Monsoonal forcing controlled cold water coral growth off south- eastern Brazil during the past 160 kyrs

Cold-water corals (CWC) constitute important deep-water ecosystems that are increasingly under environmental pressure due to ocean acidification and global warming. The sensitivity of these deep-water ecosystems to environmental change is demonstrated by abundant paleo-records drilled through CWC mounds that reveal a 20 characteristic alteration between rapid formation and dormant or erosive phases. Previous studies have identified several parameters such as food supply, oxygenation, and carbon saturation state of bottom water as central for driving or inhibiting CWC growth, yet there is still a large uncertainty about the relative importance of the different environmental parameters. To advance this debate we have performed a multi-proxy study on a sediment core retrieved from the 25 m high Bowie Mound, located in 866 m water depth on the continental slope off south-eastern Brazil, a 25 structure built up mainly by the CWC Solenosmilia variabilis. Our results indicate a multi-factorial control on CWC growth and mound formation at Bowie Mound during the past ~160 kyrs, which reveals distinct formation pulses during glacial high northern latitude cold events (Heinrich Stadials, HS) largely associated with anomalous continental wet periods. The ensuing enhanced run-off elevated the terrigenous nutrient and organic matter supply to the continental margin, and might have boosted marine productivity. The dispersal of food particles towards the CWC 30 colonies during HS was facilitated by the highly dynamic hydraulic conditions along the continental slope that prevailed throughout glacial periods. These conditions caused the emplacement of a pronounced nepheloid layer above Bowie Mound aiding the concentration and along-slope dispersal of organic matter. Our study thus demonstrates a yet unrecognized impact of continental climate variability on a highly vulnerable deep-marine ecosystem. 35 https://doi.org/10.5194/bg-2020-206 Preprint. Discussion started: 30 June 2020 c © Author(s) 2020. CC BY 4.0 License.


Introduction
Cold-water corals (CWC) are hotspots of biodiversity in the deep-sea (Roberts and Cairns, 2014), important constituents of the deep water carbon cycle (Titschack et al., 2009;White et al., 2012;Cathalot et al., 2015;Titschack et al., 2015Titschack et al., , 2016, and potent bio-engineers due to their sediment baffling capacity that allows for enormous sediment accumulation rates of up to 1500 cm/kyr during maximum CWC mound formation phases (Titschack et al., 2015;40 Wienberg and Titschack, 2017). Yet the fate of CWC reefs and associated ecosystems under global climate change is poorly constraint, because the factors driving or inhibiting their formation as well as potential thresholds in their resilience to environmental change are still under debate (Hebbeln et al., 2019;Raddatz and Rüggeberg, 2019).
The most common framework-forming CWC comprise preferences of S. variabilis, the dominant framework-building CWC discussed in this study, are still pending. Changes the species Lophelia pertusa (recently assigned to the genus Desmophyllum by Addamo et la., 2016), Macropora oculata, Solenosmilia variabilis, Bathelia candida, and 50 Enallopsammia profunda (e.g., Mangini et al., 2010;Frank et al., 2011;Muñoz et al., 2012;Hebbeln et al., 2014;Raddatz et al., 2020). Field and laboratory studies of L. pertusa, the most intensively investigated species, suggest that scleractinian CWC are non-specialists regarding food sources, which may range from POC to DOC (Kiriakoulakis et al., 2005;Duineveld et al., 2007;Gori et al., 2014;, algae, bacteria, and zooplankton (Gori et al., 2014;Mueller et al., 2014;Wienberg and Titschack, 2017). Note, however, that similar studies in the feeding in 55 the properties and spatial configuration of ambient intermediate-or deep-water masses might have a strong impact on CWC as they alter the dissolved oxygen concentration and also the seawater parameters pH, alkalinity and carbonateion concentration. All affect the capacity of CWC to build their aragonitic framework (e.g., Form and Riebesell, 2012;Maier et al., 2012;Lunden et al., 2014;Hennige et al., 2015;Büscher et al., 2017). Spatial fluctuations of intermediateto deep-water masses further influence the depth and strength of pycnoclines, which are thought to play an important 60 role for the concentration and dispersal of nutrients and food utilized by CWC (Frederiksen et al., 1992;Mienis et al., 2007;Rüggeberg et al., 2016). Aside of processes directly affecting the water-mass properties bathing CWC, several studies also point at the importance of surface productivity in providing food that is transported to the deep ocean (Davies et al., 2009;Soetaert et al., 2016). The role of adjacent continents via the input of terrestrial nutrients and POC is still a matter of debate considering the proximity of CWC situated on the continental slope to 65 land.
To systematically test the relative importance of the diverse factors potentially influencing CWC growth and mound formation, we investigated the response of CWC at Bowie Mound, a coral-bearing mound in the Campos Basin on the continental slope offshore SE Brazil Raddatz et al., 2020) to changes in paleoenvironmental conditions. CWC-bearing mounds off Brazil were first reported by Viana et al. (1998) and Sumida et al. (2004) along 70 the continental slope at intermediate water depths between 500 to 1000 m within the Antarctic Intermediate Water (AAIW). At Bowie Mound, the dominating species is S. variabilis, which is adopted to colder (as low as 3-4°C; Fallon https://doi.org/10.5194/bg-2020-206 Preprint. Discussion started: 30 June 2020 c Author(s) 2020. CC BY 4.0 License. et al., 2014;Gammon et al., 2018) and less aragonite saturated waters than mounds formed by L.
The selected location at Bowie Mound is ideally suited to test for a variety of external factors potentially driving CWC 75 growth dynamics as it is situated at the interface of distinctly different water masses (cf. Section 2) and is strongly influenced by terrigenous input from land and the broad shelf off Cabo Frio with intense seasonal upwelling. This setting allows us to test the relative importance of different factors that are potentially crucial for the CWC growth at Bowie Mound, in particular (i) intermediate water-mass variability and its impact on nutrient availability and local hydrodynamics as well as (ii) variations in nutrients and organic matter derived from upwelling and terrestrial input in 80 the context of global climatic changes. Our unique set of multi-proxy data combined with Th/U-dated CWC for the first time demonstrates that an invigorated continental hydroclimate played a so far underestimated role in triggering CWC growth at the SE Brazilian margina scenario that is likely affecting CWC-mounds worldwide.

