Distribution of coccoliths in surface sediments across the Drake Passage and calcification of Emiliania huxleyi morphotypes

. The Southern Ocean is experiencing rapid and profound changes in its physical and biogeochemical properties that may inﬂuence the distribution and composition of pelagic plankton communities. Coccolithophores are the most proliﬁc carbonate-producing phytoplankton group playing an important role in Southern Ocean biogeochemical cycles. However, knowledge is scarce about the record of (sub-)fossil coccolith assemblages in the Southern Ocean, which are constituting invaluable indi- 5 cators for paleoenvironmental reconstructions. This study investigates coccolith assemblages preserved in surface sediments of southernmost Chile and across the Drake Passage that were retrieved during R/V Polarstern Expedition PS97. We focused on the coccolith response to steep environmental gradients across the frontal system of the Antarctic Circumpolar Current and to hydrodynamic and post-depositional processes occurring in this region. We used statistical analyses to explore which environmental parameters inﬂuenced the coccolith assemblages by means of Cluster and Redundancy Analyses. We speciﬁcally 10 assessed the morphological diversity of the dominant taxa, i.e. Emiliania huxleyi , emphasizing biogeographical variability of morphotypes, coccolith sizes and calcite carbonate mass estimations. High coccolith The aims of the present study are to investigate if the biogeographical distribution of the coccolith assemblages in surface sediments across the DP reﬂect the steep environmental gradients marked by the frontal systems and to assess if (and how) they are affected by the hydrodynamic and post-depositional processes in this region. Ad- 90 ditionally, we evaluated the coccolith mass variations in the dominant taxa E. huxleyi within each different morphotype. The the approach from and Ziveri Combining the decreasing coccolith sizes and carbonate masses observed in this study with predictions of SO warming and freshening due to sea ice melting, and responses of cultured

analysis of pre-industrial assemblages by using a suite of well-preserved surface sediment allowed us to compare to the available plankton data (Saavedra-Pellitero et al., 2019) and will constitute an invaluable dataset for future potential palaeoproxy calibrations and climate models, covering an existing gap in the literature of the SO.

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The study area covers the Chilean margin south of 52°S together with the western part of the Drake Passage (DP, see Figure 1).
Off the Chilean margin, the relatively narrow but strong Cape Horn Current (CHC) transports low salinity and modified ACC waters into the Atlantic through the northern DP (Chaigneau, 2005;Strub et al., 1998). Two further poleward flowing currents, the surface Peru-Chile Countercurrent transporting equatorial waters, and the subsurface Peru-Chile Undercurrent transporting warm and high-nutrient waters (Karstensen and Ulloa, 2009;Strub et al., 1998), reach approximately the area where the 100 CHC diverges from the ACC at around 40°S (not shown in the map). The DP represents the narrowest strait through which the ACC flows, resulting in a strong concentration of the oceanic fronts Barré et al., 2011), and hence large environmental changes across a relatively small space. In general, sea surface temperature decreases southwards while nutrients (e.g., nitrate, phosphate, silicate) increase polewards across the DP. While fronts amplify vertical mixing, their associated strong jets diminish horizontal mixing (Chapman et al., 2020) and as such, they can act as biological barriers for nonmotile plankton 105 (such as coccolithophores). However, fronts are very dynamic in the DP, as they meander, merge, and split over short timescales (i.e., weeks, see Barré et al., 2011 for a detailed analysis of the DP), enhancing water exchange. The latter is also supported by emerging eddies along the fronts, which are capable of transporting water masses across fronts. Especially the Polar Front Zone (PFZ), between the SAF and PF, and the AZ, south of the PF, are both characterized by anticyclonic and cyclonic eddies Talley et al., 2011) which influence the surface water temperatures and nutrient contents in the DP depending on 110 the season. While anticyclonic eddies normally upwell deep, cold and nutrient-rich waters and enhance primary production in austral winter and spring, this mechanism seems to reverse in austral autumn and summer (Dawson et al., 2018).
The DP is furthermore known for strong bottom currents, so winnowing and trapping of sediment are common in this area (Lamy, 2016;Wu et al., 2019). Modern bottom flow speed in proximity to the frontal jets lies between 10 to 25 cm/s Donohue et al., 2016) and was estimated to be between 12 cm/s and 22 cm/s at the modern SAF location in the 115 Holocene (Toyos et al., 2020). Despite these strong bottom currents, the surface sediments in the DP show a clear trend between the composition of surface sediments and ocean productivity, terrigenous input, intensity of ocean currents, and ice proximity (Wu et al., 2019;Cárdenas et al., 2019).
A general N-S transition from carbonate-rich to opal-rich sediment is observed within the DP surface sediments (Cárdenas et al., 2019). Relatively high carbonate contents of > 45 weight % in the SAZ along the Chilean and Argentinian margins 120 decrease in the subantarctic waters of the AZ south of the PF and become extremely low (mean 2.4 weight %) in the surface sediments of the SZ and CZ south of the sACCF (Raymond, 2014;Cárdenas et al., 2019;Wu et al., 2019). Diatom concentrations at and south of the PF are generally one order of magnitude higher than north of it. Diatom distribution clearly reflects the N-S environmental gradients of sea surface temperature and sea ice extent, and the assemblage distribution characterize the different frontal zones (Raymond, 2014;Cárdenas et al., 2019). Furthermore, terrigenous sediments in the DP region mainly 125 originate from proximal terrestrial sources such as Patagonia and the Antarctic Peninsula, as shown by a comparable set of surface sediment samples (Wu et al., 2019). Estimated holocene sedimentation rates in the DP area are in the order of 3.5 cm/kyr (Ho et al., 2012) or 3.79 cm/kyr (Caniupán et al., 2011a).

