Late Cenozoic Sea Surface Temperature evolution of the South Atlantic Ocean

. At present, a strong latitudinal sea surface temperature (SST) gradient of ~16°C exists across the Southern Ocean, maintained by the Antarctic Circumpolar Current (ACC) and a set of complex frontal systems. Together with the Antarctic ice masses, this system has formed one of the most important global climate regulators. The timing of the onset of the ACC-system, its development towards modern-day strength, and the consequences for the latitudinal SST gradient around the southern Atlantic Ocean, are still uncertain. Here we present new TEX 86 -derived SST records from two sites located east of 20 Drake Passage (southwestern South Atlantic) to assist in better understanding two critical time intervals of prominent climate transitions during the Cenozoic: The Late Eocene–Early Oligocene (ODP Site 696) and Middle–Late Miocene (IODP Site U1536) transitions. Our results show temperate conditions (20–11°C) during the first time interval, with a weaker latitudinal SST gradient (~8°C) across the Atlantic sector of the Southern Ocean compared


Introduction
Today, Southern Ocean surface flow is dominated by the strongest ocean surface current on Earth, the Antarctic Circumpolar Current (ACC).This wind-driven, eastward flowing surface current is associated with strong meridional gradients in sea surface temperature (SST) (~16°C, ~45-60°S) and oceanographic conditions (Locarnini et al., 2018).Questions remain about the timing and nature of the development of the ACC and concomitant evolution the complex Southern Ocean frontal systems (Fig. 1).A primary prerequisite for the existence of a strong ACC is an unobstructed latitudinal band of (deep ocean) water (Orsi et al., 1995;Barker and Thomas, 2004;Toggweiler et al., 2006), which is largely determined by the tectonic evolution and opening of the Tasmanian Gateway as well as the Drake Passage (Huber et al., 2004).
The tectonic evolution of the Tasmanian Gateway is relatively well constrained; early southern opening of the Tasmanian Gateway started around 49-50 Ma (Huber et al., 2004;Stickley et al., 2004;Bijl et al., 2013) with a change in course of tectonic drift of Australia from the Northeast to the North (Whittaker et al., 2007).Final breakup between Australia and Antarctica started around ~35.5 Ma, with ocean crust formation between southwestern Tasmania and Wilkes Land, Antarctica, and onset of bottom-water currents around 35.5-33.5 Ma (Stickley et al., 2004;Houben et al., 2019), although the strength of this socalled 'proto-ACC' during the Oligocene remains debated.New field data reconstructing SST and water properties (Bijl et al., 2018;Hartman et al., 2018;Salabarnada et al., 2018;Evangelinos et al., 2020Evangelinos et al., , 2022;;Sauermilch et al., 2021;Hoem et al., 2021aHoem et al., , b, 2022;;Duncan et al., 2022;Hou et al., 2022) allow tracing the migration of frontal systems, which may be reconducted to opening of gateways.Furthermore, high resolution modelling exercises (England et al., 2017;O'Brien et al., 2020; strengthening of the ACC, and shifts in the frontal systems.In turn, the changes in these three processes can be linked to the throughflow of surface and deep waters through the Drake Passage. Recent drilling efforts in and around the Scotia Sea as part of International Ocean Discovery Program (IODP) Expedition 382 (Weber et al., 2021a), and a revisit of previously drilled records in the Weddell Sea during ODP Leg 113 offer improved spatial coverage of sedimentary records across two prominent climate transitions, viz.(1) the Eocene-Oligocene transition (EOT; 33.7 Ma) and (2) the transition from the Miocene Climate Optimum (MCO,~16.5Ma) to the mid-Miocene Climate Transition (MMCT, ~14.7-13.8Ma) and the late Miocene Cooling (LMC; ~7-5.4Ma).Both transitions are marked by increases in benthic foraminiferal δ 18 O, suggesting cooling and expansion of the Antarctic ice sheet (Rohling et al., 2022).Given the paucity of carbonaceous sediments in this region, typically employed for paleotemperature reconstructions, we here choose to generate lipid biomarker (TetraEther indeX of tetraethers consisting of 86 carbon atoms (TEX86)) proxy SST reconstructions, based on isoprenoid glycerol dialkyl glycerol tetraether (isoGDGT) distributions in sediments from the northern Weddell Sea at ODP Site 696 (Late Eocene-Early Oligocene) and southern Scotia Sea IODP Site U1536 (mid-late Miocene) (Fig. 1).We compare our findings to available TEX86, alkenone unsaturation index (U k' 37), and clumped isotope (∆47) derived SST records from the South Atlantic region (Fig. 1) for a reconstruction of paleoceanographic conditions over the late Cenozoic.1).