Hydrological, climatological and geological setting
The (sub)surface circulation in the western Tropical South Atlantic at the Campos Basin off southeast Brazil is 85 dominated by the southward flowing, warm Brazil Current (BC; Fig. 1). The BC forms the western portion of the anticyclonic subtropical gyre (Stramma and England, 1999), which is characterized by high evaporation rates that lead to the formation of the Salinity Maximum Water (SMW, 24ºC, σθ ~ 25.2) occupying the upper 200 m of the water column. The interaction of the BC with the coastal hydrographic system promotes subsurface upwelling of South Atlantic Central Water (SACW) on the shelf edge and on the shelf (Roughan and Middleton, 2002;Aguiar et al., 2014). 90 Upwelling is particularly strong during austral spring and summer when northeasterly winds generating upward Ekman pumping on the mid shelf (Castelao and Barth, 2006;Castelao, 2012), which fuels productivity due to the subsurface encroachment of nutrient-rich SACW. Below the SMW, the SACW is found until 500 m water depth, characterized by decreasing temperatures and salinities (20ºC, 36.0 psu to 5ºC, 34.3 psu; Fig. 1) (Raddatz et al., 2020) owing to its formation in the southwest Atlantic and the South Indian Ocean (Sverdrup et al., 1942;Stramma and England, 1999). 95 The SACW is underlain by the Antarctic Intermediate Water (AAIW; 34.3 PSU, ~4°C) (Fig. 1). North Atlantic Deep Water (NADW) is found below 1100 m water depth with higher oxygen concentrations and salinities compared to AAIW (Mémery et al., 2000). Below ~2500 m the Antarctic Bottom Water (AABW) constitutes the deepest and most dense water mass (Stramma and England, 1999) in this region.
The interaction between the north-or southward-directed flow of the different water masses with the morphology of 100 the slope at Campos Basin causes the formation of strong geostrophic currents (Viana and Faugères, 1998;Viana et al., 1998;Viana, 2001). These are responsible for enhanced sediment focusing leading to the formation of drift bodies, while internal waves at the boundary between different water masses create wide-spread erosional surfaces (Viana et al., 1998(Viana et al., , 2001. Bowie Mound itself has a total elevation of 25 m and situates within a field of mound-like structures, which are at present barren of living CWC colonies  The Campos Basin receives freshwater and sediment input primarily from the Paraíba do Sul River, which delivers between 180 to 4400 m 3 s -1 water (Carvalho et al., 2002) and 30 t yr -1 of sediment load (Jennerjahn et al., 2010). Most 110 precipitation in the hinterland of the Paraíba do Sul River occurs during austral summer, when strong atmospheric convection forms the South Atlantic Convergence Zone, an elongated band of heavy precipitation that reaches from central Amazonia into the tropical South Atlantic (Carvalho et al., 2004;Marengo et al., 2012) (Fig. 1).
115 Fig. 1 Location of core M125-34-2 (yellow star) on Bowie Mound, reference core M125-50-3 (green star) and other CWC records published in Mangini et al. (2010) and Ruckelshausen (2013) Raddatz et al., 2020); the location of the hydrographic section is indicated by a black line in the map.