Material and Methods
In total, 28 surface sediment samples from the southern Chilean and Argentinian margin and the DP were prepared and analysed 130 for this coccolithophore study ( Figure 1). All samples were retrieved with a multicorer sampling device from February to April 5 https://doi.org/10.5194/bg-2021-105 Preprint. Discussion started: 23 April 2021 c Author(s) 2021. CC BY 4.0 License. 2016 during expedition PS97 (Lamy, 2016). Datings of near-surface sediments at the southern Chilenean margin (Caniupán et al., 2011b) as well as south of the PF within the DP  give calibrated accelerator mass spectrometry (AMS) 14 C ages of 2.91 -3.06 ka BP and 4.83 ka BP respectively. We therefore assume that our studied surface sediments represent relatively modern conditions, with ages ranging most likely from mid to late Holocene.

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The uppermost centimetre of the multicores were sampled and prepared with a combined dilution/filtering technique following Andruleit (1996). Between 66 and 153 mg of dry bulk sediment per sample were suspended in demineralized water buffered with ammonia and ultrasonicated for up to 30 s. The suspensions were split to one-hundredth with a rotary sample divider, filtered through polycarbonate membrane filters with a pore size of 0.45 µm, and dried in an oven at 40°C for 24 h.
Out of the dried filters, a piece of approximately 1 cm 2 was cut out, mounted on an aluminium scanning electron microscope 140 (SEM) stub, fixed with carbon conductive tabs and sputter-coated with gold-palladium. The filters were analysed with a Zeiss DSM 940A SEM at a magnification of 3000x for coccolith species abundance counts, and of minimum 5000x for E. huxleyi morphotype identification and abundances. A minimum of 300 coccoliths per sample was counted in transects across the filter area, except for eight relatively coccolith-poor samples south of the PF and two in the SAZ in which at least 100 coccoliths were aimed to be counted. All the sampling points were considered when plotting the number of coccoliths per gram of sedi-145 ment, except for three sample with extremely low counts that were excluded in the plots of relative abundances. The number of coccoliths per gram of sediment (Coc/g sed.) were calculated using the formula from Andruleit (1996): in which, F c = effective filtration area (mm 2 ); Cc = number of counted coccoliths; Sp = split factor; A = investigated filter area, and W = weight of bulk dry sediment.

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Furthermore, a preservation index (Calcidiscus leptoporus -Emiliania huxleyi Dissolution Index, CEX) adopted from Dittert et al. (1999) was calculated in order to check whether the coccolith assemblages were influenced by carbonate dissolution. The CEX is based on the differential dissolution behaviour of the delicate E. huxleyi versus the more robust C. leptoporus and has proven to be comparable to dissolution indices based on foraminiferal tests.
The assemblage diversity was assessed using the Shannon Index: with p i = proportion of species i, and S = number of species. In morphogroup A, we identified morphotypes A, A overcalcified and R; in morphogroup B we identified morphotypes B/C and O based on the central area feature. Note the large size variation within morphotype B/C. See Table 1 for a classification summary of the different morphotypes.

Species and morphotype taxonomy
Coccoliths were classified at species level following Young et al. (2003) and the electronic guide to the biodiversity and 160 taxonomy of coccolithophores Nannotax3 by (Young et al., 2020). Specific taxonomical considerations regarding E. huxleyi specimens were taken into account, and morphotypes were differentiated as far as it was possible on single coccoliths directly during the counts.
In an additional count, we differentiated between five E. huxleyi morphotypes within morphogroups A and B. Type B/C as a mixed classification for coccoliths resembling characteristics of these 3 types, with a size ranging across the typical threshold at 3.5 µm (Young et al., 2003;Cubillos et al., 2007). In our studied samples, size was the only coherent characteristic that differed between specimens within morphogroup B (excluding morphotype O) with a normal distribution 170 maximum between approximately 3 and 4 µm and showing no indication for distinct morphotype distributions (See Table 1 and Figure 2). Hence, we classified all E. huxleyi coccoliths from morphogroup B into either Type B/C or Type O, depending on the central area (Hagino et al., 2011).  (Young, 2015) in Fiji software (Schindelin et al., 2012). Measurements were done in µm based on the scalebar of the SEM images. They were scaled to 100 % with a Coccobiom2 SEM calibration of 1.09 and the chosen magnification.
From the morphometrical measurements on E. huxleyi the coccolith masses were estimated based on two different formulas, 180 that of Beuvier et al. (2019) and that of Young and Ziveri (2000). Beuvier et al. (2019) studied coccospheres and coccoliths of 7 different Noëlaerhabdaceae species including three strains of E. huxleyi by means of X-ray nanotomography. They found that coccolith mass correlates with grid perimeter and with crystal number. They developed the following empirical formula (Equation 4) to calculate the coccolith mass m in pg from the number of segments n (with two constants k n = 4.73 * 10 −5 (± 0.28 * 10 −5 ) and β = 3.175 ± 0.251). This is based on a calcite density of 2.71 pg µm 3 . However, this formula does not account 185 for calcification of the central area itself, as found in the E. huxleyi morphotype A overcalcified observed in this study.
We additionally used the well established approach to estimate coccolith masses by Young and Ziveri (2000). Their approach (Equation 5) is based on the length l of a coccolith together with a species-specific and morphotype-specific shape factor k s , and the calcite density C (for coherency with the formula of Beuvier et al. (2019) we used here 2.71 pg µm 3 ).
The different shape factors used were based on the identified morphotype following Young and Ziveri (2000): k s = 0.02 for morphotypes A and B/C and k s = 0.04 for morphotype A overcalcified. The shape factor for morphotype O (k s = 0.15) was introduced by Poulton et al. (2011) in a plankton study along the Patagonian shelf for a morphotype with a central area described as "open or thin plate" which the authors called Type B/C but that we identified as morphotype O (see Hagino et al., 2011). The approach of performing morphometric measurements on the coccoliths followed by the estimation of their coccolith mass assuming a systematic relation between length and thickness was chosen because all data were determined on the same material using SEM. Rigual Hernández et al. (2020) compared coccolith carbonate estimates from a birefringencebased approach with the morphometrics-based approache from Young and Ziveri (2000), and showed, on average, slightly higher but largely comparable carbonate contents for E. huxleyi coccoliths using the latter approach.

