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 homogenised 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 passing the total lipid extract (TLE) over a silica column using 3 mL hexane as an 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 of 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 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 glycerol trialkyl glycerol tetraether (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 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 <3000 mV s-1 and/or peak height <3× background signal) of any of the GDGTs included in the TEX86 were excluded from analysis.

Although lipid extractions of Sites 696 and U1536 were performed at different institutions, the latest interlaboratory comparison study (Francien Peterse, personal communication, 2023) 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 for GDGT analysis (Schouten et al., 2013). Since the polar fractions from both sites were analysed using the same HPLC–MS instrument at Utrecht University, the uncertainty in our TEX86-based SSTs mostly represents the analytical uncertainty, which is ±0.3 ∘C based on long-term observation of the in-house standard.

The TEX86 SST proxy is based on the temperature dependence of the number of cyclopentane rings in GDGT membrane lipids produced by marine Thaumarchaeota and calculated as defined by Schouten et al. (2002): 1TEX86=([GDGT-2]+[GDGT-3]+[cren'])/([GDGT-1]+[GDGT-2]+[GDGT-3]+[cren']. The use of TEX86 as a proxy for SST relies upon the assumption that isoGDGTs in marine sediments are principally derived from membrane lipids of marine pelagic Thaumarchaeota (Schouten et al., 2013). However, in some environments, non-thermal factors may alter the distribution of isoGDGTs stored in the sediment and thus the temperature signal (Supplement). 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., 2004) and the fractional abundance of the crenarchaeol isomer over that of crenarchaeol to explore GDGT distribution in the sediment (fcren'; O'Brien et al., 2017), the methane index to identify contributions of methanotrophic archaea (MI; Zhang et al., 2011), the GDGT-0/crenarchaeol ratio to identify contributions of methanogenic archaea (Blaga et al., 2009), the GDGT-2/GDGT-3 ratio to assess contributions of a deep-dwelling GDGT-producing community (Taylor et al., 2013), and the Δring index to identify GDGT distributions that deviate from modern analogues (ΔRI; Zhang et al., 2016) (Fig. S1c, panel 2–8; Fig. S2c, panel 2–8; Tables S3 and S4 in the Supplement).

The empirical relationship between TEX86 values and SST has appeared to not always be 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 at both extreme ends of the core-top calibration: at or above modern SSTs and in cold polar regions. However, for the time intervals (late Eocene–early Oligocene and Middle–Late Miocene) and locations we are targeting, we expect SSTs within the intermediate temperature range. In this study, we used the regionally varying BAYSPAR SST calibration of Tierney and Tingley (2015) to reconstruct SST (±4 ∘C standard calibration error) from TEX86 values. BAYSPAR compares measured TEX86 values to modern TEX86 values obtained from surface sediments to derive linear regression parameters and propagates 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 refer to the proxy results as SSTs 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 also compare the BAYSPAR-derived SST estimates with those based on the exponential function (GDGT index-2) from Kim et al. (2010) and the linear function by O'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 of Kim et al. (2010) and O'Brien et al. (2017) and can thus be considered conservative estimates (Tables S3 and S4 and Figs. S3 and S4 in the Supplement).

A total of 71 samples from ODP Site 696 were analysed for TEX86 paleothermometry (Table S3 in the Supplement). The GDGT pool consists of 90±5 % isoGDGTs and 10±5 % branched GDGTs (brGDGTs), resulting in BIT index values <0.2 (Fig. 5b). The isoGDGT distributions indicate that GDGTs are primarily derived from surface-dwelling Thaumarchaeota (Fig. S1 in the Supplement). In total, nine samples had higher ΔRI values than the cutoff of 0.3 (Fig. S1b), indicating a potential non-thermal overprint on the GDGT distribution, and are excluded from SST analysis (triangles in Fig. 4). The TEX86-based SST record from Site 696 (607–521 m b.s.f., 36–33 Ma and ∼ 16.5 Ma; Fig. 4) thus consists of 62 data points and predominantly ranges between 12 and 18 ∘C, although the total temperature range is from 4 to 25 ∘C (±4 ∘C standard calibration error). There is a general cooling trend of ∼ 8 ∘C between the upper Eocene and the lower Oligocene (from 20 to 12 ∘C between 610 and 558 m b.s.f.). This is followed by an average ∼ 5 ∘C increase in temperature between ∼ 558 and ∼ 552 m b.s.f.. In the organic-rich interval at ∼ 555 m b.s.f. (Core 53R, 33.5–33.2 Ma), 35 sediment samples were analysed, with 1 sample every 3 cm (∼ 15 kyr resolution). In this interval, the high-resolution TEX86-SST record shows high-amplitude variability (total range of 4–21 ∘C, with most data points falling within the range of 8–17 ∘C). At ∼ 550 m b.s.f. (lower Oligocene) SSTs rapidly decrease to 10 ∘C. In the Middle Miocene (∼ 520 m b.s.f., n=2) SST values are ∼ 14 ∘C.

A total of 40 sediment samples from IODP Hole U1536E were processed for GDGT analysis (Table S4 in the Supplement), of which 14 had GDGT concentrations below the detection limit (Fig. 5). The Site U1536 GDGT pool consists of variable quantities of both isoGDGTs and brGDGTs, where brGDGTs are relatively more abundant in the middle (500–560 m b.s.f.) and top parts of the record (450 m b.s.f.) (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 Figs. 5a and 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 m b.s.f.) and 1.5–5 ∘C for the Upper Miocene (570–450 m b.s.f.).

