, the brine-related dissolution mechanism for explaining deep submarine karstic landscapes has been strongly challenged by [START_REF] Rosenberg | Erosion of the Florida and Yucatan carbonate margins of the gulf of Mexico: Subaqueous or subaerial?[END_REF]; the author proposes that all of the deep Florida and Yucatan sinkholes (water depth: -500 m to -2000 m) are related to a ~ one My-duration subaerial karstification event. This latter event would imply a ~ 2000 m relative sea-level fall related to the isolation of the Gulf of Mexico at the Paleocene that seems unrealistic for the Bahamas.

J o u r n a l P r e -p r o o f

Methodology and database

Data set was collected during the Leg 2 of the CARAMBAR 2 cruise from December 2016 to January 2017 onboard the R/V l'Atalante [START_REF] Mulder | CARAMBAR 2 cruise, RV l'Atalante[END_REF]. The survey covered the southern part of Exuma Sound (ES), the Exuma Plateau (EP), the Blake Bahama Escarpment (BBE) and the adjacent San Salvador Abyssal Plain (SSAP), including 20.395 km 2 of multibeam bathymetry and backscatter and 2149 km of High Resolution (HR) seismic profiles. Bathymetry and backscatter data have been obtained using a Konsberg EM122/EM710 multibeam echo sounder, and the high-resolution (HR) multichannel seismic system uses a 192 channels /1200 m long streamer (6.25 m group interval) and four 35/35 inc 3 GI airguns. The signal dominant frequency is between 20 and 300 Hz. Streamer and source positioning were derived from vessel Differential Global Positioning System (DGPS) and compass birds. Processing of this seismic data was performed using SolidQC software developed by Ifremer. Basic processing flow included (i) Normal Move Out (NMO) correction, (ii) 24fold stacking and (iii) constant velocity gradient migration. The obtained data are with vertical resolution approaching 2 m and usable acoustic imaging to approximately ca. 1 to 1.5s two-way travel time. Sinkholes and structural lineaments have been mapped using their morphologic expressions such as depression, local changes in isobaths trends, as well as their seismic expression where 2D seismic data were available (Fig. 2).

Results

Submarine present day physiography

The studied area is located on an ocean-continent transition that has been shaped as a transform continental margin exhibiting a steep and sharp submarine morphology [START_REF] Mullins | Carbonate platforms along the southeast Bahamas-Hispaniola collision zone[END_REF][START_REF] Mercier De Lépinay | Transform continental margins -Part 2: A worldwide review[END_REF]Fig. 1a). The Sunniland Fracture Zone (SFZ) bounding the San Salvador Abyssal Plain is parallel to the oceanic transform faults and fracture zones identified on the sea floor of the North Atlantic Plate (NAP). This major tectonic corridor of tens of km wide trends perpendicular to the magnetic anomalies of the oceanic crust and clearly shapes the eastern side of the Bahamas submarine physiography (Fig. 1a). Only few morphologies and reliefs of the studied area are parallel to the N-S striking tensional basement faults recognized in Sheridan's works (1974) that control the Blake Plateau area shape.

Based on the analysis of bathymetric maps, the Drowned Barrier Reef (DBR), the Exuma Canyon (EC), the Samana Re-entrant (SR), the Exuma Valley (EV), the Exuma Plateau, the South Exuma Plateau Valley (SEPV), the Crooked Canyon (CC), the BBE, the plunge pools, and the contouritic deposits have been named similarly to the work of [START_REF] Mulder | Into the deep: A coarse-grained carbonate turbidite valley and canyon in ultra-deep carbonate setting[END_REF] (Fig. 1b). The generally N-S trending carbonate BBE can reach up to 4200 m of submarine elevation above the SSAP and is interestingly curved over a distance of ~18 km at ~74°10'W; 23.32'N. The BBE curvature is observed in the area where the SFZ intersects the BBE. The SFZ appears to be made of different sets of tectonic features, striking N110, N145 and E-W (see inset in upper right corner of Figure 1b). The 090E structural lineaments appear to control the shape of the southern part of the J o u r n a l P r e -p r o o f BBE, the northern edge of the Samana Reentrant, the EV and the SEPV. The N110 and N145 trending lineaments (mean ~N130 direction) control the submarine morphologies of the southern edge of the Exuma Canyon (Samana Re-entrant; SR), the transition between the EV and the SR, as well as the NW-SE trending BBE curvature (Fig. 1b). Regarding the oceanic crust structure in the SSAP, the lineaments are mostly N160 to N-S oriented and control the elongation of the distal lows where sediments are preferentially deposited (right part of Figure 1b). In addition, the N-S basement-inherited BBE trend is very similar to the Crooked Canyon orientation. The contourites are present at the toe of the BBE [START_REF] Mulder | Into the deep: A coarse-grained carbonate turbidite valley and canyon in ultra-deep carbonate setting[END_REF]; e.g. Fig. 2a and2b) with the exception of the BBE curvature area (Fig. 2c).

