RECENT results1,2 from two ice cores drilled in central Greenland have revealed large, abrupt climate changes of at least regional extent during the late stages of the last glaciation, suggesting that climate in the North Atlantic region is able to reorganize itself rapidly, perhaps even within a few decades. Here we present a detailed stable-isotope record for the full length of the Greenland Ice-core Project Summit ice core, extending over the past 250 kyr according to a calculated timescale. We find that climate instability was not confined to the last glaciation, but appears also to have been marked during the last interglacial (as explored more fully in a companion paper3) and during the previous Saale–Holstein glacial cycle. This is in contrast with the extreme stability of the Holocene, suggesting that recent climate stability may be the exception rather than the rule. The last interglacial seems to have lasted longer than is implied by the deep-sea SPECMAP record4, in agreement with other land-based observations5,6. We suggest that climate instability in the early part of the last interglacial may have delayed the melting of the Saalean ice sheets in America and Eurasia, perhaps accounting for this discrepancy.

Evidence for general instability of past climate from a 250-kyr ice-core record

OXYGEN isotope measurements in Greenland ice demonstrate that a series of rapid warm-cold oscillations—called Dansgaard–Oeschger events—punctuated the last glaciation1. Here we present records of sea surface temperature from North Atlantic sediments spanning the past 90 kyr which contain a series of rapid temperature oscillations closely matching those in the ice-core record, confirming predictions that the ocean must bear the imprint of the Dansgaard–Oeschger events2,3. Moreover, we show that between 20 and 80 kyr ago, the shifts in ocean-atmosphere temperature are bundled into cooling cycles, lasting on average 10 to 15 kyr, with asymmetrical saw-tooth shapes. Each cycle culminated in an enormous discharge of icebergs into the North Atlantic (a 'Hein-rich event'4,5), followed by an abrupt shift to a warmer climate. These cycles document a previously unrecognized link between ice sheet behaviour and ocean–atmosphere temperature changes. An important question that remains to be resolved is whether the cycles are driven by external factors, such as orbital forcing, or by inter-nal ice-sheet dynamics.

Correlations between climate records from North Atlantic sediments and Greenland ice

The most prominent Holocene climatic event in Greenland ice-core proxies, with approximately half the amplitude of the Younger Dryas, occurred ∼8000 to 8400 yr ago. This Holocene event affected regions well beyond the North Atlantic basin, as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane. Widespread proxy records from the tropics to the north polar regions show a short-lived cool, dry, or windy event of similar age. The spatial pattern of terrestrial and marine changes is similar to that of the Younger Dryas event, suggesting a role for North Atlantic thermohaline circulation. Possible forcings identified thus far for this Holocene event are small, consistent with recent model results indicating high sensitivity and strong linkages in the climatic system.

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Holocene climatic instability: A prominent, widespread event 8200 yr ago

A high-resolution, absolute-dated Holocene and deglacial Asian monsoon record from Dongge Cave, China

SEDIMENTS in the North Atlantic ocean contain a series of layers that are rich in ice-rafted debris and unusually poor in foraminifera1. Here we present evidence that the most recent six of these 'Heinrich layers', deposited between 14,000 and 70,000 years ago, record marked decreases in sea surface temperature and salinity, decreases in the flux of planktonic foraminifera to the sediments, and short-lived, massive discharges of icebergs originating in eastern Canada. The path of the icebergs, clearly marked by the presence of ice-rafted detrital carbonate, can be traced for more than 3,000 km—a remarkable distance, attesting to extreme cooling of surface waters and enormous amounts of drifting ice. The cause of these extreme events is puzzling. They may reflect repeated rapid advances of the Laurentide ice sheet, perhaps associated with reductions in air temperatures, yet temperature records from Greenland ice cores appear to exhibit only a weak corresponding signal. Moreover, the 5–10,000-yr intervals between the events are inconsistent with Milankovitch orbital periodicities, raising the question of what the ultimate cause of the postulated cooling may have been.

Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period

As shown by the work of Dansgaard and his colleagues, climate oscillations of one or so millennia duration punctuate much of glacial section of the Greenland ice cores. These oscillations are characterized by 5°C air temperature changes, severalfold dust content changes and 50 ppm CO2 changes. Both the temperature and CO2 change are best explained by changes in the mode of operation of the ocean. In this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50°N. Based on this finding, we suggest that the Greenland climate changes are driven by oscillations in the salinity of the Atlantic Ocean which modulate the strength of the Atlantic's conveyor circulation.