Material
Gravity core M125-34-2 was retrieved during R/V METEOR cruise M125 from the top of the 25 m high Bowie Mound in 866 m water depth at 21°56.957'S and 39°53.117'W (exact positioning was secured by the Ultra-Short-Baseline system POSIDONIA; Fig. 1, . The core was cut into 1 m segments onboard and stored unopened at -20°C. After CT-scanning, the core was opened in frozen state (for details on the CT-scanning cf. Skornitzke et al., 130 2019;Raddatz et al., 2020). Discrete samples for X-ray Diffractometry (XRD), grain-size analyses, C and N content, and stable carbon and oxygen isotope analyses of foraminifera and organic matter were taken from the sediment matrix https://doi.org/10.5194/bg-2020-206 Preprint. Discussion started: 30 June 2020 c Author(s) 2020. CC BY 4.0 License.
avoiding the sampling of coral fragments. X-ray Fluorescence (XRF) scanning was performed on the archive halve, avoiding coral segments when defining the sampling path of the detector. The core M125-34-2 ( Fig. 2) consists of moderately to strongly bioturbated olive grey to dark grey, silty clayey sand with an alteration of coral-bearing and coral-free zones (Raddatz et al., 2020). The sediments are rich in micro-and macrofossils including pteropods, bivalves as well as benthic and planktonic foraminifers. Six intervals (CI-CVI) with particularly high coral contents of up to 31 vol.% were identified within the core located between 0-13 cm, 80-105 140 cm, 131.5-138 cm, 157-190 cm, 479.5-480.5 cm, and 549.5-568 cm (Raddatz et al., 2020) (Fig. 2). Coral-bearing intervals are characterized by the presence of S. variabilis and to a minor degree M. oculata as the major macrofossils

Quantitative interpretation of CT-scanning data
Prior to opening, the sediment core sections were scanned with a SOMATOM Definition Flash computer tomograph at the Clinic of Diagnostic and Interventional Radiology (DIR) of Heidelberg University Hospital, Heidelberg, Germany, with 140 kVp tube potential and 570 mAs tube currenttime product with a pitch of 0.4 (for details see Skornitzke et al. 2019). The raw data with a resolution of 0.5 mm in z-orientation and 0.3 mm in xy-orientation was 155 reconstructed iteratively (ADMIRE, Siemens Healthineers) using a sharp kernel (I70 h level 3) to an isotropic voxel size of 0.35 mm. Further data processing was carried out with the ZIB edition of the Amira software (Stalling et al. 2005; http://amira.zib.de). Within Amira, the core sections were virtually reunited and the core liner and marginal coring artefacts were removed (2 mm of the core rim). Furthermore, coral clasts were segmented and separated with the ContourTreeSegmentation module (threshold: 1400; persistence value: 1150), quantified and macrofossils >2 cm 160 were visualized as surfaces in 3D following the methodology of Titschack et al. (2015). As an index for bioturbation, we determined the standard deviation of the matrix sediment X-Ray attenuation within each XY-oriented CT slice.
The matrix sediment was segmented by selecting the data volume surrounding the corals, removing areas with values <500 HU (considered to represent air and water) and reducing the remaining segmented volume by three voxels to avoid marginal artefacts in the x-ray attenuation caused for example by the resolution depending averaging effect. 165

X-Ray fluorescence (XRF) scanning
XRF core scanning was performed on the archive halve of the split core. All segments of core M125-34-2 were scanned at the Heidelberg University, using an Avaatech 4th generation XRF Core Scanner. XRF core scanner data were collected every 1 cm down-core. The split core surface was cleaned and covered with a 4 micron thin SPEXCerti Prep 170 Ultralene1 foil to avoid contamination of the XRF measurement unit. Core intervals with very abundant corals had to be skipped to avoid damaging the foil covering the detector through sharp and rigid edges of coral fragments. Data were collected in two separate runs using generator settings of 10 and 30 kV and currents of 0.2 and 1.0 mA respectively. Sampling time was set to 20 seconds per measurement. To counteract artifacts derived from variations in sediment porosity, water content and surface roughness, element counts were normalized by dividing the value of the 175 component (C) by the sum of the counts for each depth (Bahr et al., 2014).

X-Ray diffractometry (XRD)
About 9 g of wet material was dried in an oven at 40°C and subsequently milled in a ball mill (Pulverisette, FRITSCH), with 300 cycles per sec for 3 minutes to obtain a powder with a grain size of 1-3 micrometres. The XRD measurements 180 were done at the Heidelberg University, Department of Geosciences, using a Bruker, D8 ADVANCE Eco diffractometer (40 kV, 25mA) with a Cu Kα diode. The samples were measured in rotating, circular, synthetic sample holders. An angular range of 2θ from 5° to 70° was measured with a step size of 0.02 increments (3338 steps per sample) for 1 sec per step. Peak positions and intensity of data was analyzed with Diffract Suite EVA (Bruker https://doi.org/10.5194/bg-2020-206 Preprint. Discussion started: 30 June 2020 c Author(s) 2020. CC BY 4.0 License. Software). For quantitative phase analysis the Rietveld refinement program DIFFRAC.TOPAS (Bruker Software) was 185 used.