Environmental parameters
We used biogeochemical parameters to test how much of the species assemblage composition could be explained by environmental factors (see also subsection 3.4). Those are annual salinity, temperature and phosphate at 10 m water depth which we extracted from the 1°GLODAPv2 mapped climatology (Lauvset et al., 2016;Key et al., 2015) to stay consistent with the calculated carbon system parameters based on the same data product. Carbon system parameters for the location of each sample 205 at its respective water depth were calculated using CO 2 SYS macro for PC (Pierrot et al., 2012) based on salinity, temperature, silicate, phosphate, alkalinity and total CO 2 . The data from GLODAPv2 and the derived carbonate system data have a seasonal bias towards southern hemisphere winter (December to March) because samples in this database were mostly taken during the austral summer (Lauvset et al., 2016). Coccolithophores in the southeast Pacific sector of the SO bloom mostly during austral spring and summer months (Balch et al., 2016;Nissen et al., 2018), and thus the coccolithophore assemblages in surface 210 sediments are biased towards the same season as the environmental dataset.
Austral winter Photosynthetic Available Radiation (PAR) at 10 m water depth was estimated using a model of light penetration (Buiteveld, 1995;Murtugudde et al., 2002), the diffuse atternuation coefficient for downwelling irradiance at 490 nm and Equation 1 in Lin et al. (2016). Mixed layer depth (MLD) was extracted from monthly 1°ARGO MLD climatology (Holte et al. (2017), based on a density algorithm). Data from austral spring and summer months (September to March) were averaged, 215 and extracted from the respective sample positions.

Statistical analyses
Prior to any statistical analysis, we excluded three samples (PS97/077-1, PS97/079-1 and PS97/071-2) because of the very low number of coccoliths counted (< 40 per sample). A detrended correspondence analysis (DCA) was applied on the species relative abundance dataset resulting in a first axis length of 1 SD suggesting a short gradient for which linear ordination methods 220 are suitable. The relative abundance data was standardized using Hellinger transformation (giving low weights to rare species, Legendre and Gallagher, 2001), using R package adespatial 0.3-8 (Dray et al., 2020).
Average-linkage (UPGMA) hierarchical clustering was performed on the assemblage data with R function hclust (R Core Team, 2020). The best number of clusters was suggested by the majority of 30 indices calculated with R package NbClust version 3.0 (Charrad et al., 2014). The significance of each cluster was assessed by multiscale bootstrap resampling with 225 10 000 replications using R package pvclust version 2.2-0 (Suzuki et al., 2019).
To perform transformation-based RDA (tb-RDA), we constrained our assemblage data (response, 16 species at 25 sites) to seven standardized environmental variables (explanatory): salinity, temperature, phosphate and PAR at 10 m water depth; CO 3 as representative variable for the carbonate system at surface sediment sample depth; the MLD and the respective surface sediment sample depth itself. The adjusted R 2 was calculated and the significance of the tb-RDA was tested at 9999 permuta-230 tions. Analysis was performed using R package vegan version 2.5-6 (decostand, rda, RsquareAdj, anova.cca, Oksanen et al., 2019). We also calculated the analogue distance of a subset of the sediment surface samples to the nearest plankton samples