Site U1536: lithology, age model and depositional setting
Site U1536 is located in the Dove Basin, in the southern Scotia Sea (59°26.4608'S,41°3.6399'W,3220 m water depth).The site was drilled to study the Neogene flux of icebergs through "Iceberg Alley", the main pathway along which icebergs calved from the margin of the Antarctic ice sheet drift into the warmer waters of the ACC (Weber et al., 2021a).Today, the site is located just south of the southern ACC front (sACCf) and the Southern Boundary (SBdy) front and is seasonally covered by sea ice (Fig. 1).The rotary drilling at Hole U1536E penetrated down to 643 mbsf.Sediments have moderate to high core disturbance and biscuiting, or brecciated core material due to the rough nature of rotary drilling and compaction of gravel rich material.The lithology of the studied interval from Hole U1536E, 640-450 mbsf (Cores 33R-13R), consists of silty clays with interbedded diatom ooze (Fig. 3), with an estimated average sedimentation rate of ~4.3 cm/kyr (3.9-6.4 cm/kyr; Pérez et al., 2021).The shipboard bio-and magnetostratigraphic age model was used to date the sediments (Weber et al., 2021b, Supplementary  sediment directly overlying Reflector-c (Weber et al., 2021b), at 617-570 mbsf (Cores 30R-26R), have an age of 8.4 Ma (Pérez et al., 2021).The sediments below Reflector-c, at 622 mbsf (Core 31R), are dated to ~14.2 Ma, and as the lithologic contact is not recovered, Reflector-c could represent a prolonged time interval of slow sedimentation rates or non-deposition or erosion.The sparse brecciated lithology fragments in the lower cores, below the Reflector-c (566 mbsf), consist of lithified mudstone and gravel-conglomerate-breccia (Weber et al., 2021b;Perez et al., 2021).3 Methods

Lipid Extraction and glycerol dialkyl glycerol tetraether (GDGT) analysis
Lipid extraction of sediments from Site 696 was performed at the Laboratoire d'Océanographie et du Climat, Expérimentations et Approches Numériques (LOCEAN-Sorbonne Université, Paris, France).First, 71 sediment samples were freeze-dried and crushed to a fine powder.Total lipids were extracted from ~9.5 to 15 g of homogenized sediment using a solvent mixture of 40 ml dichloromethane:methanol (DCM:MeOH; 3:1, v/v).The apolar fraction was separated from the polar lipids by passinfpassing the total lipid extract (TLE) over a silica column using 3 ml hexane as eluent, followed by the recovery of the polar fraction by eluting with 3 ml DCM:MeOH (1:1, v/v).The polar lipid fractions were sent to Utrecht University for GDGT analysis.Sediment samples from Site U1536E were processed for GDGT analysis by lipid extraction at Utrecht University, from 10 g freeze-dried and manually powdered sediments using a Milestone Ethos X microwave system and adding DCM:MeOH (9:1, v/v).The TLEs were first filtered through a NaSO4 column to remove potential remaining water and 270 sediments.The TLEs were then separated on an activated Al2O3 column into apolar, ketone and polar fractions, using hexane:DCM (9:1, v/v), hexane:DCM (1:1, v/v), and DCM: MeOH (1:1, v:v) as eluents, respectively.All polar fractions were dried under N2.A known amount of C46 GTGT standard was added to the polar fractions from Sites 696 and U1536, which were subsequently dissolved in hexane:isopropanol (99:1, v/v) to a concentration of ~2 mg ml -1 and filtered through a 0.45 µm polytetrafluorethylene filter.After that, the dissolved polar fractions were injected and analysed by ultra-high performance 275 liquid chromatography/mass spectrometry (UHPLC/MS) according to the method described by Hopmans et al. (2016), using an Agilent 1260 Infinity UHPLC system coupled to an Agilent 6130 single quadrupole mass detector, at Utrecht University.
Selected ion monitoring (SIM) was used to identify the GDGTs using their [M+H] + ions and integrated using ChemStation software.Samples with very low concentrations (i.e., peak area <3,000 mV/s and/or peak height <3× background signal) of any of the GDGTs included in the TEX86 were excluded from analysis.280 Although lipid extractions of Sites 696 and U1536 were performed at different institutions, the latest interlaboratory comparison study (F.Peterse personal communication) that assessed the repeatability and reproducibility of the TEX86 indicates that differences in sediment extraction and workup procedures do not affect isoGDGT distributions, and thus reconstructed SSTs.Instead, variations in reported TEX86 values appeared to be mainly introduced by the type of mass spectrometer used 285 for GDGT analysis (Schouten et al., 2013).Since the polar fractions from both sites were analyzed using the same HPLC-MS instrument at Utrecht University, the uncertainty on our TEX86-based SSTs mostly represents the analytical uncertainty, which is ±0.3°C based on long-term observation of the in-house standard.