Grain-size analysis (Sortable Silt, ̅̅̅̅ )
The preparation of the samples for the ̅̅̅ analyses followed Bianchi et al. (2001) and Stuut et al. (2002). Wet samples with a weight between 0.3 and 1 g were dried over night at 40 °C. Macroscopically visible coral fragments were 190 removed. After weighting the dry samples, 10 ml of 30 % H2O2 were added to each sample to dissolve the organic material under sub-boiling conditions on a heating plate until the reaction ceased. To remove carbonates, 5 ml HCl (10 %) were added to each sample under sub-boiling conditions for at least 10 minutes until end of the reaction. Samples were washed through standard sieves (63 μm mesh size) to remove the sand and coarser fraction. The fraction >63 µm was dried at 40°C and weighted. The material <63 μm was transferred into 1 L beakers, filled until capacity with 195 demineralized water. After a settling period of at least 8 hours (h), the supernatant water was decanted, and the sample transferred into a 200 mL beaker, topped with demineralized water and settled for 8 h. After decanting supernatant water the sample was transferred into a 50 mL plastic beaker, put into an ultrasonic bath to disintegrate aggregated sediment particles. Afterwards, 35 ml Na-Pyrophosphate were added to prevent particles from forming new aggregates and 2 ml isopropyl alcohol to keep the formation of air bubbles in the liquid to a minimum. Samples were measured 200 with a Laser Particle Sizer (LPS) ANALYSETTE 22 by FRITSCH TM in wet dispersion covering the range 0.08-2000 µm with 99 size classes. For analysis of the raw data the software MaScontrol (FRITSCH TM ) was used applying a Fraunhofer model. Results are derived from a total of five analytical runs à 100 scans per sample. Up to seven replicate samples were performed for each depth to minimize the natural variability of the samples. The high number of replicates resulted from a relatively large inter-sample variability that is likely caused by the high amount of mica 205 flakes present in the sediment distorting the laser beam. We nevertheless consider the results retrieved via the ANALYSETTE 22 as reliable (for an in-depth discussion see Jonkers et al., 2009).

Carbon and nitrogen content
Sediment samples were homogenized with a mortar and pestle and then weighted (0.5 mg for each analyses). The total 210 organic carbon (TOC) and inorganic carbon (TIC, together TC) content was analyzed with a LECO RC-412 (Institute of Geoscience, Goethe University Frankfurt). The reproducibility of the replicate analyses was < ± 0.1 %. The total nitrogen (TN) content was analysed with a LECO TruSpec Macro (Institute of Geoscience, Goethe University Frankfurt).

Stable isotope analysis of foraminiferal calcite
For stable isotope analyses of core M125-34-2, samples were taken at 5-10 cm intervals, wet-sieved (>63 μm) and dried afterwards. For each sample 1-3 tests of Uvigerina spp. or Planulina wuellerstorfi were selected from the size fraction >125 µm, depending on availability. Stable carbon and oxygen isotope ratios were measured on a Thermo 220 Standards) carbonate standard and the in-house "Standard Bremen" (Solnhofen limestone). Isotope values presented in the delta-notation are reported in permil (‰) relative to the VPDB scale. The analytic error is ±0.06‰ for δ 18 O and ±0.03‰ for δ 13 C.

Stable isotope analysis of organic matter 235
Previously homogenized samples were decarbonized with 10 % HCl to remove all inorganic carbon. Afterwards, the samples were centrifuged and washed several times with deionized water in order to remove residual HCl. The samples were then dried in an oven at 50 °C. Subsequent analyses of the carbon isotopic composition of organic carbon (δ 13 Corg) was performed by a Flash Elemental Analyzer 1112, connected to the continuous flow inlet system of a MAT 253 gas source mass spectrometer (Institute of Geosciences, Goethe University Frankfurt). Samples and standards both 240 reproduced within ±0.2‰ and are reported relative to VPDB standard.

Statistical analysis
Correlation coefficients and associated p-values were calculated using the Monte-Carlo-based SurrogateCorr function implemented in the astrochron package in R  with 1000 iterations. This method has been particularly 245 developed to assess the correlation of parameters sampled on different down-core resolutions (e.g. XRF scanning data vs. discrete grain size measurements) .
Discriminant analysis was performed with the program PAST (v3.15) (Hammer et al., 2001). Prior to analysis, the data was detrended and normalized to mean and standard deviation.