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Coccoliths were found in all the oceanographic zones bounded by the fronts, with generally high absolute numbers in the SAZ and comparatively low numbers in the AZ and SZ+CZ. The abundance ranges from 9 * 10 6 Coc/g sed. in the SZ+CZ (at PS97/077-1) to 4159 *10 6 Coc/g sed. in the SAZ (at PS97/020-1). We unexpectedly found a quite diverse coccolith assemblage with a total of 23 identified species, ranging from 6 to 15 different taxa per station.
Highest numbers of coccoliths were recorded in the deepest samples studied along the Chilean margin (1.8 to 2.5 km water 240 depth) with 2669*10 6 to 4159 *10 6 Coc/g sed. belonging to different taxa, ranging from 7 to 14 different species depending on the station (Figure 3). The shallower surface sediment samples analysed in this study, located in the northernmost area offshore Chile (0.5 to 1.3 km water depth) bore 10 to 15 different species but contained relatively low coccolith numbers of ca. 591 to 1023 * 10 6 Coc/g sed. The samples along the Argentinian margin, in an open ocean setting (from 1.6 to 4 km water depth), yielded only 199 to 472*10 6 Coc/g sed. belonging to 6 to 10 different species. The samples south of the PF, located in the AZ, 245 were retrieved from water depths of at least 1.2 km but mostly of 3 to 4 km. We found similar numbers of coccolith species than at lower latitudes (from 6 to 13 taxa), but the coccolith contents considerably decreased (22 * 10 6 to 645 * 10 6 Coc/g sed.), especially in the deeper samples, below 3.1 km. Even some of the southernmost samples in the SZ+CZ along the Antarctic margin (PS97/074-1 and PS97/052-3) yielded a content of 57 and 141 *10 6 Coc/g sed. from 12 and 9 species, respectively.

Species composition and distribution
All surface sediment coccolith assemblages in the study area consist of E. huxleyi, Gephyrocapsa muellerae and Calcidiscus leptoporus (intermediate morphotype; hereafter only referred to as C. leptoporus according to Baumann et al., 2016) and in 255 all, except in one sample, we identified Gephyrocapsa ericsonii as a main contributor. Those species make up on average Table 2. Sample main metadata (Lamy, 2016), frontal zone, total counted coccoliths, coccolith abundance and species richness. The lowermost three samples yielded very low total coccolith counts and were excluded from statistical analyses.
Station Latitude in°S Longitude in°W frontal zone water depth in m total counted abundance *10 6 species richness 94 % of the coccolith assemblages in the SAZ, 93 % in the AZ and 87 % in the SZ+CZ. Additionally, C. leptoporus small, Florisphaera profunda, Gephyrocapsa oceanica, Helicosphaera carteri and Syracosphaera spp. are present in low relative numbers in several of the DP and the Chilean margin studied stations (see Figure 4).

Cluster and Redundancy Analysis
We performed Hierarchical Cluster Analysis on the relative species assemblages resulting in two groups (see section 5). Group A consists of samples that stem from water depths above 3.1 km from the SAZ and AZ. Group B consists of samples that are either from the SZ+CZ, or from the AZ below or just above 3.1 km, and one sample from the SAZ far below 3.1 km.
Additional to sample depth, we assessed possible further drivers of the assemblage distributions using RDA. Constraining

Emiliania huxleyi morphotypes
Emiliania huxleyi coccoliths belonging to morphotypes A, A overcalcified and R are mainly found in the SAZ ( Figure 5).  2000) with an overall mean of 2.12 pg (Equation 4) and 2.29 pg (Equation 5), respectively (see Table 3 and Figure A2). The

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The surface sediments in our study area are characterized by a striking difference in the total coccoliths abundance, with higher Coc/g sed. along the Chilean margin than south of the PF (Figure 7). In total, 22 species were identified in the surface sediments samples analysed. This is rather surprising because so far only between 8 and a maximum of 15 species have been found in samples located along latitudinal transects crossing the same frontal systems in other sectors of the SO (e.g., Findlay and  2019)). Fewer species have also been found in comparable sediment settings in the northern North Atlantic (e.g., Baumann et al., 2000).
The high coccolith abundances along the Chilean margin within the SAZ clearly suggests relatively high productivity conditions in the surface water. The species composition, dominated by E. huxleyi, C. leptoporus, and Gephyrocapsa species, agrees well with those of Chilean communities in overlying plankton samples (e.g., Saavedra-Pellitero et al., 2019). Thus, the 325 general good resemblance between (sub-) fossil surface sediment sample and living communities indicate that the regional oceanography plays an important role shaping recent assemblages.
Coccolith abundances in surface sediments south of the PF are lower than at the Chilean margin, but they are still unusually high for this southern latitude (up to 650 x10 6 Coc/g sed.). As it has often been observed, coccoliths are replaced by diatom valves, which become more abundant southwards (e.g., Cárdenas et al., 2019, see Figure 7). However, a decrease in coccolith 330 diversity is not observed at and south of the PF (see Figure 3). Emiliania huxleyi remains the dominant species together with robust taxa, such as G. muellerae and C. leptoporus. In addition, other taxa are selectively enriched here and even species that are not observed in the overlying plankton samples offshore southern Chile and across the DP are recorded in the surface sediment samples. This potentially suggests that other factors than surface-ocean productivity might have affected the species composition in these samples.

