Interglacial Sea Surface Temperatures Research Articles

A Southern Portuguese Margin Perspective of Marine Isotope Stage 47—An Interglacial in the 41 kyr World

In order to better understand interglacial climate variability within the 41 kyr world, we produced high-resolution climate records for interglacial Marine Isotope Stage (MIS) 47 (1424–1452 ka) at IODP Site U1387 (36°48′ N, 7°43′ W) on the southern Portuguese margin. Using benthic and planktonic foraminifera stable isotope records, Uk’37 sea-surface temperature (SST), and plankton assemblage data we investigated Mediterranean Outflow Water (MOW) and surface water conditions. The MOW-level records indicate a poorly ventilated and sluggish bottom current during the MIS 48/MIS 47 transition in association with the insolation maximum, whereas a well-ventilated MOW formed a contourite layer during the second insolation maximum. The benthic δ18O record shows a fairly abrupt change during the deglaciation of MIS 48, while the surface waters experienced a terminal stadial event that was associated with initial cooling and freshening followed by stepwise warming until interglacial SST was reached at 1450 ka. Interglacial conditions with SST of 24 °C or higher persisted until 1427 ka, although warm SST prevailed into MIS 46. The persistent and prolonged warmth is attributed to a northward expansion of the subtropical gyre during MIS 47 as reflected by the dominance of subtropical-tropical planktonic foraminifera species and the presence of warm water coccolithophores taxa.

CO2‐Induced Decoupling of Tropical Surface and Thermocline Water Temperature at the Onset of Interglacials

AbstractTropical air‐sea interaction is important in global climate change; its behavior over geological history is poorly understood but can be explored by examining reconstructed sea surface temperature (SST) and thermocline water temperature (TWT). Here, deglacial‐interglacial air‐sea interactions over the past 500 ka were studied by comparing UK'37‐derived SST0‐30m and TEXH86‐derived TWT75m in a sediment core from the southern South China Sea. During deglacials, SST0‐30m and TWT75m varied synchronously toward interglacial peaks, while during the peak interglacials, TWT75m decreased earlier than SST0‐30m. These changes have been found ubiquitously in tropical oceans during the last two glacial‐interglacial cycles. We propose that the prolonged warm interglacial SST was probably sustained by the natural CO2 “overshoot” greenhouse effect at the end of most terminations. The early interglacial TWT decrease following local insolation is driven by the decreasing downward heat transport efficiency induced by weakening wind stirring.

Planktonic adaptive evolution to the sea surface temperature in the Neoproterozoic inferred from ancestral NDK of marine cyanobacteria

The optimum growth temperature of ancestral cyanobacteria inhabiting the sea surface in the Neoproterozoic was estimated based on the thermal stability of experimentally reconstructed ancestral NDK enzymes. Ancestral NDKs of cyanobacteria that diversified ∼1.7, ∼1.0, ∼0.9, ∼0.7, ∼0.6, and ∼0.5 billion years ago were reconstructed and analyzed, and the unfolding midpoint temperatures (Tms) ranged from ∼65°C to ∼70°C. Among the host of analyzed NDKs, the ancestors of marine, planktonic α-cyanobacteria diversified ≤ ∼1.0 Ga are highly likely to have inhabited marine environments during the Neoproterozoic, while ancestral cyanobacteria diversified ∼1.7 billion years ago were possibly marine but the habitat is less constrained compared to the others. According to the calibration curves derived from extant organisms, the obtained Tms of α-cyanobacteria diversified ≤ ∼1.0 Ga correspond to the range of optimum growth temperatures of around ∼33–48°C. The temperature range agrees well with the long-term sea temperature trend during Neoproterozoic suggested by δ18O and δ30Si records from marine cherts. Adaptation to the low temperature during the snowball glaciations in the late Neoproterozoic was not observed, implying that adaptation of optimum growth conditions to the episodic low temperature may not have been necessary. Therefore, ancestral marine plankton must have consistently adapted to the interglacial sea surface temperature in the Neoproterozoic, which was approximately 5–20°C higher than that is today. They may have survived the glaciations by acquiring cold tolerance and/or by suppressing growth rate.