GDGT indices for non-thermal overprints on TEX86
The TEX86 SST proxy is based on the temperature dependence of the number of cyclopentane rings in GDGT membrane lipids 290 produced by marine Thaumarchaeota and calculated as defined by Schouten et al. (2002): The use of TEX86 as a proxy for SST relies upon the assumption that isoGDGTs in marine sediments are principally derived 295 from membrane lipids of marine pelagic Thaumarchaeota (Schouten et al., 2013).However, in some environments, nonthermal factors may alter the distribution of isoGDGTs stored in the sediment and thus the temperature signal (Supplementary information).We assess potential non-thermal effects on isoGDGT distributions prior to translating TEX86 into SSTs.We use the branched and isoprenoid tetraether (BIT) index to assess possible overprints of terrestrial GDGT input (Hopmans et al., Deleted: was 300 Deleted: ed from the total lipid extracts (TLEs) using Deleted: over a silica column

TEX86 calibration
The empirical relationship between TEX86 values and SST has appeared to be not always straightforward, as reflected by continued revisions of the approach of TEX86 -SST calibrations (Kim et al., 2010;Tierney and Tingley, 2015;Ho and Laepple, 2016;O'Brien et al., 2017;Dunkley Jones et al., 2020).Particularly, the relationship between TEX86 and SST seems to become obscured in both extreme ends of the core-top calibration: at or above modern SSTs, and in cold polar regions.uncertainties in the surface sediment data into resulting temperature predictions (Tierney and Tingley, 2015).Even when GDGT-2/GDGT-3 ratios indicate that deeper dwelling GDGT producers do not contribute to the sedimentary signal, the GDGTs could still originate from the subsurface (50-200 m water depth) rather than the sea surface (Schouten et al., 2013).
However, since the BAYSPAR calibration translates TEX86 values to SSTs, we will, therefore, refer to the proxy results as such in the remainder of this work.For all new and existing TEX86 records discussed in this study, we applied a standard deviation of ±20°C and a prior mean of 20°C for the Late Eocene-Early Oligocene interval and 15°C for the Miocene.We  3, 4, Fig. S3, S4).

