Lloyd D. Keigwin

Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes

The Atlantic meridional overturning circulation is widely believed to affect climate. Changes in ocean circulation have been inferred from records of the deep water chemical composition derived from sedimentary nutrient proxies, but their impact on climate is difficult to assess because such reconstructions provide insufficient constraints on the rate of overturning. Here we report measurements of 231Pa/230Th, a kinematic proxy for the meridional overturning circulation, in a sediment core from the subtropical North Atlantic Ocean. We find that the meridional overturning was nearly, or completely, eliminated during the coldest deglacial interval in the North Atlantic region, beginning with the catastrophic iceberg discharge Heinrich event H1, 17,500 yr ago, and declined sharply but briefly into the Younger Dryas cold event, about 12,700 yr ago. Following these cold events, the 231Pa/230Th record indicates that rapid accelerations of the meridional overturning circulation were concurrent with the two strongest regional warming events during deglaciation. These results confirm the significance of variations in the rate of the Atlantic meridional overturning circulation for abrupt climate changes.

Comparison of Atlantic and Pacific paleochemical records for the last 215,000 years: changes in deep ocean circulation and chemical inventories

Abstract Detailed Cd/Ca and δ 13 C data have been obtained for benthic foraminifera from western North Atlantic and Equatorial Pacific sediment cores. In the modern ocean, both tracers are closely linked to nutrient distributions. The sedimentary records for both tracers indicate that bottom waters overlying the Atlantic site have been nutrient-depleted relative to those at the Pacific site over the last 215,000 years. From this evidence it is reasonable to infer that there has been a continuous net flux of nutrient-depleted water from the western North Atlantic into the Pacific. This exchange has undergone significant fluctuations, with the export of nutrient-depleted Atlantic water diminishing by about a factor of two relative to the inflow from Southern Ocean sources. Over the last 215,000 years, carbon isotope fluctuations in both regions are dominated by variable storage of isotopically light carbon on continents with a lesser contribution from these deep ocean circulation changes. The cadmium signal in the North Atlantic is dominated by changes in deep ocean circulation patterns; cadmium shows less variability in the Pacific which may reflect changes in the global average cadmium content or minor changes in deep Pacific circulation patterns. Using these records to estimate global averages, it appears that glacial ocean water had 22% more Cd and 0.46‰ less 13 C than the modern ocean. These numbers are estimates which may be revised as more data become available, although they are not likely to be as much as 20% in error for Cd or 0.2‰ for 13 C. Relative North Atlantic Deep Water (NADW) formation rates are modulated with a significant 41 kyr periodicity linked to obliquity-induced variations in high latitude insolation; NADW lags 8 ± 2 kyr behind insolation, however.

The Little Ice Age and Medieval Warm Period in the Sargasso Sea

Sea surface temperature (SST), salinity, and flux of terrigenous material oscillated on millennial time scales in the Pleistocene North Atlantic, but there are few records of Holocene variability. Because of high rates of sediment accumulation, Holocene oscillations are well documented in the northern Sargasso Sea. Results from a radiocarbon-dated box core show that SST was approximately 1°C cooler than today approximately 400 years ago (the Little Ice Age) and 1700 years ago, and approximately 1°C warmer than today 1000 years ago (the Medieval Warm Period). Thus, at least some of the warming since the Little Ice Age appears to be part of a natural oscillation.

Deep Circulation of the North Atlantic over the Last 200,000 Years: Geochemical Evidence

Variations in the cadmium/calcium ratio of North Atlantic Deep Water are recorded in the fossil shells of benthic foraminifera. The oceanic distribution of cadmium is similar to that of the nutrients, hence the cadmium/calcium ratio in shells records temporal variations in nutrient distributions. Data from a North Atlantic sediment core show that over the past 200,000 years there has been a continuous supply of nutrient-depleted waters into the deep North Atlantic. The intensity of this source relative to nutrient-enriched southern waters diminished by about a factor of 2 during severe glaciations. This evidence combined with carbon isotope data indicates that the continental carbon inventory may have been less variable than previously suggested.

Isotopic paleoceanography of the Caribbean and East pacific: role of panama uplift in late neogene time.

Comparisons of carbon isotopic data on benthic foraminifera from Deep Sea Drilling Project sites 502 (western Caribbean) and 503 (eastern Pacific) indicate that the difference between the Atlantic and the Pacific in the per mil enrichment in carbon-13 of total dissolved carbon dioxide increased about 6 million years ago and again 3 million years ago, when the difference reached the modern level (1 per mil). Comparisons of planktonic foraminiferal oxygen isotopic data for the Caribbean and the Pacific suggest that the salinity of Caribbean surface waters began increasing 4 million years ago, possibly in response to shoaling of the Panama isthmus. These results suggest that modern circulation patterns in the Caribbean and eastern Pacific developed by 3 million years ago in concert with changing tectonic, climatic, and biogeographic patterns.