Low Florida coral calcification rates in the Plio-Pleistocene

Abstract. In geological outcrops and drill cores from reef frameworks, the skeletons of scleractinian corals are usually leached and more or less completely transformed into sparry calcite because the highly porous skeletons formed of metastable aragonite (CaCO3) undergo rapid diagenetic alteration. Upon alteration, ghost structures of the distinct annual growth bands often allow for reconstructions of annual extension ( = growth) rates, but information on skeletal density needed for reconstructions of calcification rates is invariably lost. This report presents the bulk density, extension rates and calcification rates of fossil reef corals which underwent minor diagenetic alteration only. The corals derive from unlithified shallow water carbonates of the Florida platform (south-eastern USA), which formed during four interglacial sea level highstands dated approximately 3.2, 2.9, 1.8, and 1.2 Ma in the mid-Pliocene to early Pleistocene. With regard to the preservation, the coral skeletons display smooth growth surfaces with minor volumes of marine aragonite cement within intra-skeletal porosity. Within the skeletal structures, voids are commonly present along centres of calcification which lack secondary cements. Mean extension rates were 0.44 ± 0.19 cm yr−1 (range 0.16 to 0.86 cm yr−1), mean bulk density was 0.96 ± 0.36 g cm−3 (range 0.55 to 1.83 g cm−3) and calcification rates ranged from 0.18 to 0.82 g cm−2 yr−1 (mean 0.38 ± 0.16 g cm−2 yr−1), values which are 50 % of modern shallow-water reef corals. To understand the possible mechanisms behind these low calcification rates, we compared the fossil calcification rates with those of modern zooxanthellate corals (z corals) from the Western Atlantic (WA) and Indo-Pacific calibrated against sea surface temperature (SST). In the fossil data, we found a widely analogous relationship with SST in z corals from the WA, i.e. density increases and extension rate decreases with increasing SST, but over a significantly larger temperature window during the Plio-Pleistocene. With regard to the environment of coral growth, stable isotope proxy data from the fossil corals and the overall structure of the ancient shallow marine communities are consistent with a well-mixed, open marine environment similar to the present-day Florida Reef Tract, but variably affected by intermittent upwelling. Upwelling along the platform may explain low rates of reef coral calcification and inorganic cementation, but is too localised to account also for low extension rates of Pliocene z corals throughout the tropical WA region. Low aragonite saturation on a more global scale in response to rapid glacial–interglacial CO2 cyclicity is also a potential factor, but Plio-Pleistocene atmospheric pCO2 is generally believed to have been broadly similar to the present day. Heat stress related to globally high interglacial SST only episodically moderated by intermittent upwelling affecting the Florida platform seems to be another likely reason for low calcification rates. From these observations we suggest some present coral reef systems to be endangered from future ocean warming.

The last interglacial climate: comparing direct and indirect impacts of insolation changes

The last interglacial climate was influenced by substantial changes in the annual insolation cycle that led to a warmer climate state with pronounced high northern latitude warming. We analyze the impact of the insolation changes 125,000 years before present using an equilibrium snapshot simulation with the EC-Earth coupled climate model at high spatial resolution. Using additional atmosphere-only simulations, we separate the direct impact from the changed insolation from the secondary contribution from changed sea surface conditions. These simulations are forced with a combination of last interglacial sea surface temperatures and sea ice conditions and pre-industrial insolation, and vice versa. The coupled simulation yields an annual mean global warming of approximately 0.5 °C compared to pre-industrial conditions. While the warming over the continents follows the annual cycle of the insolation anomalies, two regions exhibit persistent responses throughout the year: The tropical region exhibits lower temperatures and stronger monsoonal systems, while the Arctic region shows a warming of more than 2 °C in all seasons. The hybrid simulations reveal that the changed sea surface conditions dominate the response at high northern latitudes, including the North Atlantic region and Europe, while the direct insolation impact is more dominant in the tropics.

Parallelisms between sea surface temperature changes in the western tropical Atlantic (Guiana Basin) and high latitude climate signals over the last 140 000 years