Summarizing these observations, the physiographic sketches of Figure 1b reveal that ocean-continent transition inherited structural lineaments that control and shape the BBE trends and curvatures, the submarine steep-walled giant canyon orientations [START_REF] Mulder | Into the deep: A coarse-grained carbonate turbidite valley and canyon in ultra-deep carbonate setting[END_REF] as well as the abyssal topographies, clearly expressed by the structurally-controlled isobath contours. This structural architecture is consistent with the conceptual view of [START_REF] Sheridan | Conceptual Model for the Block-fault origin of the North American Atlantic Continental Margin Geoscyncline[END_REF] arguing for the presence of structural blocks compartmentalizing the Atlantic continental margin by N-S tensional faults and major strike-slip features accommodating block rotation.

Regional distribution of submarine depressions

Twenty-nine abyssal submarine depressions have been mapped at the toe of the BBE, in the western part of the SSAP lying at ~ -4900 m of depth (Fig. 1b and 2); they occupy ~20% of the base of the carbonate escarpment, i.e. a total of 21.5 km / 102 km long (Fig. 1b). The water depths of the depressions range from -4584 m to -4967 m, exclusively below the Carbonate Compensation Depth of the area (~ -4500 m; Heath and Mullins, 1984) (Fig. 2; Table 1). They are all located between 2 km and 5 km seaward of the toe of the BBE (isobath -4000m). The mean depression diameters range from 255 m to 1819 m and form depressions ranging from 30 m to 185 m of depth without any clear preferential direction of elongation (refer to the 4.4 section for details). The depressions of this study (n = 29) are so called 'giant depressions' considering that their diameters are wider than 250 m, following the pockmark classification of [START_REF] Foland | Pockmarks along the California continental margin; implications for fluid-flow[END_REF]. Depressions are not randomly distributed in the studied area and show a high concentration nearby to the BBE curvature (Fig. 1b and2c). Depression density increases where (i) the structural lineaments of the SFZ intersect the BBE and (ii) where contouritic deposits are absent. The depressions have been classified into two families: inner depressions (n=7; d escarpment = 2 -3 km where 'd' is the shortest distance to the 4000 m isobath) and outer depressions (n=22; d escarpment = 3 -5 km). In order to facilitate the description of the bathymetric map (Figs. 1 and2), the depressions have been numbered from south to north (Fig. 2a to 2d); In the cases of 14a, 14b, 17a and 17b, each overlapping depression has the same number. This study distinguishes four areas of interest where a detailed analysis of abyssal depressions is considered (Fig. 2a, b, c andd).

Depression 1 is located in contouritic deposits and clearly appears isolated from the other depressions (7 km south of depression 2; Fig. 2a and2b). It seems aligned with the E-W trending structural lineament bounding the northside of the Exuma Canyon and the southern depression described as a plunge pool in Mulder et al.

(2019) (Fig. 1b). This is also, together with the depressions 23 to 27, one of the depressions of the study area that is collapsing contouritic deposits. Depressions 2, 4, 6, 7 and 8 appear to be N-S aligned and located at ~3500 m from the BBE (Fig. 2b).

In contrast, the depressions 3 and 5 are 1000 m to 2000 m closer to the BBE than 2, 4, 6, and 7, belonging to the J o u r n a l P r e -p r o o f inner depressions (Fig. 2b). The spacing between the depressions is between 0 m for the overlapping depressions (i.e. #7 and #8) and 1200 m (i.e. #2 and #4). Depressions 9,10,11,12,13,14a,14b,15,16,17a,17b,18,19,20 and 21 are located near the BBE curvature (~74°10'W; 23.32'N; (Fig. 2c)). More than half of the studied depressions (15/29) are therefore located where structural lineaments cross the BBE. Depressions 14a, 14b, 17a and 17b are called inner depressions because they are located at 2709 m and 2838 m from the BBE (Table 1). In contrast, the other depressions form a chain more distant to the BBE (around 4000 m; Fig. 2c). Contourites seem to be absent in this area (Fig. 1b).

Depressions 22, 23, 24, 25, 26 and 27 are located in the northern part of the studied area where a contouritic drift has been mapped (Fig. 1b). The depressions water depths range from -4710 m to -4812 m whereas the inner depression 26 is located at only 2211 m from the BBE, which contrasts with the others located at around 4000 m from the BBE (Fig. 2d; table 1).

Seismic profile analysis

2D Seismic profiles and their interpretations are given in Figures 3, 4 and5. These figures exhibit two BBE-perpendicular-trending seismic lines (Fig. 3a, 5a) and one single BBE-parallel trending seismic line (Fig. 3b, 5b and 5c), which are all shown in Figures 1, 2 and3.