4.2
Site U1536 GDGT distributions and TEX86-SST trends A total of 40 sediment samples from IODP Hole U1536E were processed for GDGT analysis (Supplementary Table 4), of which 14 had GDGT concentrations below detection limit (Fig. 5).The Site U1536 GDGT pool consists of variable amounts of both isoGDGTs and brGDGTs, where brGDGTs are relatively more abundant in the middle (500-560 mbsf) and top parts of the record (450 mbsf) (Fig. S2A), resulting in high BIT-index values in these intervals of the record (Fig. 5b).GDGT distributions in 12 sediment samples were outside the range of what is considered reliable for multiple indicator proxies (triangles in Fig. 5a, Supplementary information, Fig. S2).For the remaining 14 samples TEX86 values were translated into SSTs using the BAYSPAR calibration (Fig. 5a).The obtained record shows temperatures of 5-11°C for the Middle Miocene (619-640 mbsf) and 1.5-5°C for the Upper Miocene (570-450 mbsf).Seymour Island (Douglas et al., 2014), in addition to our new SST record from Site 696 (Fig. 6) to put the SST records into a broader regional context and discuss the surface oceanographic development during the Late Eocene-Early Oligocene.Our 410 SST record from Site 696 (yellow in Fig. 6A) shows warm-temperate conditions (SST range: 22-14°C) during the latest Eocene (~36.5-33.6Ma) and on average decreasing SSTs (~15-9°C) in the earliest Oligocene (33.6-33.2Ma).The cooling of South Atlantic surface waters across EOT is in broad agreement with the average Southern Ocean-wide temperature drop (Kennedy-Asser et al., 2020;Tibbett et al., 2023), the increase in benthic foraminifer δ 18 O values as a result of deep sea cooling, a drop in atmospheric pCO2 levels (https://www.paleo-co2.org;Pearson et al., 2009;Steinthorsdottir et al., 2016;Hoenisch, 415 2021) and the growth of a continent-wide Antarctic ice sheet across the EOT (e.g.Bohaty et al., 2012).Sites 511 and 1090 also show a step-wise cooling across the EOT (Hutchinson et al., 2021), where the first step occurs around 34.1 Ma and coincides with common IRDs at Site 696, indicating the onset of marine-terminating glaciers in the region (López-Quirós et al., 2021), and the second step coincides with the Earliest Oligocene Oxygen Isotope Step (EOIS, 33.65 Ma; Hutchinson et al., 2021).Miospores at Site 696, believed to be of local origin from the South Orkney Microcontinent, changed concomitantly 420 from southern beech, Nothofagus-dominated vegetation to an increase in gymnosperms and cryptogams, accompanied by a rapid rise in taxon diversity after the EOIS (~33.65 Ma, 568.82 mbsf; Thompson et al., 2022).This shift in vegetation to a cooler and dryer climate occurred after the onset of earliest glacial expansions (~34.1 Ma).Sedimentological investigations by López-Quirós et al. (2021) for the same interval showed deepening of the South Orkney Microcontinent shelf and enhancement of biological production, possibly due to upwelling along the shelf, leading to low oxygen conditions at the seafloor.The high-425 amplitude SST variability (~4-8°C) in our TEX86-SST record would suggest that this upwelling regime was strongly variable.
The variability in upwelling conditions could be induced by strong fluctuating ice sheet expansion and retreat and shifts in wind patterns and ocean frontal systems.Low-resolution palynological investigations on Late Eocene -Early Oligocene sediments from the southern South Atlantic (Houben et al., 2019;Hoem et al., 2022) show a highly diverse and variable dinocyst assemblage, which includes Antarctic derived, open ocean, temperate and high nutrient indicative species, 430 respectively, and indeed infers a fluctuation in surface ocean conditions, potentially related to shifts in frontal systems and upwelling regions.Alternatively, the high variability in the TEX86 signal could be introduced by the input of reworked isoGDGTs.Such inputs were previously found to be high at ice proximal sites where the onset of large-scale Antarctic glaciation across the EOT caused reworking of pre-Eocene deposits, such as e.g., Prydz Bay (Tibbett et al., 20201).However, Tibbett et al. (2021) found that this had little impact on the overall TEX86-SST trend.Furthermore, we record very low BIT 435 Deleted: with Deleted: or…Site 696 (yellow in Fig. 6A) shows warm-temperate conditions (SST range: 22-14°C) during the latest Eocene (~36.5-33.6Ma) and on average decreasing SSTs (~15-9°C) in the earliest Oligocene (33.6-33.2Ma).The cooling of South Atlantic surface 500 waters across EOT is in broad agreement with the average Southern Ocean-wide temperature drop (Kennedy-Asser et al., 2020;Tibbett et al., 2023), the increase in benthic foraminifer δ 18 O values as a result of deep sea cooling (Bohaty et al., 2012) .

Moved down [2]:
Step-wise cooling across the EOT (Hutchinson et al., 2021) is evident at Deleted: in the U k' 37 record at

Moved (insertion) [2]
Deleted: S…tep-wise cooling across the EOT (Hutchinson et al., 2021), where is evident at.Bijl et al., 2013), because of the well-developed soils on Antarctica at the time (Inglis et al., 2022).The lack of brGDGTs in our record thus suggests little influence of reworked Eocene GDGTs.We, therefore, assume that the TEX86 record from Site 696 represents an in-situ pelagic signal and that the variability in the SST record is introduced by upwelling.545 The clumped isotope (∆47)-based SST datapoint from Seymour Island (34 Ma, purple star in Fig. 6B) shows a similar temperature (13°C) to that derived from the TEX86 proxy at the same site (Douglas et al., 2014), as well as from nearby Site 696.The correspondence of the biomarker-derived SSTs with that from Eurhomalea (bivalve) ∆47 (mean annual temperature), 570 adds reliability to the temperature proxies accurately reflecting SST in this region.Interestingly, the SSTs from Seymour Island and Site 696 are very similar to Site 511, even though there was a paleolatitudinal difference of ~12° between Site 511 and Seymour Island (Fig. 6).The subtropical Site 1090 is the warmest site (~19-27°C) in our compilation, which is expected given its lower paleolatitude.However, the absolute SSTs of Sites 696 and 511 are strikingly similar across the EOT (Fig. 6A).The southwestern South Atlantic temperature gradient (between Sites 511 and 696) decreased from ~1.8°C in the Late Eocene to 575 ~0°C in the Early Oligocene.A larger throughflow through Drake Passage would increase the temperature gradient between Sites 696 and 511.Today both sites are separated by the strong ACC and an SST gradient of >7°C (Locarnini et al., 2018).
We therefore imply that the throughflow changes induced by the opening Drake Passage did not change the South Atlantic Ocean circulation across the EOT.Tectonic evidence suggests that the Drake Passage was narrow, with little deep-water connection from the Pacific to the Atlantic around the EOT (Livermore et al., 2007;Eagles and Jokat, 2014;van de Lagemaat 580 et al., 2021), which may explain the lack of regional oceanographic response.Model experiments (Huber et al., 2004;Hill et al., 2013;England et al., 2017;Sauermilch et al., 2021) show that a Southern Ocean without deep gateways featured winddriven clockwise gyres in the South Pacific and South Indian/Atlantic Ocean basins (Fig. 6B) that would advect warm surface waters toward the Antarctic coast.Specifically, eddy resolving ocean model simulations by Sauermilch et al., (2021) for the Eocene show that a restricted (depths <600 m) Drake Passage throughflow would sustain the subpolar gyre and lead to SSTs 585 reaching 19°C in the Australian-Antarctic Basin and 15-17°C in the subpolar Pacific and Atlantic.This is very similar to our Site 696 TEX86-SST EOT record (~11-18°C).We thus propose that the small differences in SSTs between Sites 511 and 696 are the result of a restricted Drake Passage during the latest Eocene -earliest Oligocene, facilitating a persistent wind-driven gyral circulation that connected the southern South Atlantic sites in our compilation.