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Because of the scarcity of coccolith studies in surface sediments across similar latitudinal transects in the SO, the determination of the ecological drivers of the coccolithophore assemblages in this region, and potential implications for paleorecord interpretations have not been extensively explored. In the following sections we will interpret and discuss the potential in-situ and post-depositional factors that may govern the coccolith abundance and species composition in southern high latitudes, with special focus on E. huxleyi and its different morphotypes. The observed high numbers of coccoliths in the surface sediments offshore southern Chile are in good agreement to previously reported higher coccolith accumulation rates off central Chile (Saavedra-Pellitero et al., 2013), an area that is however, influenced by an active coastal upwelling system. The generally high numbers of coccoliths along the Chilean margin within 345 the SAZ also suggest relatively high productivity conditions in the surface waters. For the present study area, elevated nutrient supply via freshwater runoff by precipitation and seasonal glaciers' melting (e.g., Dávila et al., 2002;Saldías et al., 2019), and relatively warm surface water temperatures in the CHC in comparison to the overall study area are plausible causes for an elevated coccolithophore production and the related increase in coccolith sedimentation off southern Chile. Slight differences in coccolith abundances between deeper mid slope sediments (between 1.8 and 2.5 km depth, > 2500*10 6 Coc/g sed.) and 350 shallower sediments from the upper slope (0.6 -1.2 km depth, < max. 1000*10 6 Coc/g sed.) in the SAZ are probably due to dilution of the shallow samples by high sedimentation rates via freshwater runoff near the coast. However, steadily increased coccolithophore abundances and diversity from coastal to oceanic regions as described for areas further to the north (Menschel et al., 2016;von Dassow et al., 2018) may also account for variations in coccolith abundances in the studied slope sediments off southern Chile. , and are more diverse than those found in the AZ sediment traps (Rembauville et al., 2016;Rigual Hernández et al., 2018). Thus, the general good agreement between (sub-) fossil sediment sample and living assemblages indicate that the regional oceanography plays an important role shaping the structure of the coccolithophore community offshore Chile and broadly north of the PF.

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South of the PF coccolith abundances in surface sediments are strikingly lower than at the Chilean margin, but coccoliths are still relatively common in this area (up to 650 x10 6 Coc/g sed.). It is also noticeable that a decrease in coccolith diversity is not observed in the present surface sediment samples ( Figure 3). Thus, the situation south of the PF contrasts with the rapid decline in diversity and coccolithophore abundance observed in plankton samples in the DP and in other sectors of the SO (e.g., Gravalosa et al., 2008;Saavedra-Pellitero et al., 2014;Malinverno et al., 2015;Charalampopoulou et al., 2016;Saavedra-410 Pellitero et al., 2019). At the same time, no analogues were found between the species assemblages of a subset of surface samples and the nearest plankton samples from Saavedra-Pellitero et al. (2019) (see Figure A3). This discrepancy could be explained by the episodic nature of coccolithophore bloom events south of the PF (Balch et al., 2014;Winter et al., 2014), which might not have coincided with the timing of the plankton sampling in the DP, while the sediments record in surface samples averages hundreds or even thousands of years.

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Emiliania huxleyi remains the dominant species in the DP, and is -as off southern Chile -accompanied by G. muellerae and C. leptoporus, but also by relatively dissolution resistant taxa such as G. oceanica and F. profunda. While the latter species are selectively enriched here, some other species are not even observed in the overlying plankton samples offshore southern Chile and across the DP. These subordinate taxa found in DP sediments are more typical of lower latitudes where they usually live in warmer surface waters. For example, one of the taxon found in unusual abundances at these high-latitudes is F. profunda, 420 which is a typical subtropical-temperate species that dwells in the lower photic zone and is rarely present in high abundance at latitudes outside 30°N and S (Hernández-Almeida et al., 2019). Some of these low-latitude species were found in surface water samples retrieved via the ship's water pump system by Winter et al. (1999) during a cruise in austral autumn 1992 in the Weddell Sea, east of the DP. Species belonging to the genus Syracosphaera were mostly found offshore Chile and in the SAZ, in agreement with previous plankton studies in the DP (Charalampopoulou et al., 2016;Saavedra-Pellitero et al., 2019), but 425 also in the AZ. Although different authors also found taxa of this genus in the SO (e.g., Findlay and Giraudeau, 2000;Saavedra-Pellitero et al., 2014), it was never observed so close to Antarctica, except by Winter et al. (1999). However, Syracosphaera species have been described from a very similar setting off south-east Greenland (Balestra et al., 2004). There, a quite diverse upper photic zone assemblage dominated by E. huxleyi and Syracosphaera spp. once occurred, despite the harsh environmental conditions with sea surface water covered by ice most of the year. Therefore, the higher diversity in sediment samples may be 430 explained by low-latitude coccolithophores species occasionally thriving south of the PF.
However, although the settling of biogenic material is directly related to surface production and reflects the seasonality of that production (Deuser et al., 1990), the sinking of the particles can strongly be influenced by their drifting due to strong surface and deep currents in the DP. It should be noted that at least the largest part of the coccolith material sinks to the sea floor, incorporated into faecal pellets or in macro-aggregates (marine snow, e.g., Steinmetz, 1994;Honjo, 1976). Factors 435 such as dilution and resuspension processes or drifting of the coccolith material due to strong surface and deep currents may have further influenced the surface sediment assemblage, as it has been observed for other microplankton groups which are deposited in the sea-floor (e.g., van Sebille et al., 2015;Nooteboom et al., 2019Nooteboom et al., , 2020. Nooteboom et al. (2019) showed that bias in microfossil assemblages in surface sediments occur in most oceanic regions and are dependent on current strength and direction, sinking speed and sample depth. The strong ACC flow and frequent eddie formation in the area of the DP (Barré 440 et al., 2011) are likely to influence the sinking pathways of coccoliths. Thus, temperate taxa observed might not be in situ but could have been transported by currents, mostly the CHC, Peru-Chile Countercurrent and the Peru-Chile Undercurrent, which flow southward along the Chilean Margin and which carry relatively warm water masses towards and into the SO and the DP through eddy circulation. These taxa may also originate around Patagonia, the southernmost region of South America, or the Patagonian shelf in the southwesternmost South Atlantic. Patagonia is at least one of the most important sources for terrigenous 445 fine-grained sediments, which is predominantly transported by bottom currents into the deep DP (Wu et al., 2019). The same transport mechanism can also be assumed for similar-sized coccoliths from the southern Patagonian shelf, from where species such as G. muellerae, C. leptoporus and Gephyrocapsa small (G. ericsonii) were described in surface sediments (Rivas et al., 2019). Thus, transportation via surface and deep ocean currents is another factor possibly influencing the surface sediment assemblages south of the PF.