Sedimentary reflectors -The offshore seismic reflectors are believed to exhibit respectively from the subsurface to the surface, the top of the oceanic crust (basement), the top Kimmeridgian sedimentary package and the top of the Tithonian sedimentary package, consistent with the work of [START_REF] Schlager | Early Cretaceous platform re-entrant and escarpment erosion in the Bahamas[END_REF] (Figs. 3a and3a). These interpreted sedimentary packages are westwards dipping and thickening, despite the fact their clear seismic expressions are blurring towards the BBE. The oceanic crust is around 0.3 s two-way travel time (TWT) deep in the eastern part of the profile whereas it is 1.2 s TWT deep in the western part of the profile. Due to this basement structural dip and the high sedimentary supply nearby the platform, the thickness of the sedimentary package reaches a maximum at the toe of the BBE (Fig. 3a). The Oligocene unconformity (A u ) has also been identified according to its high-amplitude seismic reflections, probably related to a strong lithologic contrast, as well as the recognized "downlaps" in the overlying Miocene deep-sea deposits (Figs. 3a;[START_REF] Paull | Erosional origin of the Blake Escarpment: An alternative hypothesis[END_REF].

Buried bench -A 6 km wide buried carbonate bench exhibiting a typical blind facies is clearly identifiable in Figures 3a and5a and is similar in size and shape to the one described at the toe of the BBE in [START_REF] Schlager | Early Cretaceous platform re-entrant and escarpment erosion in the Bahamas[END_REF]. The top of this structure is located at around 6.5 s TWT and is overlain by contourites (Figs. 4a,4b), except in the BBE curvature surrounding where it lies under a 0.2 to 0.3 s thick toe of slope hemipelagic cover (Fig. 5a, 5c, 5a and 5c). The seismic expression of the bench is also similar to the wider one (10-15 km) recognized at the toe of the BBE [START_REF] Paull | Erosional origin of the Blake Escarpment: An alternative hypothesis[END_REF]. Subvertical seismic discontinuities within the bench suggest the presence of well-defined linear, antithetic and therefore "brittlestylized" collapse faults (Figs. 4a,4b). These faults have interestingly a "softer" shape in the overlying contouritic body, this latter being probably less lithified than the bench carbonates.

Bright spot -A seismic bright spot is defined as "a segment of a reflection on a near-vertical seismic reflection section which exhibits anomalously-high reflections amplitudes" (Makovsky and Klemperer, 1999, p. J o u r n a l P r e -p r o o f 10795). The BBE -parallel seismic line crosses the BBE twice and exhibits diffuse high amplitude seismic anomalies (bright spot) between 6.4 s and 7.0 s TWT (Fig. 3b). The absolute depth of this feature is independent of the seafloor relief, the water depth or the vertical distance between the seafloor and this anomaly. This independency demonstrates that none of these parameters is a major controlling one, therefore suggesting that this feature probably originates from subsurface processes. The perpendicular trending seismic line exhibits a V-Shape structure (cone of deformation) which is located right above the buried carbonate bench and terminates on the flank of the depression #1. The V-Shape structure is given by antithetic normal faults with sub seismic offsets that form a network apparently accommodating downwards collapse movement (Fig. 4b). The base of the V-shaped structure is located within the bench (Fig. 4b), between 0.6 s and 0.9 s underneath the seafloor depression. This bucket-shaped seismic bright spot shows a significant contrast between high-amplitude layered facies recognized in its upper part, the chaotic facies identified at its base and the low amplitude reflectors in the surrounding sedimentary rocks (Fig. 4a).

Contourites and depressions -

The fine-grained deep-sea sediments represent the contouritic drift and are crosscut by depression #1 (fig. 4b). This depression is bounded by at least four antithetic curved normal faults and shows a chaotic/disorganized facies in its center. The flanks of the depression are made of at least three phases of contourite deposition and show increasing sedimentary thicknesses at the vicinity of the depression (Fig. 4b). At least the upper package exhibits a hyperbolized seismic facies interpreted as a potential flank destabilization of the contourite drift (Fig. 4b).

We made a record of the acoustic response in the water column using the multibeam echosounder by positioning the boat just above depression #1 on the 23 th of December 2016 (20h13 -20h23 UT). There was no evidence for acoustic anomalies (upward motion of fluid plume) in the overlying water columns of this depression. The same measurement has been repeated on 28 th of December (15h34 -15h47 UT), within the water column above depression #15 with an identical result (Appendix 1).