Age model refinement
The age model of core M125-34-2 used in this study represents a refined version of the stratigraphy published in Raddatz et al. (2020), which is based on 230 Th/U dates of CWCs. The six coral-bearing intervals, as described in Section 3.1, exhibited a mean accumulation rate of 30 cm kyr -1 with an overall range from 2 cm kyr -1 (CV) to 80 cm kyr -1 (CIII) (Raddatz et al., 2020). 255 To better constrain the age of CWC-barren intervals and evaluate the chronostratigraphic duration of potential hiatuses, we compared the  18 O record of Core M125-34-2 with the benthic isotope record of adjacent core M125-50-3. The clear glacial-interglacial pattern of  18 OUvi in Core M125-50-3 allows to construct a robust age model for this site by https://doi.org/10.5194/bg-2020-206 Preprint. Discussion started: 30 June 2020 c Author(s) 2020. CC BY 4.0 License. tuning its benthic isotope record to the LR04 benthic stack (Lisiecki and Raymo, 2005), indicating that it reaches down to ~135 ka (Fig. 3). Its stratigraphic range is thus only slightly shorter than the one of M125-34-2 (158 ka based on 260 Th/U dates) and therefore suitable as an off-mound reference site. As both cores are situated in similar water depths and are thus bathed by the same water mass (today the AAIW, see Section 2), we expect the respective  18 O values not only to follow a common glacial/interglacial pattern but also to be comparable in their absolute values, allowing to further constrain the chronostratigraphy of M125-34-2.
As neither shallow infaunal Uvigerina spp. and epibenthic P. wuellerstorfi were consistently present throughout core age of the top of Unit 3 is relatively well defined by Th/U dates of 34 ka and older while its base lacks any absolute dating. As values of δ 18 Ouvi of Unit 3 are more depleted than MIS 2 values but less than MIS 5 samples of reference site M125-50-3, we infer that lithological Unit 3 was most likely deposited during MIS 3 and potentially MIS 4 but did not reach into MIS 5. It hence appears that MIS 2 deposits are not present in core M125-34-2, either due to nondeposition or subsequent erosion (note the prominent erosive surface between Units 2 and 3). This age assignment 280 would also imply that the CWC-free portion of Unit 3 from 200 to 465 cm has been deposited within a short period of approximately 8 kyr (62.20 ka as the oldest Th/U dates and ~70 ka as the MIS 4/5 boundary). This would yield a sedimentation rate of 33 cm kyr -1 , which is typical for contouritic sediments as common at the south-eastern Brazilian margin (Viana et al., 1998;Rebesco et al., 2014). Unit 4 has distinctively more depleted δ 18 Ouvi values in the range of MIS 5a-d values in reference core M125-50-3, but less depleted as those of MIS 5e. 285 Th/U dates of ~107 ka of CWC at the top of Unit 4 support a deposition of this Unit in MIS 5d and further indicate a prolonged interval of non-deposition or erosion leading to the absence of MIS 5a-c in core 125-34-2. As δ 18 O values of Unit 5 are again on glacial levels, this unit can be assigned to MIS 6 in line with Th/U dates of 152.6-158.4 ka.
Hence the penultimate interglacial MIS 5e is likely not recovered and falls into the hiatus between Units 4 and 5. In summary, deposition at Bowie Mound core M125-34-2 appears to be focused on glacial intervals of intermediate ice 290 volume, such as MIS 3, contrary to the reduced presence in interglacial deposits.
The erosive horizons present in core M125-34-2 provide evidence for extreme variability in the hydrological regime at the south-eastern Brazilian Margin (Viana and Faugères, 1998;Viana et al., 1998;Viana, 2001), produced by winnowing due to strong current activity and internal waves. While winnowing has the capacity to remove the sediment matrix leading to lag deposits, it is rather unlikely that it would also remove coral fragments. Hence, it is feasible to 295 https://doi.org/10.5194/bg-2020-206 Preprint. Discussion started: 30 June 2020 c Author(s) 2020. CC BY 4.0 License. assume that the dated intervals of CWC presence represent the complete sequence of CWC presence at this part of Bowie Mound.