We have compiled available South Atlantic SST records from Site 1090 (Liu et al., 2009), Site 511 (Houben et al., 2019) and 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 SST record from Site 696 (yellow in Fig. 6a) shows warm–temperate conditions (SST range: 22–14 ∘C) during the latest Eocene (∼ 36.5–33.6 Ma) and on average decreasing SSTs (∼ 15–9 ∘C) in the earliest Oligocene (33.6–33.2 Ma). The cooling of South Atlantic surface waters across the 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 δ18O values as a result of deep-sea cooling, a drop in atmospheric pCO2 levels (https://www.paleo-co2.org, last access: 1 August 2023; Pearson et al., 2009; Steinthorsdottir et al., 2016; Hoenisch, 2021; Rae et al., 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 stepwise 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 from southern beech, Nothofagus-dominated vegetation to a more abundant gymnosperm and cryptogam vegetation, accompanied by a rapid rise in taxon diversity after the EOIS (∼ 33.65 Ma, 568.82 m b.s.f.; Thompson et al., 2022). This shift in vegetation to a cooler and drier climate occurred after the onset of the 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-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, 2022) show a highly diverse and variable dinocyst assemblage, which includes indicative Antarctic-derived, open-ocean, temperate and high-nutrient species, 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, for example, Prydz Bay (Tibbett et al., 2021). However, Tibbett et al. (2021) found that this had little impact on the overall TEX86-SST trend. Furthermore, we record very low BIT index values (<0.2; Table S3 in the Supplement, Fig. 4b) throughout the record, whereas Eocene sediments around Antarctica are commonly high in brGDGTs (e.g. 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.

The clumped-isotope-based (Δ47) SST data point 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) Δ47 (mean annual temperature) 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 south-western South Atlantic temperature gradient (between Sites 511 and 696) decreased from ∼ 1.8 ∘C in the late Eocene to ∼ 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 of 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 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 wind-driven clockwise gyres in the South Pacific and South Indian Ocean/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 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.

To investigate the cooling step in the Middle to Late Miocene, the new Miocene SST records from the Antarctic-proximal south-western 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 sedimentary records of these sites (Weber et al., 2021a; Pérez 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 south-western Pacific Ocean (Leutert et al., 2020) and Subtropical Front, ODP Site 1088 (Herbert et al., 2016) in the south-eastern Atlantic, and Site ODP 1168 west of Tasmania (Hou et al., 2023). Further, we compare our new Site U1536 SST record to clumped-isotope bottom-water temperatures (BWTs) from South Indian 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 Climatic Optimum (MCO, ∼ 16.5 Ma) and Site U1536 (red, ∼7±4 ∘C, n=5) in the Middle Miocene Climate Transition (MMCT, ∼ 14.7–13.8 Ma). 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 south-western South Atlantic Antarctic-proximal Sites 696 and U1536 and the subantarctic Site 1171 (south-western Pacific Ocean), with Site 1171 clumped-isotope (Δ47) 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 alkenone-based 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., 2023). This subtropical cooling is weaker than at the Antarctic-proximal sites, which indicates that the cooling was amplified at high latitudes. The clumped-isotope (Δ47) 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 south-western 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 south-western 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 δ18O (Figs. 7a and 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), paleoclimate sensitivity (1.5–4.5 ∘C per pCO2 doubling; Martínez-Botí et al., 2015) and polar amplification factors (2–3; Holland and Bitz, 2003), the 7 ∘C cooling in the south-western South Atlantic records (Sites 696 and U1536) could have been completely induced by pCO2 decline in the Middle Miocene. This also means that regional cooling was not amplified through strengthening of the ACC and destruction of the subpolar gyre, which occurred later in the Miocene (Evangelinos et al., 2022).

By the latest Miocene (∼ 6.4–5.2 Ma), temperatures at Site U1536 cooled to an average SST of 3.4 ∘C (n=8), which matches those at SHALLDRILL II for the earliest Pliocene (5.1–4.3 Ma, average 2.8 ∘C, n=3). The 2–5 ∘C SST decrease at Site U1536 between the MMCT (∼ 14 Ma) and the latest Miocene (∼ 6.4–5.2 Ma) is relatively 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 during this time interval because south-western South Atlantic surface waters (Site U1536) were already cold (5–7 ∘C) at the end of the MMCT (Fig. 7a) and thus could not cool as much as the subtropics. Instead, the remaining warmer part of the Southern Ocean experienced pronounced cooling during this time interval. Based on the Southern Ocean SST records (Fig. 7a), the south-western South Atlantic was already possibly the coldest region of the Southern Ocean 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 paint a full 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 conclude here that the south-western South Atlantic 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 south-western 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.

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 records (Site 696 and 511) cooled by ∼ 8 ∘C and subtropical records (Site 1090) cooled by ∼ 5 ∘C. The strong, large and persistent South 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 (Figs. 7 and 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 latest Miocene.

The cooling phases in the South Atlantic across the EOT and from the MCO to the MMCT represent two climatic transitional phases, both characterised 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 δ18O 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 the present-day ice sheet (Bohaty et al., 2012). The MCO is considered to be a global warm phase, with warm–temperate 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 south-western 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, to those for the MCO, when ice sheets were profoundly reduced. Keeping in mind the higher-than-modern Antarctic paleotopography in the Oligocene (Wilson and Luyendyk, 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.