Abstract. Sea surface temperatures (SST) in the Guiana Basin over the last 140 ka were obtained by measuring the C37 alkenone unsaturation index Uk'37 in the sediment core MD03-2616 (7° N, 53° W). The resulting data set is unique in the western tropical Atlantic region for this period. The SSTs range from 25.1 to 28.9 °C, i.e. glacial–interglacial amplitude of 3.8 °C, which is in the range of change of other tropical areas. During the last two interglacial stages (marine isotope stages; MIS1 and MIS5e) and warm long interstadials (MIS5d-a), a rapid transmission of climate variability from Arctic–tropical latitudes is recorded. During these periods, the MD03-2616 SSTs show a conspicuous parallelism with temperature changes observed in Greenland and SST records of North Atlantic mid-latitude cores (Iberian Margin 38° N, Martrat et al., 2007). The last deglaciation in the Guiana Basin is particularly revealing. MIS2 stands out as the coldest period of the interval analysed. The events recorded in Guiana parallel northern latitude events such as the Bølling–Allerød warming and the Younger Dryas cooling which ensued. These oscillations were previously documented in the δ18O of the Sajama tropical ice core (Bolivia) and are present in Guiana, with rates of ca. 6 °C ka−1 and changes of over 2 °C. During the glacial interval, significant abrupt variability is observed, e.g. oscillations of 0.5–1.2 °C during MIS3, which is about 30 % of the maximum glacial–interglacial SST change. In the MD03-2616 record, it is possible to unambiguously identify either the Dansgaard–Oeschger oscillations described in northern latitudes or the SST drops associated with the Heinrich events characteristic of North Atlantic records. Although these events form the background of the climate variability observed, what truly shapes SSTs in the Guiana Basin is a long-term tropical response to precessional changes, which is modulated in the opposite way to Northern Hemisphere variability. This lack of synchrony is consistent with other tropical records in locations to the north or south of the Guiana Basin and evidences an Arctic–tropical decoupling when a substantial reduction in the Atlantic meridional overturning circulation (AMOC) takes place.

Similarities and dissimilarities between the last two deglaciations and interglaciations in the North Atlantic region

Sea surface temperatures (SSTs) recorded by alkenones and oxygen isotopes in the Alboran basin are used here to describe, at an unprecedented fine temporal resolution, the present interglaciation (PIG, initiated at 11.7 ka BP), the last interglaciation (LIG, onset approximately at 129 ka) and respective deglaciations. Similarities and dissimilarities in the progression of these periods are reviewed in comparison with ice cores and stalagmites. Cold spells coeval with the Heinrich events (H) described in the North Atlantic include multi-decadal scale oscillations not previously obvious (up to 4 °C in less than eight centuries within the stadials associated with H1 and H11, ca 133 ka and 17 ka respectively). These abrupt oscillations precede the accumulation of organic rich layers deposited when perihelion moves from alignment with NH spring equinox to the summer solstice, a reference for deglaciations. Events observed during the last deglaciation at 17 ka, 14.8 ka and 11.7 ka are reminiscent of events occurred during the penultimate deglaciation at ca 136 ka, 132 ka and 129 ka, respectively. The SST trend during the PIG is no more than 2 °C (from 20 °C to 18 °C; up to −0.2 °C/ka). The trend is steeper during the LIG, i.e. up to a 5 °C change from the early interglaciation to immediately before the glacial inception (from 23 °C to 18 °C; up to −0.4 °C/ka). Events are superimposed upon a long term trend towards colder SSTs, beginning with SST maxima followed by temperate periods until perihelion aligned with the NH autumn equinox (before ca 5.3 ka for the PIG and 121 ka for the LIG). A cold spell of around eight centuries at 2.8 ka during the PIG was possibly mimicked during the LIG at ca 118 ka by a SST fall of around 1 °C in a millennium. These events led interglacial SST to stabilise at around 18 °C. The glacial inception, barely evident at the beginning ca 115 ka (North Atlantic event C25, after perihelion passage in the NH winter solstice), culminated with a SST drop of at least 2 °C in two millennia (event C24, ca 111 ka). The Little Ice Age (0.7 ka) also occurred after the latest perihelion passage in the NH winter solstice and could be an example of how a glacial pre-inception event following an interglaciation might be.

Northern and southern hemisphere controls on seasonal sea surface temperatures in the Indian Ocean during the last deglaciation

Different proxies for sea surface temperature (SST) often exhibit divergent trends for deglacial warming in tropical regions, hampering our understanding of the phase relationship between tropical SSTs and continental ice volume at glacial terminations. To reconcile divergent SST trends, we report reconstructions of two commonly used paleothermometers (the foraminifera G. ruber Mg/Ca and the alkenone unsaturation index) from a marine sediment core collected in the southwestern tropical Indian Ocean encompassing the last 37,000 years. Our results show that SSTs derived from the alkenone unsaturation index (UK′37) are consistently warmer than those derived from Mg/Ca by ~2–3°C except for the Heinrich Event 1. In addition, the initial timing for the deglacial warming of alkenone SST started at ~15.6 ka, which lags behind that of Mg/Ca temperatures by 2.5 kyr. We argue that the discrepancy between the two SST proxies reflects seasonal differences between summer and winter rather than postdepositional processes or sedimentary biases. The UK′37 SST record clearly mimics the deglacial SST trend recorded in the North Atlantic region for the earlier part of the termination, indicating that the early deglacial warming trend attributed to local summer temperatures was likely mediated by changes in the Atlantic Meridional Overturning Circulation at the onset of the deglaciation. In contrast, the glacial to interglacial SST pattern recorded by G. ruber Mg/Ca probably reflects cold season SSTs. This indicates that the cold season SSTs was likely mediated by climate changes in the southern hemisphere, as it closely tracks the Antarctic timing of deglaciation. Therefore, our study reveals that the tropical southwestern Indian Ocean seasonal SST was closely linked to climate changes occurring in both hemispheres. The austral summer and winter recorded by each proxy is further supported with seasonal SST trends modeled by Atmosphere–ocean General Circulation Models for our core site. Our interpretation that the alkenone and Mg/Ca SSTs are seasonally biased may also explain similar proxy mismatches observed in other tropical regions at the onset of the last termination.