Intermediate water mass properties and hydraulic dynamics
Intermediate water mass properties might have played a crucial role for the development of Bowie Mound. First, Bowie Mound lies within the nutrient-rich AAIW, which might have boosted CWC growth at the East Brazilian slope. Second, 320 nutrients and POC typically concentrate within nepheloid layers at water mass boundaries and provide a prolific food source for CWCs Dullo et al., 2008;Raddatz et al., 2014;Rüggeberg et al., 2016;Magill et al., 2018). In the case of Bowie Mound it might thus be suspected that enhanced production (Pahnke and Zahn, 2005;Pahnke et al., 2008) and/or nutrient-enrichment (i.e. increasing phosphate and nitrate concentration) of the AAIW (Poggemann et al., 2017) during phases of weak Atlantic Meridional Overturning Circulation (AMOC) could have 325 triggered its episodic formation phases. A more prominent AAIW might have further strengthened internal waves hitting the slope at the AAIW/SACW boundary in the Campos Basin (Viana et al., 1998;Viana, 2001) and thus fueled the nepheloid layer by enhanced resuspension.
Here, we use the benthic  13 C obtained on M125-34-2 as an indicator of the relative contribution of nutrient-rich (hence, 13 C depleted) southern-sourced intermediate water masses (Kroopnick, 1985;Curry and Oppo, 2005). As we 330 had to use different species (shallow infaunal Uvigerina spp. and epibenthic C. wuellerstorfi) we combined both records. To adjust for intra-species offsets, we followed the approach of Kaboth et al. (2017) and normalized the δ 13 C values of each species to the respective mean and standard deviation. The resultant normalized data, although exhibiting a considerable scatter, do not provide compelling evidence for distinctly depleted values during phases of CWC growth compared to CWC-barren intervals (Fig. 4). Albeit the  13 C of Uvigerina spp. might be influenced by 335 isotopic variations of dissolved inorganic carbon of pore-water (Zahn et al., 1986), we nevertheless consider it as appropriate to reconstruct profound changes in the bottom-water signature. Even when only considering the  13 C values obtained on epibenthic C. wuellerstorfi no systematic difference between CWC-bearing and CWC-barren intervals becomes apparent. The continuous, monospecific Uvigerina spp.  13 C record obtained on off-mound core M125-50-3 also lacks negative excursions during times of CWC growth at Bowie Mound (Fig. 5C) formation. Hydrodynamic conditions at core M125-34-2 can be reconstructed by using the variation in ̅̅̅ , a wellestablished proxy for bottom current speed reflected by the mean grain size of the 10-63 µm fraction (McCave et al., 1995), and the sedimentary Zr/Al ratio (Fig. 4). The latter proxy follows the rationale that heavy minerals accumulate relative to aluminosilicates during high bottom current flow speed (e.g., Turnewitsch et al., 2004;Bahr et al., 2014;Miramontes et al., 2019). The significant correlation (r=+0.49; p<0.05) of both proxies may therefore predominantly 375 reflect the hydraulic regime at Bowie Mound, despite certain intervals where both parameters deviate (e.g. between 60-115 cm with high ̅̅̅ and low Zr/Al values), which might be due to small-scale hydraulic effects created by the CWC branches themselves (Mienis et al., 2019). Based on both hydrodynamic proxies, CWC at Bowie Mound tend to accumulate in intervals with elevated flow speed. Phases of low flow speed are accompanied by low CWC abundances (e.g., between 140-150 cm and 465-550 cm), in line with the notion that active bottom currents play a significant role 380 in distributing nutrients and food towards CWC colonies (e.g., Thiem et al., 2006;Dorschel et al., 2007;Davies et al., 2009;Raddatz et al., 2011). High current speeds will also increase sediment and POC supply, thereby providing food and at the same time increase accumulation rates due to the baffling capacity of CWC. Our data, however, suggest that

Terrestrial and marine organic matter supply
The prerequisite for enhanced CWC growth is sufficient food supply to Bowie Mound. To assess the potential impact of enhanced POC and/or DOC supply as an incentive for enhanced CWC growth we compared TOC measurements to CWC abundances (Fig. 6). It appears that CWC abundances are highest in intervals with elevated OM presence, which stresses its importance as a prerequisite for mound aggregation. While statistically significant, the down-core pattern 395 of CWC abundances and TOC (r = 0.55; p < 0.05) also indicates that this correlation is not straight forward; an example is the interval between 15-60 cm with high TOC contents that is barren of corals (Fig. 6). As argued also in the previous section, there have been multiple factors necessary to stimulate coral growth at Bowie Mound. https://doi.org/10.5194/bg-2020-206 Preprint. Discussion started: 30 June 2020 c Author(s) 2020. CC BY 4.0 License. To investigate the origin of the organic matter within the sediment matrix we analyzed its Corg/Ntotal ratio and stable carbon isotopic composition ( 13 Corg). Marine organic matter has lower Corg/Ntotal ratios (~6.6) compared to terrestrial 410 organic matter (>20) (Holtvoeth et al., 2003). Organic matter in core M125-34-2 exhibits Corg/Ntotal ratios of 14 to 52 ( Fig. 6), indicating that land-derived material has been an significant source of organic matter throughout the record. https://doi.org/10.5194/bg-2020-206 Preprint. Discussion started: 30 June 2020 c Author(s) 2020. CC BY 4.0 License.
Intervals with CWC abundances are thereby characterized by elevated Corg/Ntotal ratios, namely at the top and at the base of the core with ratios >40, pointing at an overwhelming contribution of terrigenous matter. Notably, organic matter derived from upwelling on the shelf off Cabo Frio apparently plays a subordinate role as this predominantly 415 marine organic material has low Corg/Ntotal ratios typically below 6.5 (Albuquerque et al., 2014) in line with the upwelling center being confined to the shelf without reaching northward to the slope where Bowie Mound is situated (Albuquerque et al., 2014. A significant terrigenous admixture of terrestrial organic matter is also indicated by the relatively low  13 Corg (-23.2 to -20.2 ‰) as terrigenous organic material has a more depleted signature (-27 ‰) compared to typical marine  13 Corg values (-19 ‰) (Holtvoeth et al., 2003). In this regard, low  13 Corg values during 420 CWC-bearing intervals in the upper 50 cm and below 549.5 cm might be interpreted as enhanced marine productivity, which is in contradiction to parallel Corg/Ntotal ratios as high as 52. This conflicting evidence might be resolved by interpreting the high  13 Corg values as representing enhanced input of POC from C4 plants (typically grasses), which are characterized by an endmember of ca. -12 ‰ (Holtvoeth et al., 2003). Palynological evidence in fact point to the establishment of grassland biomes in the catchment of the Paraiba do Sul during the last glacial caused by generally 425 drier conditions (Behling et al., 2002). This is in accord with the presumed deposition of the intervals 0-50 cm and 549.5-568 cm during the last deglaciation and MIS 6, respectively (Fig. 5).