Middle to Late Miocene 590
To investigate the cooling step in the Middle to Late Miocene, the new Miocene SST records from the Antarctic-proximal southwestern South Atlantic Sites 696 and U1536 are compared to records from the Antarctic Peninsula (SHALDRILL II, Core 5D; Tibbett et al., 2022) and Wilkes Land (Site U1356, Sangiorgi et al., 2018).However, there is still a lack of well constrained and overlapping records form the Southern Ocean Middle to Late Miocene due to glacial expansions and erosion causing discontinuous records.Due to the large age uncertainties and gaps in the in the sedimentary records of these sites 595 (Weber et al., 2021a;Perez et al., 2021;Bohaty et al., 2011), we present the TEX86-derived SSTs as average temperatures within two broad time intervals corresponding to the age uncertainty.We also compare SST trends from the above-mentioned sites to those from the subantarctic zone; ODP Site 1171 from the southwest Pacific Ocean (Leutert et al., 2020) and subtropical front; ODP Site 1088 (Herbert et al., 2018) in the Southeast Atlantic, and Site ODP 1168 west of Tasmania (Hou et al., 2022).
Deleted: TEX86-and U k' 37-based SST reconstructions from ODP Site 511 differ slightly in trends across the EOT, with U k' 37 showing amplified cooling compared to TEX86.U k' 37-temperature data reflect surface water temperature following the niche of the haptophyte 830 algae that produce the alkenones on which the U k' 37 is based, while the GDGTs that mostly contribute to the TEX86 signal stored in the sediments are likely produced in the subsurface (50-200 m water depth) (Schouten et al., 2013).Although U k' 37 is calibrated to mean annual temperature (Müller et al., 1998), in some settings it reflects a 835 seasonally biased temperature (e.g., Ternois et al., 1997).In fact, modern high-latitude alkenone producing haptophyte communities primarily bloom in early spring (Herbert et al., 1998).Thus, at Site 511, the amplified cooling in U k' 37 might reflect a shift towards early spring blooming, which could explain its amplified cooling trend 840 across EOT, relative to that in TEX86.As TEX86-based SSTs are consistently higher than U k' 37-based SSTs at this site, we argue that TEX86 represents a warmer season than U k' 37 and that it does seem to be reflecting a surface water signal rather than a subsurface signal.¶ ... Further, we compare our new Site U1536 SST record to clumped isotope bottom water temperatures (BWT) from South Indian 845 Ocean ODP Site 747 (Leutert et al., 2021) (Fig. 7).The data compilation of South Atlantic SSTs (Fig. 7A) shows a 7°C cooling between Site 696 (yellow dots, ~14°±4°C standard calibration error, n=2) during the Miocene Climate Optimum (MCO, ~16.5 Ma) and Site U1536 (red, ~7±4°C, n=5) in the Middle Miocene Climate Transition (MMCT, ~14.7-13.8Ma).Both sites were located at comparable paleolatitudes (albeit with a 2.5° latitudinal difference, Fig 8A ) during the Miocene (Fig. 7B).SSTs at Wilkes Land Site U1356 were warmer (17°C around 17 Ma) than at Site 696 (~14°C), even though Site U1356 was situated closer to the cooler East Antarctic ice sheet.Additionally, a less pronounced cooling (SST ~16-12°C) occurred across the MMCT at Site U1356 (Sangiorgi et al., 2018), than the ~7°C cooling shown in the South Atlantic low-resolution dataset.Thus, the South Atlantic sector was colder with a likely more proximal ice mass at the onset of MMCT than the Wilkes Land Antarctic margin.There was a ~6-10°C temperature difference between the southwest South Atlantic Antarctic proximal Sites 696/U1536 and the subantarctic Site 1171 (southwest Pacific Ocean), with Site 1171 clumped isotope (∆47) SSTs of 14-12°C and TEX86 SSTs of 18-13°C during MMCT (Leutert et al., 2020).This suggests a relatively strong SST gradient between the coldest Antarctic-proximal regions and the subantarctic zone in the Middle Miocene, even though the subantarctic zone was likely situated at lower paleolatitudes in the Australian-Antarctic Gulf than in the South Atlantic due to the more southerly position of Australia.The increase in the temperature gradient in the complied SST records (Fig. 7A) during the Middle Miocene indicates breakdown of the dominant gyral circulation at the early Oligocene and a subsequent strengthening of the ACC during the Middle-Late