Quantitative analysis

Water depths for the studied depressions are comprised between -4584 m and -4967 m (Fig. 6a). The deepest depressions are located within the BBE curvature where contourites are absent. The shallowest depression is the southern one (1) which is isolated (-4584 m) and perched on a contouritic drift. The inner depressions in the vicinity of the BBE curvature (i.e. 14a, 14b, 17a, 17b) are clearly shallower than the outer ones (9, 10, 11, 12, 13, 15, 16, 18, 19, 20, and 21). Consistently, the inner depression 26 is shallower than the 24, 25 and 27 outer ones.

All the giant depressions of this study are located between 2211 m (#26) and 4648 m (#25) away from isobath "-4000 m", within the toe of BBE (Fig. 6b). The 7 inner depressions are located between 2211 m (#26) and 2894 m (#3) from the BBE whereas the outer depressions are located between 2943 m (#9) and 4648 m (#25) (mapping of inner and outer depressions is presented in Figure 2). The greatest distances between the BBE and the outer depressions are systematically shorter than the BBE erosional retreat (5-6 km) proposed in [START_REF] Freeman-Lynde | Erosional modification of Bahama Escarpment[END_REF] and Schlager et al. (1980). This suggests that depressions appear to be always located immediately above the buried carbonate bench.

J o u r n a l P r e -p r o o f

Journal Pre-proof Depression mean diameters are related to depression shortest distances to the BBE according to the positive correlation of Figure 6c. Two main trends can be identified on this graph, both including inner and outer depressions. For both relations, the closer to the BBE, the smaller is the depression.

Elongations have been measured for outer and inner depressions (Fig. 6d). Most of the depressions are slightly elongated with the exception of the inner depression #5, which probably originated from several underlying coalescent depressions. The related rose diagram highlights a N-S (from N020 to N160) and a N095 preferential direction of slight elongation (Fig. 6d; n max elongation = 3). The inner depressions seem to be generally smaller than the outer ones.

The depression depths positively correlate with the depression mean diameters (Fig. 6e). The depression depths seem to be around 1/10 of the depression mean diameters. This scaling law is consistent with pockmark's scaling law from [START_REF] Pilcher | Mega-pockmarks and linear pockmark trains on the West African continental margin[END_REF].

Interpretations and discussion

The studied abyssal depressions show strong morphologic similarities (size, depression depth and shape) with the submarine karstic structures described by [START_REF] Land | Submarine karst belt rimming the continental slope in the Straits of Florida[END_REF] along the carbonate continental slope of Florida (Fig. 6b). In addition, the morphologic sharpness of the studied depressions appears related to seafloor collapse (Figs. 5a and5c). The depressions do not exhibit any evidence for fluid-assisted physical excavation that are commonly reported for pockmarks (Figs. 4b; Appendix 1). In consequence, despite being located in deeper water depths than the Florida depressions, we name these structures "abyssal giant sinkholes" (Figs. 7 and8). The water depths (> 4500 m) are important, so that we can confidently exclude a subaerial origin for the studied sinkholes and associated karstic landscapes. These newly discovered sinkholes are most probably related to an underlying hypogenic speleogenesis [START_REF] Klimchouk | morphogenesis of hypogenic caves[END_REF] that could be related to either a brineassisted dissolution of carbonates, as partly suggested by [START_REF] Paull | Continental margin brine seeps: Their geological consequences[END_REF], or to the dissolution of an hypothetical salt layer that has been not observed on seismic profiles (e.g. [START_REF] Warren | Salt dissolution and pointers to vanished evaporites: Karst, breccia, nodules and cement in: Evaporites[END_REF]Fig. 3).

The physiographic sketch in Figure 1 highlights that structural lineaments have most probably controlled and localized through times persistent and well-defined sedimentary pathways from the platform to the SSAP such as the Great Abaco Canyon [START_REF] Mulder | Carbonate slope morphology revealing a giant submarine canyon (Little Bahama Bank, Bahamas)[END_REF]2019). The studied abyssal sinkholes occupy 20% of the BBE toe and are preferentially localized where the inherited structural lineaments of the area cross the BBE (Fig. 1a). These observations demonstrate that a probable structurally-driven fluid circulation (leading to carbonate dissolution) along the lineaments and through the carbonate platform has likely played a key role during the erosional process leading to the retreat of the BBE. The deep-sea sinkholes growth have most likely destabilized the platform toe which is supported by the direct observations that this type of carbonate escarpments currently exhibits unstable submarine slopes steepening with depth, leading to one of the most pronounced topographic configurations on Earth [START_REF] Paull | Continental margin brine seeps: Their geological consequences[END_REF]Fig. 7). The BBE erosional retreat has therefore preferentially reused the inherited faults initially formed by the existing transform continental margin to reach its present shape (Fig. 7a).