A salt oscillator in the glacial Atlantic? 1. The concept

Radiocarbon ages for benthic and planktonic foraminifera from the late glacial sections of two Atlantic and two Pacific cores are reported. The differences for benthic-planktonic pairs suggest that the radiocarbon age for deep Atlantic water was somewhat larger than today's (i.e., 600±250, as opposed to 400 years) and that the radiocarbon age for deep Pacific water was also slightly larger than today's (2100±400, as opposed to 1600, years). Our results suggest that during glacial time, the deep Pacific was, as it is today, significantly depleted in radiocarbon relative to the deep Atlantic. As many questions remain unanswered regarding the reliability of this approach, these conclusions must be considered to be preliminary.

Preliminary estimates for the radiocarbon age of deep water in the glacial ocean

Abrupt changes in climatic conditions have been seen at high latitudes in the North Atlantic1 and the Antarctic2,3 at 13 kyr BP. It is important to determine whether this abrupt change was confined to high-latitude regions or whether it was global. Here we present results demonstrating an abrupt change in the rate and character of sedimentation in the South China Sea at the close of the last glacial period. Radiocarbon dating and its position in the oxygen isotope shift suggest that this change may be coincident with the changes found at high latitudes.

New evidence from the South China Sea for an abrupt termination of the last glacial period

[RADIOCARBON, VOL 31, No. 2, 1989, P 157-162] ANOMALOUS AMS RADIOCARBON AGES FOR FORAMINIFERA FROM HIGH-DEPOSITION-RATE OCEAN SEDIMENTS WALLACE S BROECKER*, SUSAN TRUMBORE***, GEORGES BONANI**, WILLY WOLFLI** and MILLIE KLAS* ABSTRACT. Radiocarbon ages on handpicked foraminifera from deep-sea cores are reveal- ing that areas of rapid sediment accumulation are in some cases subject to hiatuses, reworking and perhaps secondary calcite deposition. We present here an extreme example of the impacts of such disturbances. The message is that if precise chronologies or meaningful benthic plank- tic age differences are to be obtained, then it is essential to document the reliability of radio- carbon ages by making both comparisons between coexisting species of planktomc forami- nifera and detailed down-core sequences of measurements. INTRODUCTION et al, 1988) we document In a previously published paper (Broecker 14C ages for G sacculifera and P that systematic differences exist between obliquiloculata from sediments in the South China Sea. In that paper, a case is made against three of the obvious causes for such differences, ie, calcifi- cation depth, abundance changes and dissolution effects. In an attempt to come to grips with this problem, we made measurements on planktonic foraminifera pairs from two other cores from this region: V24-135 from the Sulu Sea (7° 21' N, 120° 21' E, 4276m) and V33-88 from the western Pacific Ocean (2° 42' N, 127° 50' E, 3237m). Both cores have sufficiently high sedimentation rates 1 Ocm/l 03 yr) so that impacts associated with bioturbation are minimized. The results listed in Tables 1 and 2 were obtained on targets prepared by the cobalt catalyzed reduction method (Vogel et al, 1987) and measured ETH/AMS Facility in by accelerator mass spectrometry (AMS) at the Q14C difference has been Zurich, Switzerland. No correction for the air-sea made. To do this, 400 years should be subtracted. SULU SEA CORE V24-135 We chose core V24-135 because carbon and oxygen isotope records are available (Lindsley et al, 1985) allowing us to select glacial age horizons. Although nearby core V28-322 (7° 58' N, 120° 11' E, 4102m) is reported to contain mud turbidites, no mention is made of similar layers in V24-135 (Lindsley et al, 1985). Nor could we find evidence for such layers in our examination of the core. The results (see Table 1) came as such a shock that we immediately repicked and redated the two samples from the 101-102cm horizon. As agreement with the first set was achieved, we are convinced that the differ- ences are geological rather than experimental in origin. It is clear that neither abundance changes nor growth effects can account for the bizarre pattern of ages. As the foram shells are well pre- served and deposition rate is high, we doubt whether dissolution is the vil- lain, especially in a core with such a high deposition rate. One way to look at the results is to assume the process creating the age *Lamont-Doherty Geological Observatory of Columbia University Palisades, New York **Institut fur Mittelenergiephysik, ETH Honggerberg, 8093 Zurich, Switzerland

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Anomalous ams radiocarbon ages for foraminifera from high-deposition-rate ocean sediments

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Millie Klas

Papers

A salt oscillator in the glacial Atlantic? 1. The concept

Preliminary estimates for the radiocarbon age of deep water in the glacial ocean

New evidence from the South China Sea for an abrupt termination of the last glacial period

Anomalous ams radiocarbon ages for foraminifera from high-deposition-rate ocean sediments

The impact of calcite dissolution on radiocarbon dating of deep-sea sediments.