Miocene.The alkenonebased SST reconstructions for Site 1088 and TEX86-based SST reconstructions for Site 1168, together representing subtropical front conditions, show MCO temperatures between 32-27°C that progressively cooled during the MMCT (~24-14°C) (Hou et al., 2022).This subtropical cooling is weaker than at the Antarctic-proximal sites, which indicates that the cooling was amplified at high latitudes.The clumped isotope (∆47) BWT record from Site 747 in the South Indian Ocean (Leutert et al., 2021) shows strikingly similar temperatures to the reconstructed SSTs at Site U1536, which may suggest that the Weddell Gyre in the southwestern South Atlantic was an important region of deep water formation in the Miocene, like today (e.g., Orsi et al., 1999), which furthermore is in line with what modelling studies suggest for the Miocene (e.g., Herold et al., 2012).The 7°C cooling of the southwestern South Atlantic between the MCO and MMCT occurs during a time of declining pCO2 (Foster et al., 2012;Greenop et al., 2014) (Fig. 8D) and increasing benthic foraminiferal δ 18 O (Fig. 7A, 8C), reflecting an increasingly colder climate with larger (in area) ice sheets on the Antarctic continent with oceanward expansion (Lewis et al., 2008;Shevenell et al., 2008;Holbourn et al., 2018;Levy et al., 2019;Leutert et al., 2020).Given estimates of pCO2 decline (by 100-300 ppm; Sosdian et al., 2018;Super et al., 2018), paleo climate sensitivity (1.5-4.5°C per pCO2 doubling; Martínez-Botí et al., 2015) and polar amplification factors (2-3;Holland et al., 2003), the 7°C cooling in the southwestern South Atlantic records (Sites 696 and U1536) could have been completely induced by pCO2 decline in the Middle Miocene.This also means Deleted: We, therefore, infer that in the Middle Miocene, the large(???) degree of cooling in the southern South Atlantic as 920 recordeds at Sites 696 and U1536 indicates that SSTs were sensitive to the decrease in respond sensitively to the pCO2 in the Middle Miocene-induced cooling.
that regional cooling was not amplified through strengthening of the ACC and destruction of the sub polar gyre, which occurred later in the Miocene (Evangelinos et al., 2021).925 By the latest Miocene (~6.4-5.2Ma), temperatures at Site U1536 cooled to a average SST of 3.4°C (n=8) which match those at SHALLDRILL II for the earliest Pliocene (5.1-4.3Ma, average 2.8°C, n=3).The 3-5°C SST decrease at Site U1536 between the MMCT (~14 Ma) and the latest Miocene (~6.4-5.2Ma) is perhaps surprisingly small compared to the ~10°C cooling at the subtropical front Sites 1168 and 1088 at the same time.We surmise that high-latitude cooling was subdued 930 during this time interval because southwestern South Atlantic surface waters (Site U1536) were already cold during the MMCT (Fig. 7A).Instead, the remaining part of the Southern Ocean experienced pronounced cooling during this time interval.Based on the Southern Ocean SST records (Fig. 7A), the southwestern South Atlantic was possibly the coldest region of the Southern Ocean already since at least the Middle Miocene.However, there is a strong lack of Antarctic proximal records (e.g., no records from the Ross Sea; Levy et al., 2019) that cover the MMCT to Late Miocene, partly due to glacial advances, to draw a full 935 picture of circum-Antarctic cooling since the MMCT.
Site U1536 SSTs and Site 747 BWTs were low during the MMCT (Leutert et al., 2021), with minor cooling thereafter.Leutert et al. (2021) attributed the subdued post MMCT cooling to the growing Antarctic ice sheet, which could have led to increased stratification and shielding of deeper waters in the Southern Ocean.We here conclude that the southwestern South Atlantic 940 regions already reached cold conditions during the MMCT, because of the proximity to ice sheets, and, as a result, could not cool much more given the global cold climate of the Late Miocene.The cooling of the subtropics (Sites 1088 and 1168) is much more pronounced than the southwestern South Atlantic, because of the gradual northward expansion of the westerly winds, ACC, and cold subantarctic waters (Leutert et al., 2020) in response to the expansion of the Antarctic ice sheet in the Late Miocene.This process likely also further promoted cooling in other sectors of the Antarctic-proximal Southern Ocean.945