The last interglacial ocean

The final effort of the CLIMAP project was a study of the last interglaciation, a time of minimum ice volume some 122,000 yr ago coincident with the Substage 5e oxygen isotopic minimum. Based on detailed oxygen isotope analyses and biotic census counts in 52 cores across the world ocean, last interglacial sea-surface temperatures (SST) were compared with those today. There are small SST departures in the mid-latitude North Atlantic (warmer) and the Gulf of Mexico (cooler). The eastern boundary currents of the South Atlantic and Pacific oceans are marked by large SST anomalies in individual cores, but their interpretations are precluded by no-analog problems and by discordancies among estimates from different biotic groups. In general, the last interglacial ocean was not significantly different from the modern ocean. The relative sequencing of ice decay versus oceanic warming on the Stage 6/5 oxygen isotopic transition and of ice growth versus oceanic cooling on the Stage 5e/5d transition was also studied. In most of the Southern Hemisphere, the oceanic response marked by the biotic census counts preceded (led) the global ice-volume response marked by the oxygen-isotope signal by several thousand years. The reverse pattern is evident in the North Atlantic Ocean and the Gulf of Mexico, where the oceanic response lagged that of global ice volume by several thousand years. As a result, the very warm temperatures associated with the last interglaciation were regionally diachronous by several thousand years. These regional lead-lag relationships agree with those observed on other transitions and in long-term phase relationships; they cannot be explained simply as artifacts of bioturbational translations of the original signals.

Glacial‐age hydrography of the far northwest Pacific Ocean

Stable isotope data on benthic foraminifera from more than 30 cores on the northern Emperor Seamounts and in the Okhotsk Sea are synthesized in paleohydrographic profiles spanning the depth range 1000–4000 m. Holocene (core-top) benthic foraminiferal δ18O and δ13C data are calibrated to modern hydrographic properties through measurements of δ13C of ΣCO2 and δ18O of seawater. Cibicidoides stable isotope ratios are close to the δ13C and equilibrium δ18O of seawater, whereas Uvigerina δ18O and δ13C are variably offset from Cibicidoides. Glacial maximum δ13C of Cibicidoides displays a different vertical profile than that of the Holocene. When results are adjusted by +0.32‰ to account for the secular change in δ13C during the last glacial maximum, the data coincide with the modern seawater and foraminiferal curves deeper than ∼2 km. However, at shallower depths δ13C gradually increases by as much as 1‰ above the modern value. Furthermore, above 2 km the benthic δ18O decreases by ∼0.5‰. These results are consistent with a benthic front at ∼2 km in the North Pacific [see Herguera et al., 1992], but they differ from interpretations based on trace metal data which indicate a source of nutrient-depleted deep water during glaciation. The isotopic data suggest that during glaciation there was a better ventilated watermass at intermediate depths in the far northwestern Pacific, it was relatively fresher than deep waters there, and deep waters were as nutrient-rich as today.

Radiocarbon Variability in the Western North Atlantic During the Last Deglaciation

We present a detailed history of glacial to Holocene radiocarbon in the deep western North Atlantic from deep-sea corals and paired benthic-planktonic foraminifera. The deglaciation is marked by switches between radiocarbon-enriched and -depleted waters, leading to large radiocarbon gradients in the water column. These changes played an important role in modulating atmospheric radiocarbon. The deep-ocean record supports the notion of a bipolar seesaw with increased Northern-source deep-water formation linked to Northern Hemisphere warming and the reverse. In contrast, the more frequent radiocarbon variations in the intermediate/deep ocean are associated with roughly synchronous changes at the poles.

Deglacial Meltwater Discharge, North Atlantic Deep Circulation, and Abrupt Climate Change

High-resolution paleogeochemical data from the North Atlantic Ocean indicate that in the interval 15,000 to 10,000 14C years before present (B.P.) North Atlantic Deep Water (NADW) production was decreased or eliminated four times: at about 14,500 (and probably older), 13,500, 12,000 and 10,500 years B.P. Each of these changes occurred at the same time as abrupt events of meltwater discharge to the surface ocean (inferred from oxygen isotope studies of planktonic foraminifera and from glacial geological studies on land). In addition, each of these times may be associated with brief episodes of cooler climate in the North Atlantic region, the best example of which is the Younger Dryas cooling of 10,500 years ago. These results support models linking meltwater discharge, decreased NADW production, and decreased North Atlantic heat flux.

Variability of the North Atlantic thermohaline circulation during the last interglacial period

Studies of natural climate variability are essential for evaluating its future evolution. Greenland ice cores suggest that the modern warm period (the Holocene) has been relatively stable for the past 9,000 years. Much less is known about other warm interglacial periods, which comprise less than 10% of the climate record during the past 2.5 million years. Here we present high-resolution ocean sediment records of surface and deep-water variables from the Bermuda Rise spanning the last interglacial period, about 118,000–127,000 years ago. In general, deep-water chemical changes are coincident with transitions in surface climate at this site. The records do not show any substantial fluctuations relative to the much higher variability observed during the preceding and subsequent cool climates. The relatively stable interglacial period begins and ends with abrupt changes in deep-water flow. We estimate, using 230Th measurements to constrain the chronology, that transitions occur in less than 400 years.