Influence of the continental hydrological cycle
A major source of terrigenous material and thus a potent organic-matter source for the Brazilian Margin are rivers 430 draining the densely vegetated hinterland, especially the Paraiba do Sul (Fig. 1). To investigate if enhanced riverine input due to more humid conditions in the hinterland contributed to enhanced terrestrial organic-matter supply to Bowie Mound, we studied the mineralogical and geochemical composition of the terrigenous fraction of the sediment.
Changes in the water availability in the hinterland should impact the degree of weathering and thus leave an imprint in the mineralogical composition of the terrigenous sediments. The composition of the non-carbonaceous mineral 435 phases is typical for soils that underwent different degrees of chemical weathering, comprising intermediate weathering products such as hydrobiotite (11.5 %, up to 24 %) (Coleman et al., 1963;Wilson, 1970;Meunier and Velde, 1979), as well as typical constituents of soils that have been deeply weathered under tropical humid conditions such as kaolinite (7.9 % up to 15.3 %), gibbsite (on average 21.7 %) and Ca, K, Al-rich Zeolite (6.7 %) (e.g. Weaver, 1975;Hughes, 1980;Ibrahim and Hall, 1996;Furian et al., 2002) (cf. Appendix Table A). When comparing minerals typical 440 for residual soils such as kaolinite, gibbsite, and zeolite with feldspar and mica, both groups are anti-correlated (Table   XRD) and exhibit distinct fluctuations throughout the core (cf. the albite vs. kaolinite ratio is depicted in Fig. 6).
Relatively high abundances of weathering residuals like kaolinite are particularly present in the CWC-bearing interval between ca. 60 to 200 cm and (to a lesser extend) below 560 cm. This XRD-based data is also partly supported by the high-resolution K/Al record obtained via XRF scanning, which is particularly low during the interval 60-120 cm and 445 below 530 cm (Fig. 6). Low K/Al ratios are in line with high kaolinite contents reflecting periods of strong K-removal from soils due to chemical weathering under humid conditions in the hinterland. As the Corg/Ntotal ratio is also elevated during periods of low K/Al ratios and high kaolinite contents (Fig. 6), it can be inferred that high precipitation in the hinterland fostered chemical weathering in combination with elevating terrigenous organic matter transport to the continental slope. Stronger chemical weathering would have also enhanced the input of Fe to the continental slope 450 https://doi.org/10.5194/bg-2020-206 Preprint. Discussion started: 30 June 2020 c Author(s) 2020. CC BY 4.0 License. (Govin et al., 2012) leading to fertilization of the surface waters, which might have additionally invigorated surface productivity and thus food for CWC at Bowie Mound. To objectively evaluate the relative importance of terrestrial matter supply over changes in the hydraulic conditions potentially influencing CWC proliferation at Bowie Mound we performed a discriminant analysis over n=34 samples using (i) ln(Alb/Kao) as a weathering proxy, (ii) ̅̅̅ for bottom current speed, (iii)  13 Corg, and (iv) Corg/Ntotal for organic matter provenance as predictors for the occurrences of CWC abundances above 7 % (reflecting representative CWC 460 accumulations). Note that the  13 C of benthic foraminifera had not been included due to the high scatter of the data.
The restriction on those four parameters was also done to avoid unreliable results by over-prediction. The outcome of the discriminant analysis yields a correct classification of 76 % with highest skills in ln(Alb/Kao) (loading of -0.68) and Corg/Ntotal (+0.58), while loadings for ̅̅̅ (+0.06) and  13 C org (0.00) are insignificant (see Appendix Table B for details). This is in line with peak CWC abundances synchronous to phases of a strong hydrological cycle as reflected 465 by intensified chemical weathering (i.e. low ln(Alb/Kao) ratios) which triggered a strong input of terrigenous organic matter with a high Corg/Ntotal ratio to Bowie Mound directly or indirectly fueling CWC proliferation.