Alteration of the coccolith assemblages in the DP
One of the potential factors that could influence coccolith assemblages in surface sediment samples is the depth at which coccoliths are settled. In order to explore the geographical effect of this variable on our dataset, we performed a hierarchical cluster analysis considering the coccolith relative abundances (Figure 9). The cluster analysis identifies two groups of samples, which belong to different oceanic regions and sample depths. Samples in cluster A are mostly found in the SAZ zone, with 455 sample depths above 3.1 km, while samples in cluster B are mostly located south in the AZ, at depths below 3.1 km. Hence, strongest assemblage dissimilarity between clusters is concomitant with sample depth shallower and deeper as 3.1 km. This pattern is exactly matching the distinction of samples by frontal zone and depth above or below the calcite saturation horizon (CSH, where the water becomes undersaturated with respect to calcite, i.e., Ωcalcite = 1, Zeebe, 2012) shown in Figure 8, with the exception of samples PS97/052-3 and 074-1. These samples fall out of line as they are located in the AZ above CSH 460 (2.8 km), but appear grouped with samples in Cluster B (located below the CSH). Thus, the clustering of the samples located along the DP transect indicates that not only oceanographic variables, but also sample depth, which controls the calcium carbonate preservation, are influencing the composition of the coccolith assemblage, particularly south of the PF.
Based on the clustering findings and considering that other potential factors than only surface-ocean productivity seem to be affecting the coccolith assemblages, we decided to further explore and test the potential influence of the sediment sample 465 depth compared to other environmental variables driving the coccolithophore species composition, with a Redundancy Analysis (RDA). The scores of the surface sediment samples located along RDA1 (which explains almost 68 % of the variance) are separated in two groups which correspond to the two clusters, A and B, identified with the hierarchical analyses. The assemblages located in a more open ocean setting in the SAZ are comparable to those south of the PF (Figure 11). In both regions (southern SAZ and AZ) we found consistent differences in the assemblages in samples above and below 3.1 km (Fig-470 ure 11). At depths deeper than 3.1 km, assemblages are characterized by having lower abundances of the relatively fragile E. The samples were clustered into two groups A and B as suggested by the majority of 30 indices (see section 3). Cluster A consists of all but one sample from the SAZ and some samples from the AZ that stem from water depths above 3.1 km. Cluster B consists of all samples from the AZ that lie below 3.1 km water depth (except sample PS97/046-6, which stems from a water depth of 2.8 km), the two samples from the SZ+CZ and one deep sample from the SAZ (PS97/094-1). Note the frontal zone and the water depth of the respective samples on the right hand side of the dendrogram.
huxleyi and higher abundances of more robust species such as G. muellerae, G. oceanica and C. leptoporus. This feature is even more striking at the deepest sample, PS97/094-1, located at 4 km water depth in the SAZ, in which the proportion of E.

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Wigwamma antarctica, etc., that were found in the plankton Saavedra-Pellitero et al., 2019) can be attributed to selective dissolution through mechanical destruction when grazed by zooplankton or to dissolution when sinking through the water column (Young et al., 2005).
Based on these observations as well as on the clustering and RDA results, we suggest that carbonate preservation is influencing the species composition at greater depth north and south of the PF. This is supported by the Calcidiscus leptoporus 480 -Emiliania huxleyi Dissolution Index (CEX), an indicator for dissolution processes in coccolith assemblages (Dittert et al., Figure 10. Redundancy analysis based on Hellinger distances (tb-RDA) with an explained variation in species composition by the input environmental variables (adjusted R 2 ) of 38.3 %. Biplot of the first two RDA axes explaining 84.3 % of the variance between the environmental parameters (black arrows), the species (blue dots) and the samples (coloured dots). Gray dashed hulls outline the clusters (A and B) from the hierarchical cluster analysis, coloured hulls outline the samples from different frontal zones and above or below a depth of 3.1 km.
Considered explaining variables are sample depth (depth), mixed layer depth (MLD), temperature (temp), salinity (sal), phosphate (phosp) and photosynthetic available radiation (mean from december to february, PAR) at 10 m water depth and CO 2− 3 (CO3) at sample depth. 1999). Typically, CEX values below 0.6, indicate selective dissolution of E. huxleyi compared to the more robust C. leptoporus (Dittert et al., 1999). However, in our case, CEX < 0.75 (below around 3.1 km, see Figure 11) would already suggest preservational issues, coincident with the CSH threshold, which occurs in the DP at a water depth of around 3 to 3.25 km (Bostock et al., 2013). Below this level, we argue for a selective species-specific dissolution of the delicate E. huxleyi in favour of G.

