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Evolution of Ocean Temperature and Ice Volume Through the Mid RESEARCH ARTICLE iniferal calcite extends from marine isotope stage (MIS) 50, 1550 thousand years ago (ka), to the Holocene (Fig. 1A). It shows features in com- Evolution of Ocean Temperature mon with the LR04 stack with stable interglacial values (except for the anomalous interglacials MIS 5, 9, and 11) and lower glacial values be- and Ice Volume Through the fore MIS 22. There are, however, differences between our Mid-Pleistocene Climate Transition d18O record and the LR04 stack, in particular over the period thought to encompass the MPT. Ben- 18 H. Elderfield,* P. Ferretti,† M. Greaves, S. Crowhurst, I. N. McCave, D. Hodell, A. M. Piotrowski thic d OC shows an abrupt increase from about 950 ka to 870 ka, whereas LR04 shows a more Earth’s climate underwent a fundamental change between 1250 and 700 thousand years ago, gradual increase to values 0.5 per mil (‰) lighter the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from than for ODP 1123. This is not an artifact of ben- 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing. Over this thic species analyzed, because we have veri- time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic fied this observation by measuring d18Oalsoon ratios, traditionally interpretedasdefiningthemainrhythmofice ages although containing large the epibenthic species Cibicidoides wuellerstorfi effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature (fig. S3A). and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was What is the importance of this difference? initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence LR04 is thought to represent a global average from of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum. which regional variability has been removed. Ne- vertheless, LR04 is not an area-weighted stack he deep-ocean sediment oxygen isotopic a low-resolution benthic temperature record for and is biased to the Atlantic Ocean and eastern 18 composition record (d OC), commonly the past 440 thousand years (ky) (15). We now equatorial Pacific. Only one site samples the ma- Tconsidered to be a record of global ice have generated a 1.5-million-year record of deep- jor flow to over half the volume of the world volume and thus a chronicle of ice ages, also is sea temperature and sea level with a resolution ocean and that is ODP 1123. The LR04 stack is “heavily contaminated” (1, 2) by the effect of (≤1000 ky) sufficient to define their behavior over widely used, but to discuss the implications for on September 24, 2012 deep-water temperature variability. The contribu- the period of the MPT, including a comparison changing global ice volume we need to delve 18 tions of ice volume and temperature to d OC with the climate of the last 800 ky derived from further. Because of the possible bias in the stack, have been debated for more than 40 years [e.g., ice-core records (16) and information on changes it makes more sense to compare our site 1123 (1–5)], and definition of the trajectories that each in ocean carbon chemistry. record with a few other high-quality records from followed during the mid-Pleistocene transition The site studied—Ocean Drilling Program (ODP) the world oceans. The more abrupt change in 18 (MPT) has been elusive. Here, we present a high- leg 181, site 1123—is located on the Chatham Rise, d OC is seen in some records but not others (fig. resolution record of deep-water temperature, which east of New Zealand at 41°47.15′S, 171°29.94′W, S3, B to D). Some records show a stepped in- 18 we constructed based on foraminiferal Mg/Ca at a depth of 3290 m (fig. S2). It lies under the crease in d OC over the period 1000 to 600 ka. 18 paleothermometry (see supplementary text and Deep Western Boundary Current (DWBC) that If interpreted as ice volume, d OC shows that www.sciencemag.org fig. S1). Only one previous deep-sea temperature feeds the Pacific and comprises Lower Circum- the first large ice sheet was at MIS 16 in the record extending over the MPT and obtained in- polar Deep Water (CDW), which forms from dense Atlantic but at MIS 22 in the Pacific, as indicated dependently of oxygen isotopes exists, and this waters sinking around Antarctica, particularly by ODP 1123. It is likely that the difference of is at orbital-scale resolution from the North At- cold waters from the Weddell and Ross seas and individual sites from each other and from LR04 lantic using Mg/Ca ratios from benthic forami- Adelie Coast. The modern hydrography of the reflects changes in hydrography, smoothed in nifera (6). However, it has been recognized site shows a slight contribution from North At- the stack. 