MIS 7 interglacial sea-surface temperature and salinity reconstructions from a southwestern subtropical Pacific coral

We generated a 5.5-yr snapshot of biweekly-to-monthly resolved time series of carbon and oxygen isotope composition (δ13C and δ18O) and Sr/Ca and Mg/Ca from annually banded aragonite skeleton of a ~197ka pristine Porites coral collected at Niue Island (19°00′S, 169°50′W) in the southwestern subtropical Pacific Ocean. This report is the first of a high-resolution coral-based paleoclimate archive during the Marine Isotope Stage (MIS) 7 interglacial. Statistical results suggest that annual averages of sea-surface temperature (SST) and salinity (SSS) at ~197ka were not significantly different from and ~1.2 higher than at present, respectively. Monthly mean variations showed increased SSS at ~197ka that was higher (1.4–1.9 relative to today) in the austral summer than in the austral winter. Monthly SST and SSS anomalies at ~197ka indicated smaller amplitudes by ~0.3°C (11%) and ~0.3 (24%) relative to the present, possibly suggesting less influence of interannual climate variability around Niue. Our results, taken together with other climate proxy records, imply seasonal and interannual modulation of thermal and hydrological conditions, different from today, in the southwestern subtropical Pacific Ocean associated with the Western Pacific Warm Pool and the South Pacific Convergence Zone variability during the MIS 7 interglacial.

Warm climate isotopic simulations: what do we learn about interglacial signals in Greenland ice cores?

Measurements of Last Interglacial stable water isotopes in ice cores show that central Greenland δ18O increased by at least 3‰ compared to present day. Attempting to quantify the Greenland interglacial temperature change from these ice core measurements rests on our ability to interpret the stable water isotope content of Greenland snow. Current orbitally driven interglacial simulations do not show δ18O or temperature rises of the correct magnitude, leading to difficulty in using only these experiments to inform our understanding of higher interglacial δ18O. Here, analysis of greenhouse gas warmed simulations from two isotope-enabled general circulation models, in conjunction with a set of Last Interglacial sea surface observations, indicates a possible explanation for the interglacial δ18O rise. A reduction in the winter time sea ice concentration around the northern half of Greenland, together with an increase in sea surface temperatures over the same region, is found to be sufficient to drive a >3‰ interglacial enrichment in central Greenland snow. Warm climate δ18O and δD in precipitation falling on Greenland are shown to be strongly influenced by local sea surface condition changes: local sea surface warming and a shrunken sea ice extent increase the proportion of water vapour from local (isotopically enriched) sources, compared to that from distal (isotopically depleted) sources. Precipitation intermittency changes, under warmer conditions, leads to geographical variability in the δ18O against temperature gradients across Greenland. Little sea surface warming around the northern areas of Greenland leads to low δ18O against temperature gradients (0.1–0.3‰ per °C), whilst large sea surface warmings in these regions leads to higher gradients (0.3–0.7‰ per °C). These gradients imply a wide possible range of present day to interglacial temperature increases (4 to >10 °C). Thus, we find that uncertainty about local interglacial sea surface conditions, rather than precipitation intermittency changes, may lead to the largest uncertainties in interpreting temperature from Greenland ice cores. We find that interglacial sea surface change observational records are currently insufficient to enable discrimination between these different δ18O against temperature gradients. In conclusion, further information on interglacial sea surface temperatures and sea ice changes around northern Greenland should indicate whether +5 °C during the Last Interglacial is sufficient to drive the observed ice core δ18O increase, or whether a larger temperature increases or ice sheet changes are also required to explain the ice core observations.