485
muellerae dependent on the sample depth.

Emiliania huxleyi morphotypes and their calcite mass contributions
In the surface sediment samples from the DP, the distribution of E. huxleyi reaches unexpectedly high latitudes south of the PF. Except for morphotypes A overcalcified and R, the rest of E. huxleyi morphotypes are present in all bioregional zones.
Type A overcalcified is absent in most samples from the SZ+CZ, and type R is absent from AZ and SZ+CZ. While E. huxleyi 490 morphotypes belonging to morphogroup A are much more abundant in the SAZ, morphotypes within morphogroup B dominate in each of the biogeographic zones. This pattern of dominance is relatively similar to studies of extant communities from the SO, in which E. huxleyi morphotype A is typically restricted to relatively warm waters, north of the PF (usually north of the SAF) and morphotype B/C dominates in cooler waters, in some cases even south of the PF (Cubillos et al., 2007;Poulton et al., 2011;Malinverno et al., 2015;Patil et al., 2014;Saavedra-Pellitero and Baumann, 2015;Saavedra-Pellitero et al., 2019) or it 495 is even the only morphotype present (Charalampopoulou et al., 2016;Findlay and Giraudeau, 2000).
In a plankton study along the Patagonian shelf, Poulton et al. (2011) suggested that the morphotypes A and B/C are different ecotypes because of their respective dominance in warmer nutrient-poor and higher calcite saturation state water versus cooler, nutrient rich water with a lower calcite saturation state, respectively. Therefore, shifts in E. huxleyi morphotype assemblage composition seem to correlate to some extent with changes in carbonate system parameters. On the other hand, Beaufort et al.  Charalampopoulou et al., 2016). This morphotype is, however, also often ignored even in more recent studies or integrated into the B/C morphogroup (e.g., Saavedra-Pellitero et al., 2014;Saavedra-Pellitero and Bau-515 mann, 2015;Rigual-Hernández et al., 2020). Up to now, however, it has only been occasionally differentiated in plankton studies and not at all in coccolith assemblages in sediments (Malinverno et al., 2015(Malinverno et al., , 2016Saavedra-Pellitero et al., 2019).
Such a differentiation is possible, although the presence of an open central area might also be an artefact of dissolution, especially in sedimented coccoliths. It is therefore possible, that coccoliths classified as morphotype O would have been originally morphotype B/C. However, such a differentiation is of importance, since different E. huxleyi morphotypes exhibited different 520 sensitivities in regards to seawater carbonate chemistry in cultures (Müller et al., 2015). Therefore, documenting the diversity of E. huxleyi morphotypes, in the SO in general and in the DP in particular, and showing their biogeographical distribution in relation to changing environmental conditions is of critical importance to assess their response to projected environmental change in the SO.
The coccolith carbonate mass estimations obtained are in close agreement, with an overall mean mass of 2.12 pg using 525 Equation 4 and of 2.29 pg using Equation 5 (see Table 3). Highest weights were estimated for coccoliths in the SAZ ( (mostly under 2 pg) using the approach from Young and Ziveri (2000). Combining the decreasing coccolith sizes and carbonate masses observed in this study with predictions of SO warming and freshening due to sea ice melting, and responses of cultured strains to those changes (e.g., Saruwatari et al., 2016), it seems likely that produced coccoliths south of the PF will get smaller and lighter in the future.