Here, we (i) hierarchize the importance of the different controlling parameters explaining the sinkholes locations and we discuss (ii) the dissolving fluid origin and (iii) the platform-scale circulation. The structural, J o u r n a l P r e -p r o o f physical, chemical and time modalities leading to the current-day observed abyssal physiography are also discussed.

Controlling parameters for sinkhole's regional distribution

The distance to the BBE (first-order)

All of the discovered sinkholes are located at less than 4.7 km from the toe of the BBE, in this study expressed by the isobath -4000 m (Fig. 6b). 6 km is the width of the buried carbonate bench described in [START_REF] Schlager | Early Cretaceous platform re-entrant and escarpment erosion in the Bahamas[END_REF] that is consistent with the amount of 5-6 km of erosional retreat proposed in Freeman- [START_REF] Freeman-Lynde | Erosional modification of Bahama Escarpment[END_REF]. In this study, the 2Dseismic lines analysis corroborated that the aforementioned buried carbonate bench is present all over the area and in particular underneath the studied giant sinkholes (e.g. Fig. 5a). These results highlight the close relationship between (i) the toe of the escarpment, (ii) the sinkhole occurrence and (iii) the presence of the carbonate bench. Moreover, an N-S elongated sinkhole-underlyingseismic-anomaly (high amplitude) appears to be developed into the buried carbonate bench (e.g. Fig. 3b).

We here propose that a dissolution-related-cave-system at the origin of the overlying abyssal giant sinkholes would be currently expressed by the recognized underlying high-amplitude seismic anomalies and originates from the buried carbonate bench (Fig. 4a). This bright spot would represent either a collapse cave system related to dissolution such as extensively documented in [START_REF] Lu | New insights into the carbonate karstic fault system and reservoir formation in the Southern Tahe area of the Tarim Basin[END_REF] for the Tarim basin deeply buried carbonates (~ 5500 m), or a non-understood seismic artifact (Fig. 4a). In addition to the carbonate dissolution, the bench may host evaporite layers that could also be easily dissolved and may therefore form large-scale caves [START_REF] Warren | Salt dissolution and pointers to vanished evaporites: Karst, breccia, nodules and cement in: Evaporites[END_REF]. No evidence for acoustic anomalies in the overlying-sinkhole water columns (#1 and #15; Appendix 1) suggests that either no fluid was presently seeping from the sinkholes #1 and #15, or the fluid was not a gaseous gas or in adequate concentrations to be detected (appendix 1).

5.1.2

The regional faults (second-order)

The highest density of sinkholes has been identified where the BBE is structurally controlled and shaped by the Sunniland Fracture Zone (SFZ; [START_REF] Kindler | Discovery of Miocene to early Pleistocene deposits on Mayaguana, Bahamas: Evidence for recent active tectonism on the North American Margin[END_REF], leading to the BBE curvature (23°30'0''N in Figure 1b, depressions #9 to #21 in Figure 5a). This fact implies a causal connection between sinkhole location and structural lineaments in which the focused fluid migration is much more effective than non-focused flow through matrix porosities of the carbonate sedimentary column (Fig. 1; e.g. [START_REF] Pisani | Structurally controlled development of a sulfuric hypogene karst system in a fold-and-thrust belt (Majella Massif, Italy)[END_REF]. The permeability of fracture corridors in carbonates, so called structural porosity, is commonly higher than the matrix permeability (e.g. [START_REF] Rotevatn | Fault linkage and damage zone architecture in tight carbonate rocks in the Suez Rift (Egypt): Implications for permeability structure along segmented normal faults[END_REF]. This is consistent with the preferential pathways used by the fluids to corrode and dissolve the buried bench (Fig. 7b). Strike-slip faults are more efficient than other faults to allow fluid flow along a vertical direction [START_REF] Cavailhes | Influence of fault rock foliation on fault zone permeability: The case of deeply buried arkosic sandstones (Grès d'Annot, southeastern France)[END_REF]) that is consistent with the SFZ inherited architecture which is considered as an inherited strike-slip tectonic structure (transform fault tip) related to the Atlantic Ocean opening. For instance, warm saline waters currently migrate upwards from the basement and through the SFZ in offshore Florida sinkholes [START_REF] Mitchell-Tapping | Mud Hole: A Unique Warm-Water Submarine Spring, Located Offshore Southwestern Florida[END_REF]. In addition, [START_REF] Privalov | Structural control of a dissolution network in a limestone reservoir forced by radial injection of CO2 saturated solution: Experimental results coupled with X-Ray computed tomography[END_REF] showed that, J o u r n a l P r e -p r o o f at the petrophysical sample scale (cm-scale), dissolution and corrosion in carbonates are both eased and preferentially localized along fractures, along which cavities more easily form and grow. At the karst scale, the fractures and faults are enlarged by the solutional widening (e.g. [START_REF] Kaufmann | Karst aquifer evolution in fractured rocks[END_REF]. In our case study, the structural heterogeneities are expressed at the large scale by the structural lineaments that control the dissolution distribution at the platform scale (sinkholes and dissolution-derived plunge pools).