Sea surface temperatures of the western Arabian Sea during the last deglaciation

In this study we present a sea surface temperature (SST) record from the western Arabian Sea for the last 20,000 years. We produced centennial‐scale δ18O and Mg/Ca SST time series of core NIOP929 with focus on the glacial‐interglacial transition. The western Arabian Sea is influenced by the seasonal NE and SW monsoon wind systems. Lowest SSTs occur during the SW monsoon season because of upwelling of cold water, and highest SSTs can be found in the low‐productivity intermonsoon season. The Mg/Ca‐based temperature record reflects the integrated SST of the SW and NE monsoon seasons. The results show a glacial‐interglacial SST difference of ∼2°C, which is corroborated by findings from other Arabian Sea cores. At 19 ka B.P. a yet undescribed warm event of several hundred years duration is found, which is also reflected in the δ18O record. A second centennial‐scale high SST/low δ18O event is observed at 17 ka B.P. This event forms the onset of the stepwise yet persistent trend toward Holocene temperatures. Highest Mg/Ca‐derived SSTs in the NIOP929 record occurred between 13 and 10 ka B.P. Interglacial SST is ∼24°C, indicating influence of upwelling. The onset of Arabian Sea warming occurs when the North Atlantic is experiencing minimum temperatures. The rapid temperature variations at 19, 17, and 13 ka B.P. are difficult to explain with monsoon changes alone and are most likely also linked to regional hydrographic changes, such as trade wind induced variations in warm water advection.

Planktic foraminiferal and sea surface temperature record during the last 1 Myr across the Subtropical Front, Southwest Pacific

Planktic foraminiferal faunas and modern analogue technique estimates of sea surface temperature (SST) for the last 1 million years (Myr) are compared between core sites to the north (ODP 1125, 178 faunas) and south (DSDP 594, 374 faunas) of the present location of the Subtropical Front (STF), east of New Zealand. Faunas beneath cool subtropical water (STW) north of the STF are dominated by dextral Neogloboquadrina pachyderma, Globorotalia inflata, and Globigerina bulloides, whereas faunas to the south are strongly dominated by sinistral N. pachyderma (80–95% in glacials), with increased G. bulloides (20–50%) and dextral N. pachyderma (15–50%) in interglacials (beneath Subantarctic Water, or SAW). Canonical correspondence analysis indicates that at both sites, SST and related factors were the most important environmental influences on faunal composition. Greater climate-related faunal fluctuations occur in the south. Significant faunal changes occur through time at both sites, particularly towards the end of the mid-Pleistocene climate transition, MIS18–15 (e.g., decline of Globorotalia crassula in STW, disappearance of Globorotalia puncticulata in SAW), and during MIS8–5. Interglacial SST estimates in the north are similar to the present day throughout the last 1 Myr. To the south, interglacial SSTs are more variable with peaks 4–7 °C cooler than present through much of the early and middle Pleistocene, but in MIS11, MIS5.5, and early MIS1, peaks are estimated to have been 2–4 °C warmer than present. These high temperatures are attributed to southward spread of the STF across the submarine Chatham Rise, along which the STF appears to have been dynamically positioned throughout most of the last 1 Myr. For much of the last 1 Myr, glacial SST estimates in the north were only 1–2 °C cooler than the present interglacial, except in MIS16, MIS8, MIS6, and MIS4–2 when estimates are 4–7 °C cooler. These cooler temperatures are attributed to jetting of SAW through the Mernoo Saddle (across the Chatham Rise) and/or waning of the STW current. To the south, glacial SST estimates were consistently 10–11 °C cooler than present, similar to temperatures and faunas currently found in the vicinity of the Polar Front. One interpretation is that these cold temperatures reflect thermocline changes and increased Circumpolar Surface Water spinning off the Subantarctic Front as an enhanced Bounty Gyre along the south side of the Chatham Rise. For most of the last 1 Myr, the temperature gradient across the STF has been considerably greater than the present 4 °C. During glacial episodes, the STF in this region did not migrate northwards, but instead there was an intensification of the temperature gradient across it (interglacials 4–11 °C; glacials 8–14 °C).