South Atlantic SST gradient evolution
Compiling all available SST records for the two time slices discussed above (Sect.5.2 and 5.3) yields a unique insight into the long-term temperature trends in the South Atlantic Ocean (Fig. 8).The SST records from the South Atlantic region show unidirectional temperature drops across the EOT, with a small degree of polar amplification where Antarctic-proximal (Site 696 and 511) cooled by ~8°C and subtropical records (Site 1090) cooled by ~5°C.The strong, large and persistent South 950 Atlantic subpolar gyre kept the latitudinal SST gradient low in the southernmost part of the South Atlantic across the EOT (Huber et al., 2004;Houben et al., 2019).The latitudinal temperature gradient in the South Atlantic increased during the MMCT (Fig. 7, 8), due to the largest cooling at high latitudes, almost reaching modern temperatures, followed by a subdued cooling during the Late Miocene.Meanwhile the SSTs at the subtropical front (Sites 1090 and 1088) remained relatively stable from the earliest Oligocene (Site 1090, 33 Ma) until the Late Miocene (Site 1088) (Fig. 8), with minimal cooling until the 955 latest Miocene.
Formatted: Font colour: Black Deleted: The (late) middle Miocene (12.8-11.7 Ma) TEX86 data from SHALDRILL II Hole 5D (Tibbett et al., 2022)   The cooling phases in the South Atlantic across the EOT and from the MCO to the MMCT represent two climatic transitional phases, both characterized by declining atmospheric pCO2 concentrations (Pearson et al., 2009;Foster et al., 2012;Greenop et al., 2014;Steinthorsdottir et al., 2016) and increasing benthic foraminiferal δ 18 O values (Westerhold et al., 2020), indicating deep-sea cooling and/or ice sheet expansion (Flower and Kennett, 1993;Zachos et al., 1996) (Fig. 8).The EOT marks the first installation of a continent-wide Antarctic ice sheet (Deconto and Pollard, 2003;Coxall et al., 2005), with a volume between 60 and 130% of that of present-day ice sheet (Bohaty et al., 2012).The MCO is considered as a global warm phase, with warmtemperate ice proximal conditions (Sangiorgi et al., 2018) and a profoundly reduced Antarctic ice volume (Shevenell et al., 2008;Foster et al., 2012), and the MMCT is a strong and stepwise transition towards a larger Antarctic ice sheet (Rohling et al., 2022).Surprisingly, although southwestern South Atlantic records (Sites 696 and U1536) are of low resolution with notable age uncertainties, they do suggest similar Antarctic-proximal SSTs (~12-14°C) for the early Oligocene, when a large, predominately terrestrial ice sheet with marine terminating glaciers was installed, as for the MCO, when ice sheets were profoundly reduced.Keeping in mind the higher-than-modern Antarctic paleotopography in the Oligocene (Wilson et al., 2009;Duncan et al., 2022), with a gradual subsidence during the Miocene (Paxman et al., 2019), this still puts both climate phases into perspective: apparently the Oligocene Antarctic ice sheet could coexist with warm ice-proximal surface ocean conditions, while the Middle Miocene Antarctic ice sheet could be strongly reduced despite a relatively cold ice-proximal South Atlantic Ocean.