18 recently that Mg/Ca ratios of epifaunal benthic lantic Deep Water (NADW) expressed by a small The benthic d OC record has a temperature foraminifera are affected by carbonate saturation salinity anomaly marker (17). More than half of component and a seawater d18O component, and Downloaded from effects (7, 8), requiring a “regional calibration,” the flux of cold bottom water entering the major the latter may reflect a combination of a global and, consequently, the reliability of this approach basins of the world ocean does so through the ice volume signal and a local hydrographic signal has been both challenged (9) and defended (10). Southwest Pacific Ocean, as the Pacific DWBC relating, for example, to changes in proportions The increasing interest in applying inverse mod- (17, 18). CDW is drawn to the surface by up- of colder low d18O AABW versus warmer high els to deconvolve temperature and ice volume in welling with descending return flows south of the d18O NADW. If our signal may reflect “local” climate records [e.g., (11–14)] adds to the interest upwelling as Antarctic Bottom Water (AABW) hydrographic changes, those changes are related in separating these signals from the oceanic and north of it as Antarctic Intermediate Water. to Antarctica because deep-water masses at site sediment record using geochemical methods. The connection of this site to the regions where 1123 originate from waters sinking around Ant- Previously, we carried out a calibration study deep Antarctic water masses form enables com- arctica. Detailed records in addition to those in using the shallow-infaunal benthic foraminifera parison with Southern Ocean and Antarctic atmo- Fig. 1 at key hydrographic locations would be Uvigerina spp., showing that it is little affected by spheric records and is important for determining required to fully assess variability in the global bottom-water carbonate saturation, and generated the role of the deep ocean in climate evolution. seawater d18O signal. Our assessment is that the 18 Benthic d OC, deep-sea temperature, and hydrographic component at the site of 1123 is 18 Godwin Laboratory for Palaeoclimate Research, Department Antarctic temperature. We determined d OC and small: (i) benthic isotopic depth profiles at the of Earth Sciences, University of Cambridge, Downing Street, Mg/Ca on 1650 and 1485 samples of Uvigerina site of ODP 123 are related to the structure of Cambridge CB2 3EQ, UK. spp. (supplementary text, table S1, and Fig. 1). Data water masses at present and inferred for the past, *To whom correspondence should be addressed. E-mail: are also archived at doi:10.1594/PANGAEA.786205 with no apparent changes in the depths of water- [email protected] d18 † The O data were correlated to an orbitally tuned mass boundaries between glacial and intergla- Present address: Consiglio Nazionale delle Ricerche, Istituto 18 per la Dinamica dei Processi Ambientali (CNR-IDPA), Calle Larga benthic d Ostack,LR04(19), to provide an ini- cial states (17); (ii) estimates of oxygen isotopic 18 18 Santa Marta 2137, Venice I-30123, Italy. tial time scale. The d Orecordofbenthicforam- composition of seawater (d OW) from corals and 704 10 AUGUST 2012 VOL 337 SCIENCE www.sciencemag.org RESEARCH ARTICLE pore water modeling discussed below show global arctica from ice cores (16) (Fig. 1B). For this Glacial-interglacial model comparisons (23) values. Ideally, what is needed is a global array comparison, we converted Mg/Ca temperature suggest that Antarctic temperature changes rep- of seawater d18O records that could be stacked data to units of standard deviation and transposed resent global-scale temperature changes with a po- to produce a mean ocean d18Osignal. themtotheEDC3(EpicaDomeCicecore)time lar amplification factor of ~2: glacial-interglacial There is also considerable millennial and sub- scale (20). The pattern seen in the ice-core dD temperature change (Antarctic/Global) = 2.07. millennial scale variability in the records, more (deuterium isotope-based temperature) record is The comparison of Antarctic and our deep-sea 18 in Mg/Ca than d OC (supplementary materials matched by the Mg/Ca–based deep-sea tempera- temperature data shows the same amplification andFig.1,AandB).Toaidincomparison,ox- ture: (i) glacial temperatures are similar throughout (Antarctic/deep sea) = 2.1 T 0.08 (fig. S5). There- ygen isotope and Mg/Ca data have been smoothed this deep-sea record, (ii) interglacial temperatures fore, it seems that the temperature of the bottom in the time domain using a Gaussian filter hav- are cooler prior to 450 ka, and (iii) interglacials water reflects the air temperature at southern high ing a total width of 5 ky. The range in Mg/Ca of 5.5, 7.5, 9.3, and 11.3 are exceptionally warmer.
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