Amplitude and timing of sea-surface temperature change in the northern South China Sea: Dynamic link to the East Asian monsoon

Magnesium/calcium (Mg/Ca) ratios of foraminiferal shells from a sediment core from the northern South China Sea, a semi-enclosed basin in the western tropical Pacific, document variations in sea-surface temperature (SST) during the past 145 k.y. Glacial SSTs were 4 °C colder than interglacial SSTs. During the last deglaciation, most of the warming was accomplished in a single abrupt step after continental ice-sheet decay had already begun, but warming and ice-sheet demise were nearly synchronous during the penultimate deglaciation. Abrupt SST changes of the past 15 k.y. were apparently synchronous with events in East Asian monsoon rainfall, suggesting that variations in monsoon winds and their influence on surface circulation of the western Pacific exerted a strong control on northern South China Sea SSTs. We suggest that this link persisted for the previous 130 k.y., during which time orbital-scale 2-3 °C SST changes and several small (≤2 °C) abrupt SST events occurred in the northern South China Sea. The similar timing of northern South China Sea SST, on a benthic δ 1 8 O time scale, to a well-dated speleothem record from eastern China suggests that the demise of ice sheets associated with the penultimate deglaciation did not precede Northern Hemisphere summer insolation increase. Our results suggest that surface waters had higher δ 1 8 O values during times of strong summer monsoon than during times of weak monsoon, likely reflecting a redistribution of 1 8 O-depleted rainfall from land during times of strong summer monsoons, to the western Pacific during times of weaker summer monsoons.

A High-Resolution Sea-Surface Temperature Record from the Tropical South China Sea (16,500–3000 yr B.P.)

The timing and magnitude of sea-surface temperature (SST) changes in the tropical southern South China Sea (SCS) during the last 16,500 years have been reconstructed on a high-resolution, 14C-dated sediment core using three different foraminiferal transfer functions (SIMMAX28, RAM, FP-12E) and geochemical (Uk′37) SST estimates. In agreement with CLIMAP reconstructions, both the FP-12E and the Uk′37 SST estimates show an average late glacial–interglacial SST difference of 2.0°C, whereas the RAM and SIMMAX28 foraminiferal transfer functions show only a minor (0.6°C) or no consistent late glacial–interglacial SST change, respectively. Both the Uk′37 and the FP-12E SST estimates, as well as the planktonic foraminiferal δ18O values, indicate an abrupt warming (ca. 1°C in <200 yr) at the end of the last glaciation, synchronous (within dating uncertainties) with the Bølling transition as recorded in the Greenland Ice Sheet Project 2 (GISP2) ice core, whereas the RAM-derived deglacial SST increase appears to lag during this event by ca. 500 yr. The similarity in abruptness and timing of the warming associated with the Bølling transition in Greenland and the southern SCS suggest a true synchrony of the Northern Hemisphere warming at the end of the last glaciation. In contrast to the foraminiferal transfer function estimates that do not indicate any consistent cooling associated with the Younger Dryas (YD) climate event in the tropical SCS, the Uk′37 SST estimates show a cooling of ca. 0.2–0.6°C compared to the Bølling–Allerød period. These Uk′37 SST estimates from the southern SCS argue in favor of a Northern Hemisphere-wide, synchronous cooling during the YD period.

North Atlantic sea-surface temperatures for the last 1.1 million years

Summary A 1.2 Ma time series of north-east Atlantic sea-surface temperature (SST) has been assembled from the spliced record of piston core K 708-7 (0.68-0 Ma) and hydraulic piston cores taken in nearby DSDP Hole 552A (1.2–0.68 Ma). A no-analogue fauna precludes making credible SST estimates in the interval 1.2–1.1 Ma, but the record above 1.1 Ma is suitable for SST estimates and related time-series analysis. The amplitude of SST variation is considerably higher in the Brunhes than in the upper Matuyama. This is due both to colder glacial SST minima after 0.85 Ma, and to increasingly warm interglacial SST maxima from 0.7 to 0.4 Ma. The dominant periodicity in the SST signal is centred near 95 000 years; it increases in amplitude by a factor of four from the bottom of the record to the top, with the largest increase occurring between 0.7 and 0.4 Ma. This suggests a relatively gradual evolution of ‘100 000-year’ power over half a million years of the late Quaternary, rather than a single abrupt change at 0.9 Ma. Significant, but progressively smaller, spectral peaks occur at periods of 54 000, 41 000, 31 000, 23 000 and 19 000 years. Both the 54 000-year and 31 000-year signals may be a response to insolation forcing by minor obliquity terms. Long-term δ 18 O records show small responses by the ice sheets at or near these two periods, indicating the probable initial response of the climate system to these insolation rhythms. In-phase relationships between δ 18 O and SST at these periods suggest that the ice sheets then impose these rhythms on the ocean with no lag. The mechanism by which such small insolation and ice-volume signals become enhanced in the SST record is not clear.