540
Our study suggests that well preserved (sub-) fossil coccolith assemblages in the DP mirror the overlying extant coccolithophore communities which respond to environmental (i.e., physical, chemical and biological) variables, specifically north of the PF. Therefore these assemblages constitute a robust and valuable dataset for qualitative and quantitative calibrations and subsequent reconstructions of surface ocean conditions (i.e. transfer functions and oceanographic/climate models).
The phytoplankton dynamics in the SO is complex, and coccolithophores and diatoms (among other planktonic groups) 545 coexist in this region (e.g., Smith et al., 2017). The coccolithophore diversity and abundance in surface sediment samples located located south of the PF is high compared to plankton studies in the SO (e.g., Mohan et al., 2008;Balch et al., 2011;Malinverno et al., 2016), DP (Charalampopoulou et al., 2016;Saavedra-Pellitero et al., 2019), and surface sediment samples in the Pacific sector of the SO (Saavedra-Pellitero and Baumann, 2015). This demonstrates that calcareous phytoplankton can successfully thrive and be incorporated to the geological record at this latitude and that conditions for their preservation 550 south of the PF are rather variable depending on the region. Although the similarity between living (plankton) coccolithophore communities and fossil surface sediment assemblages is high at the Chilean margin, there are some differences south of the PF. In particular, we observe a southwards enrichment in dissolution resistant species, such as C. leptoporus, at the expense of a decrease of more fragile species, such as E. huxleyi. Combination of statistical techniques (hierarchical clustering and RDA) and dissolution-sensitive indices (CEX) indicate that this shift in the composition is due to the preferential dissolution 555 of calcium carbonate occurring mainly below 3.1 km, which coincides with the depth of the CSH in this region (Bostock et al., 2013). In addition, a proportion of the fossil coccoliths (i.e. temperate taxa) found in sediments in the southernmost samples are most likely non in-situ. Coccolithophores and detached coccoliths are subject to transport from lower latitudes over long distances while settling in the water column, as it has been shown by other microplankton groups, such as planktonic foraminifera or dinoflagellates (e.g., van Sebille et al., 2015;Nooteboom et al., 2019). Therefore, the hydrodynamic and post-560 depostional processes, which altered the original composition of the coccolith assemblages in some of the DP stations, provide insights into the deep ocean biogeochemistry and hydrography of the study area. These processes distort the original ecological information and limit the potential of coccolith assemblages in surface sediments as surface ocean indicators. These processes need to be taken into account when interpreting downcore coccolith records at high latitudes.
Remote sensing studies over the last decades have detected coccolithophore blooms further north in the North Atlantic linked 565 to anthropogenic-induced climate change (e.g., Smyth et al., 2004;Neukermans et al., 2018). Furthermore, although plankton studies on coccolithophores are mostly limited to the Norwegian-Greenland Seas and the Fram Strait south of the central Arctic Ocean (e.g., Baumann et al., 2000;Dylmer et al., 2015), coccoliths have been found in sediments of the central Arctic Ocean and have intensively been used for stratigraphic purposes (e.g., Gard, 1993;O'Regan et al., 2020). Their occurrences in those sediments were interpreted as an indicator of partly ice-free conditions during at least some summers. Furthermore, 570 by expanding poleward and doubling its areal extent in the northernmost Barents Sea, the occurrence of E. huxleyi attests the ongoing "Atlantification" of the Arctic Ocean (Oziel et al., 2020). The primary driver of the dynamics of this species seem to be, in fact, stronger surface currents, which in turn intrinsically shape the temperature field and frontal structures.
In the DP, the occurrence of coccoliths in surface sediments indicate that, despite the lack of observations in the plankton, coccolithophores seem to be continuously present south of the PF or are at least continuously drifted south via eddies during 575 the Holocene. From the available data, however, it is difficult to deduce whether it is an increasing process or an increased shift towards the south. Plankton samples covering the last five decades, however, have shown a gradual poleward expansion of E. huxleyi in the SO (Cubillos et al., 2007;Winter et al., 2013), which currently seems a more permanent member of the malformations (e.g., Bednaršek et al., 2012;Gardner et al., 2018). This loss in carbonate fossils in the geological record obeys to an increase in anthropogenic acidification which is particularly significant in the SO, and it is dissolving the most recent calcium carbonate geological record (Sulpis et al., 2018). Distinguishing E. huxleyi morphotype O in future plankton, sediment trap and surface sediment studies will provide new insights into calcium carbonate dissolution processes due to increasing CO 2 dissolved in the SO, which may affect coccolithophores communities either living in the water column or incorporated 590 immediately to the fossil record.
The higher than expected occurrence of coccoliths in surface sediments south of the PF as well as the aforementioned preservational limitations open the possibility that sediment records in sub-polar ice distal regions could have also born more coccoliths during older time intervals. Currently, these type of records in the SO are very scarce, and some of them only show coccoliths during deglaciations or interglacials, which is especially evident for instance during Termination IV or Marine 595 Isotope Stage 11 (e.g., Flores et al., 2003Flores et al., , 2012Saavedra-Pellitero et al., 2017;Saavedra-Pellitero et al., 2017).
It is imperative to assess the contribution of coccolithophores to changes in present and past processes (Nissen et al., 2018) with in situ SO observations and fossil datasets in order to develop high resolution and well constrained regional and global climate models. Model simulations projecting future coccolithophore growth and calcification in an acidified ocean have also proposed that coccolithophores will expand with increasing CO 2 availability, but they will become more lightly calcified, with 600 even "naked"coccolithophores (i.e., without coccoliths) dominating in polar areas (e.g., Krumhardt et al., 2019). To test these future projections, further in situ field observations will be needed during the upcoming years in the already changing polar realm.

Conclusions
Our knowledge about coccolithophore biogeographical distribution in the Southern Ocean is still patchy and rather limited. We 605 tried to fill this gap through the analysis of a series of surface sediment samples offshore Chile and across the Drake Passage (from 52°S to 63°S). Based on our data the following conclusions can be drawn: 1. Surface sediment assemblages are very similar to living coccolithophore communities, especially offshore Chile in the Subantarctic Zone. This suggests that the regional oceanography and related physical and chemical parameters play an important role shaping recent assemblages in this region. The occurrence of temperate to subtropical species in these surface sediments suggests that other factors than surface-ocean 615 conditions might have affected the species composition in these samples. This could be explained by temperate coccolithophore species occasionally thriving south of the Polar Front, drifted polewards via eddies, or by transport of detached coccoliths via surface and deep oceanic currents.
4. We observe a selective dissolution of less calcified species (Emiliania huxleyi) and enrichment of heavier calcified taxa (e.g., Calcidiscus leptoporus), mainly south of the Polar Front at depths >3.1 km in the Drake Passage (CEX dissolution index 620 < 0.75).
5. The potential drifting, transport and/or dissolution processes distort the original ecological information and limit the potential of coccolith assemblages as surface ocean indicators south of the Polar Front, but provide valuable information about hydrodynamics and post-depositional processes. This needs to be considered when interpreting downcore coccolith records at high latitudes. Appendix A