The studied sinkholes show at least three morphometric similarities (i.e. diameter (~ 1 km), water depth (-4485 m; -4058 m) and presence along a structural lineament) with the plunge pools previously mapped in [START_REF] Mulder | Carbonate slope morphology revealing a giant submarine canyon (Little Bahama Bank, Bahamas)[END_REF] (Fig. 1b). These latter only differ in terms of (i) location in the geological system i.e., located into the platform incising canyon and (ii) the depression depth (> 300 m for plunge pools; < 200 m for sinkholes).

Contourites, oceanic currents and buried sedimentary bodies (third-order)

Sinkholes can be present whether contouritic deposits are present (i.e. cover collapse sinkholes) or not (i.e. bedrock collapse sinkholes; see [START_REF] Gutiérrez | A genetic classification of sinkholes illustrated from evaporite paleokarst exposures in Spain[END_REF] for the onshore terminology), therefore demonstrating that abyssal contouritic deposits did not favor or inhibit seafloor collapses at the toe of the BBE.

In addition, Figure 4b shows that the sinkhole #1 growth and collapse both occur during the multiple phases of contourite deposition. This is consistent with an underlying carbonate dissolution by brines that would be independent of surface sedimentary processes (Fig. 7b). The studied sinkholes are elliptical in shape (Fig. 6) without any clear relation with the southwards stream bottom direction of the Deep Western Boundary Current [START_REF] Lee | Moored observations of Western Boundary Current variability and thermohaline circulation at 26.5°N in the subtropical North Atlantic[END_REF]. This means that based on our data, abyssal currents did not modify the studied sinkhole geometries. Post-sinkhole contourites may hide some underlying buried sinkholes, not identified neither on 2D seismic lines nor on the bathymetric map. Based on our 2D seismic analysis, no underlying buried sedimentary channels have been recognized underneath the abyssal giant sinkholes of this study. This means that there is no additional buried sedimentary body (e.g. channels) at the origin of the storage of brines, with the exception of the carbonate bench.

.

Fluid origin and platform-scale circulation

Relation between the underlying cave depth and the sinkhole diameter

The inner sinkholes mean diameters are smaller than the outer sinkholes (Fig. 6c). This observation suggests a geometric relationship between the sinkhole diameters and their distances to the BBE. Indeed, for a similar buried cave size, the shallower the buried sinkhole cave, the smaller the diameter of the sinkhole should be because it (i.e. "the collapse structure") is bounded by normal faults (Fig. 7b; [START_REF] Gutiérrez | A review on natural and human-induced geohazards and impacts in Karst[END_REF]. In consequence, we propose that the top of the buried bench, trapping the fluids, exhibits its own eastwards dipping slope, probably acquired during the Oligocene erosion phase [START_REF] Paull | Erosional origin of the Blake Escarpment: An alternative hypothesis[END_REF]. Assuming a similar sinkhole collapse fault dips for all cases (either 60°, 70° or 80° in Figure 7c) and using the Pythagoras theorem , we propose to quantitatively estimate the depth of the underlying dissolution-related cave location for the inner sinkholes. The base of the seismic anomaly (bright spot) where the sinkhole 1 roots is located at ~0.9 s below J o u r n a l P r e -p r o o f the seafloor i.e. 882 m -1035 m in depth, assuming Vp velocities in the range of 1.96-2.30 km/s (Hollister et al., 1972a;1972b). These estimations seem consistent with a depth of ~ 1 km inferred from the 60° dipping normal faults and a sinkhole radius around 600 m [START_REF] Anderson | The dynamics of faulting and dyke formation with applications to Britain[END_REF]Fig. 7c) commonly bounding collapse-sinkholes [START_REF] Al-Halbouni | Sinkholes, subsidence and subrosion on the eastern shore of the Dead Sea as revealed by a close-range photogrammetric survey[END_REF].

Fluid circulation at the Platform scale

At this stage, we do not have any direct evidence regarding the involved fluid nature, which would be responsible for such carbonate bench dissolution leading to sinkhole formation. However, based on the literature, the best candidate in terms of fluid chemistry would be the dense brine which is expected to seep at the toe of the BBE according to Paull and Neumann's work (1987). Our study suggests that these brines would be present down to 4.9 km of water depth according to the recognized geomorphological dissolution-markers (sinkholes).