CWC at Bowie Mound in the paleoclimatological context
As discussed above, phases of enhanced CWC proliferation at Bowie Mound occurred parallel to increased POC and/or 470 DOC input from land due to enhanced run-off. Within age model uncertainties, the most distinct CWC proliferation phases in fact took place during phases of anomalously humid conditions in E Brazil associated with Heinrich Stadials (HS) 1, 4, and 6 ( Fig. 5F, G). The prominent growth phase during HS is well in agreement with published CWC occurrences during the same time frame along the SE Brazilian Margin (Mangini et al., 2010;Ruckelshausen, 2013) (Figs. 1, 5G), corroborating that our results are representative for a larger geographic area. The slow overturning 475 circulation during HS indicated by high 231 Pa/ 230 Th ratios (McManus et al., 2004;Böhm et al., 2015) (Fig. 5 D) caused enhanced precipitation over south-eastern and eastern Brazil (Waelbroeck et al., 2018) as it led to heat accumulation in the southern hemisphere and thereby strengthening of the South Atlantic Convergence Zone (Stríkis et al., 2015) ( Fig 1). Humid phases during HS in otherwise dry eastern Brazil are documented by growth phases of speleothems and travertines (Fig. 5G), increased terrigenous matter input (core M125-95-3; Fig. 5F) (Campos et al., 2019), and a 480 https://doi.org/10.5194/bg-2020-206 Preprint. Discussion started: 30 June 2020 c Author(s) 2020. CC BY 4.0 License. slight expansion of forest cover (Gu et al., 2018). It might thus be argued that the riverine suspension load from rivers in eastern Brazil was advected southward by the BC and added to the enhanced river load from rivers adjacent to Bowie Mound (mostly the Paraiba do Sul). Due to their baffling capacity, the additional sedimentary input would have aided mound formation. This nutrient and organic-rich suspension potentially also enhanced marine surface productivity and thus directly and/or indirectly boosted food supply to the CWC. As freshwater admixture to the SMW 485 and SACW might have caused a more pronounced water column stratification and hence increased density contrast to the AAIW, the concentration of sediment and food particles at the nepheloid layer might have been more pronounced as well.
Notably, HS occurred during phases of intermediate and low sea level, which facilitated the bypass of sediment across the shelf and thus allowed for an efficient supply of terrestrial OM to Bowie Mound (Fig. 5A). As discussed in Raddatz 490 et al. (2020), a low sea level might have also forced water-mass boundaries to migrate downslope during glacials, when sea level was considerably lower by up to 120 m than today (Waelbroeck et al., 2002;Rohling et al., 2014). Such a displacement of water-mass boundaries would have moved the SACW/AAIW interface and the corresponding nepheloid layer from its present position at ~500 m closer to the depth of Bowie Mound (~860 m) aiding the concentration and dispersal of food along the slope towards the CWC colonies. This suspected influence of sea level 495 on the bottom-current dynamics in the depth of Bowie Mound is in fact evident from a sortable silt record obtained on off-mound site M125-50-3, which shows high (low) current speeds during low (high) sea level (Fig. 5B).
Hence, it might be argued that for CWC to flourish at Bowie Mound a dynamic hydraulic regime was required with a pronounced nepheloid layer such as prevailing throughout glacial periods . The development of wide-spread glacial unconformities and extensive drift bodies at the south-east Brazilian margin (Viana et al., 1998;500 Viana, 2001) are evidence for such intense bottom-current activity. This is notably different from the slope south of the Abrolhos Bank where reference core M125-50-3 is situated. Here, contouritic sediments and distinct hiatuses are largely absent in water depths affected by the AAIW and SACW testify for less dynamic hydrological conditions , potentially explaining the lack of CWC mounds at this location. At the same time, ̅̅̅ and Zr/Al data from neither Bowie Mound core M125-34-2 (Fig. 4B) nor off-mound site M125-50-3 (Fig. 5B) indicate that bottom-current 505 speed has been anomalously high during HS relative to the glacial background level. Hence, the distinct, pulse-like CWC growth phases at Bowie Mound could have been ultimately initiated by enhanced nutrient and organic matter supply from land as evidenced by the high Corg/N ratio of the organic matter. Due to ensuing fertilization of the surface waters, marine primary productivity might also have been enhanced and contributed to higher export production, additionally fueling CWC growth. As discussed before, marine productivity derived from upwelling on the shelf of 510 Cabo Frio was apparently of minor importance. Based on planktic foraminiferal assemblages (Lessa et al., 2019), enhanced upwelling occurred between HS 3 and 4, at around 35 kys, when a small peak of CWC occurs (Fig. 5G).
However, at all other instances of CWC occurrences, upwelling at Cabo Frio appears rather low and there is no evidence that it reached as far north as Bowie Mound (Albuquerque et al., 2014. We hence infer that CWC proliferation at Bowie Mound occurred simultaneously with an enhanced delivery of 515 terrestrial organic matter towards the continental slope under glacial boundary conditions of low sea level and enhanced hydrodynamic activity along the slope. As this temporal coincidence could point at the direct utilization of terrestrial https://doi.org/10.5194/bg-2020-206 Preprint. Discussion started: 30 June 2020 c Author(s) 2020. CC BY 4.0 License. organic matter by the corals, it clearly stresses the necessity for future in-depth studies of the food preferences of S. variabilis.

Conclusions
Here we present a comprehensive multi-proxy study of a cold-water coral (CWC)-bearing core retrieved off SE Brazil with the aim to assess the relative importance of different environmental factors that supported and/or prohibited CWC proliferation and coral mound formation. We find that intervals of high CWC abundances are primarily related to millennial-scale high latitude cold events (Heinrich Stadials) that were characterized by major reconfigurations of the 525 deep-ocean circulation and an enhanced continental hydrological cycle in eastern Brazil. The dominance of terrigenous-over marine-derived organic matter during phases of fast CWC proliferation indicate that strong run-off