Conclusions
Our lipid biomarker records from IODP Site U1536 and ODP Site 696 have generated new insights for the understanding of the SST evolution of the South Atlantic Ocean, viz.: • The EOT in the South Atlantic is characterized by a relatively small latitudinal SST gradient of ~5 degrees between the subtropical front and the western Weddell Sea and a regional decrease in SST (4-6°C) as global pCO2 declined.
• The South Atlantic SST gradient remains constant across the EOT, which we ascribe to a gyral circulation that connects all South Atlantic sites and can persist in the absence of a strong throughflow through Drake Passage.
• Southwestern South Atlantic SSTs at the earliest Oligocene glaciation were similar to those of the warm MCO, implying that Antarctic proximal SSTs are not the only determining factor for the extent of the Antarctic ice sheet.
• The southwestern South Atlantic experienced cold polar climate conditions (SSTs of ~5°C) already during the MMCT.This made it the coldest oceanic region around Antarctica and the likely region of deep-water formation.
• Due to the already relatively cold conditions in the southwestern South Atlantic in the Middle Miocene, it experienced little further cooling during the Late Miocene.This is in contrast to subtropical sites and other sectors of the Southern

Figure 1 .
Figure 1.Present day map of the Southern Ocean showing the location of the drill sites used in this study.Grey areas represent present-day land masses.The colors show average Summer (January) SSTs from 1971-2000 (Reynolds et al., 2002).The white areas lack modern SST data, as they are covered by ice shelves.The white lines represent the smoothed, simplified position of circumpolar fronts interpreted by Orsi et al. (1995).From north to south: The Subtropical front (STF); the Subantarctic front (SAF); the Polar Front (PF); the southern ACC Front (sACCf); and the Southern Boundary (SBdy) front.SI=Seymour Island, SD=SHALLDRILL, WS= Weddell Sea, SS=Scotia Sea, SOM= South Orkney Microcontinent.
As the TEX86 is based on specific GDGT ratio and not on a single compound concentration, the estimated SSTs is therefore not 320 biased by the extraction/separation method performed in the two labs.
also compare the BAYSPAR-derived SST estimates with those based on the exponential function fromKim et al. (2010) and the linear function byO'Brien et al. (2017).The SST records all show similar trends, but BAYSPAR-derived SSTs are usually cooler compared to those obtained from the functions ofKim et al. (2010) andO'Brien et al. (2017) and can thus be considered conservative estimates (Supplementary Table In the Middle Miocene (~520 mbsf, n=2) SSTs values are ~14°C.370Deleted: distribution, and Deleted: , but with high sample-to-sample variability

Figure 7 .
Figure 7. A. TEX86-based SST data from Site 696 and U1536 (this study) compared to Southern Ocean wide SST and BWT records.The bars on the left indicate the standard calibration error of the SST proxies.Data of Site U1536 is displayed as two bar plots (red) showing the temperature ranges for the Middle Miocene (16-14 Ma) and late Miocene (7-5.3Ma), individual data points are shown as red dots.We compare our data to SST records from Wilkes Land Site U1356 (Sangiorgi et al., 2018), SHALDRIL II Core 5D (Tibbett et al., 2022; grey bars indicate the age uncertainty), Site 1168 (west of Tasmania; Hou et al., 2022), Site 1171 (southwest Pacific Ocean; (Leutert et al., 2020), U k' 37-SST data from Site 1088 (Herbert et al., 2016) and clumped isotope bottom water temperature (BWT) data from Leutert et al. (2021) (Site 747).The arrows indicate the temperature gradient between the TEX86-SST record from the southwest South Atlantic and the subtropical front Site 1168.The black line is the benthic foraminiferal δ 18 O compilation, smoothed by a locally weighted function over 20 kyr (thin blue curve) (CENOGRID; Westerhold et al., 2020).Thick blue curve is the LOESS smoothed (span = 0.2).The stippled vertical line indicates the age for the paleogeographic map below.B. Paleogeographic reconstruction at 16 Ma, based on the GPlates reconstruction of van de Lagemaat et al. (2021) in the paleomagnetic reference frame of Torsvik et al. (2012).All sites from data compilation in A are shown as stars.Dashed black line represents the Miocene surface ocean currents derived from Herold et al. (2012).
For the location of all sites in the data compilation in A 895 see Fig. 1.Formatted: Line spacing: At least 15 pt Deleted: for Deleted: at a higher latitude (59°S) than Sites 696 and U1536 (and, more permanent ice sheets on the Antarctic continent

Deleted: conditions 240 Deleted: SOM Deleted: in prep Deleted: F
It is …he rest …emaining part of the Southern Ocean that experienced pronounced cooling in …uring this time interval.Based on the Southern Ocean SST records (Fig.7A), the southwestern South Atlantic record the coldest SSTs and …as therefore ...[25]