Whatever the fluids' chemistry, these inferences demonstrate that fluid circulation is active, significant and able to dissolve the carbonates [START_REF] Hiller | The creation of collapse dolines: A 3D modeling approach[END_REF] through the entire thickness of the platform (>4.5 km of water depth). This fluid circulation is active and efficient in the area probably in response to the chemical/thermal fluids gradients as well as the high density of available structural pathways, in particular the Sunniland Fracture Zone (SFZ; Fig. 1). The purpose of this section is to qualitatively describe, synthesize and speculate on the architecture and the mechanisms of the fluid circulation in Bahamas-Florida Carbonate Platform (Fig. 8) such as proposed in [START_REF] Paull | Continental margin brine seeps: Their geological consequences[END_REF]. Surface seawater and meteoric waters both infiltrate into faults and fracture corridors of the platform hosting carbonates (limestone, dolomite) and evaporites (anhydrites, halites) (Fig. 8). They may form a ~1 km-deep aquifer of fresh/sea water mixture in which part of the storage is matrixlocated whereas water supplies are most probably structurally guided; indeed, relatively cold fresh water entrance has been for instance recognized at ~3600 m of depth into the Great Bahama Bank [START_REF] Walles | Tectonic and diagenetically induced seal failure within the south-western Great Bahamas Bank[END_REF] Fig. 8). Anhydrites such as the Cedar Key formation [START_REF] Chanton | Control of pore-water chemistry at the base of the Florida escarpment by processes within the platform[END_REF] can be located at ~1 km depth into the platform [START_REF] Hughes | Three-dimensional flow in the Florida platform: Theoretical analysis of Kohout convection at its type locality[END_REF] and have a transitory barrier effect on the downwards fluid flow, efficiently compartmentalizing most of the fresh water circulation into the first kilometer. A fraction of this water would dissolve these anhydrites, therefore renewing its aggressiveness by incorporating acidic fluids (C0 2 , H 2 S, SO 2 ), by increasing sulfate content in waters as well as water densities (Ford and Williams, 2013); this consequently promotes downward water movement within the platform. These brine-rich waters (SO 4 2-) would migrate horizontally eastwards to the free edge of the platform (~10.6 cm a -1 ; [START_REF] Henderson | Fluid flow through carbonate platforms: constraints from 234 U/ 238 U and Clin Bahamas pore-waters[END_REF], along the relatively high-permeability fault and fracture systems (Fig. 8; e.g. sinkholes and seeps along the SFZ in Mitchell-Tapping et al. ( 2009)). The brines would keep on flowing through the structural heterogeneities, towards the bench, due to their abnormal high density. They would be subsequently "trapped" into the bench where the host-rock platform carbonates facies laterally and vertically changes to low-permeability hemipelagic/contourite deposits. This phenomenon results in an identified area where the dissolution is probably focused [START_REF] Corbella | Hydrothermal mixing, carbonate dissolution and sulfide precipitation in Mississipi valley-type deposits[END_REF], in particular along the faults and fractures. An upward bounded solution feature forms at the top of the carbonate bench (Oligocene A u -unconformity), inducing the formation of an overlying sinkhole as a surface expression of collapse in fine-grained sediments (Fig. 8; cover collapse sinkholes). Energyrich fluids may seep from the sinkholes and fuel chemosynthetic food chains [START_REF] Chanton | Control of pore-water chemistry at the base of the Florida escarpment by processes within the platform[END_REF][START_REF] Mitchell-Tapping | Mud Hole: A Unique Warm-Water Submarine Spring, Located Offshore Southwestern Florida[END_REF]. Fluid-escape features that have been recognized by [START_REF] Paull | Continental margin brine seeps: Their geological consequences[END_REF] are decametric in scale whereas the mega-sinkholes of this study are 10 to 100 times larger. This is consistent with

Conclusions

This work details and quantifies the distribution, the morphology and the vertical architecture of 29 newly discovered abyssal giant submarine depressions located at the toe of the BBE. Their origins and the mechanisms leading to a karstic landscape development are discussed together with the tectonic setting and the geomorphology of the area. The results and interpretations are summarized below:

-The studied submarine depressions are all giant (>250 m), abyssal (> 3000 m of water depth) and atypically occur in carbonate settings below the CCD (> 4500 m of water depth).

-The water depths (> 4500 m) clearly exclude a subaerial origin for the studied sinkholes.

-Based on their bathymetric, their seismic profile expressions as well as their similarities with submarine Florida sinkholes, we propose that the studied submarine depressions are related to carbonate dissolution and can therefore be named collapse 'sinkholes'. Sinkholes are preferentially located where the transform continental margin inherited structural lineaments controlling the Blake Bahama Escarpment morphology intersect the > 4 km-high and steep submarine carbonate escarpment (BBE). The erosional retreat has preferentially reused the inherited faults, initially formed during the activity of the transform continental margin (such as the Sunniland Fracture zone).

-Sinkholes are all located along the BBE, at less than 5 km from its toe (-4000 m) and immediately above a buried carbonate bench shaped during the BBE erosional retreat.

-The dissolution of the carbonate buried bench, along faults and fractures, implied several stages of overlying collapse structures in the fine-grained deposits (contourites) and probable buried cave systems expressed by the recognized underlying seismic anomalies.

-According to Paull and Neumann's work (1987), the best candidate for explaining such carbonate dissolution at the toe of the BBE would the brines. The dense brines most probably circulate along the structural heterogeneities of the entire Bahamian platform to finally occur at >4.5 km of water depth in this studied area. DBR: Drowned Barrier Reef. DSDP-99: Deep Sea Drilling Project, Site 99 (Hollister et al., 1972a).

Figure caption

J o u r n a l P r e -p r o o f

J

  o u r n a l P r e -p r o o f Mitchell-Tapping et al. (1999)'s observations as well as the model of Gay et al. (2019) showing sporadic decametric seeps within a giant submarine depression that act at different times as a function of available pathways in the sedimentary column and imply fluids lateral migration.

Figure 1 .

 1 Figure 1. (a) Structural Sketch of the Bahamas, Cuba and Hispaniola region. GGB: Great Bahama Bank, TO: Tongue of the Ocean, ES: Exuma Sound. SFZ: Sunniland Fracture Zone; NAP: North America Plate; CI Crooked Islands; Ca I: Caicos Islands. SSAP: San Salvador Abyssal Plain. PRT: Puerto Rico Trench. Dark grey is used for both platform water depth < 100 m, and the emerged areas. (b) Bathymetric data in the Exuma Canyon area where all the giant submarine depressions of this study have been recognized; -3600 m isobath is located within the BBE slope. The main structural lineaments (dashed straight lines) have been mapped according to their bathymetric, magnetic and gravimetric expressions.

Figure 2 .

 2 Figure 2. (a) Details of the submarine depression 1 area where contourites are present. (b) Details of the depressions 2 to 8 area where inner chains and outer chains appear. (c) Details of depressions 9 to 21 area within the BBE curvature where inner and outer chains have been identified. (d) Details of the depressions 22 to 27 area. Slope maps are also provided for each bathymetric map. J o u r n a l P r e -p r o o f

Figure 3 :Figure 4 :Figure 5 :Figure 6 :Figure 7 :

 34567 Figure 3: 2D seismic lines Two-Way Travel Time (TWT) of the study area and associated interpretations: (a) N080 trending line respectively showing from the west (left) to the east (right) the BBE, the depression #1 and the San Salvador Abyssal Plain. The top basement, the top Kimmeridgian, the top Tithonian, the A u unconformity and the post Miocene deposits have been interpreted. (b) N170 trending line respectively showing respectively from the south (left) to the north (right), the Exuma Canyon, the depression #1, the BBE, the depression #2 and the layered northern contouritic deposits.

Figure 8 :

 8 Figure 8: 3D block diagram expressing the general understanding of the Blake Bahama Escarpment including our observations and the previous literature understanding (modified from Paull and Neumann, 1987). The brines are denser than sea water and would flow toward edge of platform. They would be trapped into the buried carbonate bench where the massive dissolution and related collapses both create the overlying observed sinkholes.

Figure 1 Figure 2 Figure 3

 123 Figure 1

Figure 4

 4 Figure4

Figure 5

 5 Figure5

Figure 6

 6 Figure6

Figure 7

 7 Figure 7

Figure 8

 8 Figure8

Table 1 :

 1 Quantification of sinkholes geomorphologic attributes

				DISTAN
	SINKHO LE NUMBE R (From N to S)	SINKHO LE WATER DEPTH (m)	SINKHO LE DEPTH (m)	CE (m) TO THE ISOBAT H "-4000	SINKHO LE LENGTH (m)	LENGTH (°N) ORIENTATI ON	SINKHO LE WIDTH (m)	WIDTH (°N) ORIENTATI ON	SINKHO LE MEAN DIAMET ER (m)
				m"

Graphics Abstract

Acknowledgements

We thank Ifremer and Genavir (France) for cruise organization and technical support. We sincerely thank CharlesK. Paull, Jo De Waele, Francisco Gutiérrezand three anonymous reviewers for their constructive and accurate reviews on this manuscript. We acknowledge IHS for granting University of Bordeaux with academic Kingdom® software licences. We thank Yannick Thomas and Bruno Marsset for assistance in on board HR seismic reflection data processing with SolidQC.

J o u r n a l P r e -p r o o f

 Features aligned to Blake-Bahama escarpment, concentrated at structural intersections.

 Interpreted as collapse sinkholes, linked to brine circulation and dissolution.

J o u r n a l P r e -p r o o f