Christopher D. Charles

El Niño/Southern Oscillation and tropical Pacific climate during the last millennium

Any assessment of future climate change requires knowledge of the full range of natural variability in the El Niño/Southern Oscillation (ENSO) phenomenon. Here we splice together fossil-coral oxygen isotopic records from Palmyra Island in the tropical Pacific Ocean to provide 30–150-year windows of tropical Pacific climate variability within the last 1,100 years. The records indicate mean climate conditions in the central tropical Pacific ranging from relatively cool and dry during the tenth century to increasingly warmer and wetter climate in the twentieth century. But the corals also document a broad range of ENSO behaviour that correlates poorly with these estimates of mean climate. The most intense ENSO activity within the reconstruction occurred during the mid-seventeenth century. Taken together, the coral data imply that the majority of ENSO variability over the last millennium may have arisen from dynamics internal to the ENSO system itself.

Climate connections between the hemisphere revealed by deep sea sediment core/ice core correlations

Abstract Correlation of Southern Ocean deep sea sediment core records with ice core records of polar climate delineates with unprecedented detail the relationship between high latitude climate and the oceans thermohaline circulation over the last 80,000 years. Our observations suggest that, while North Atlantic Deep Water variability manifests itself clearly in Southern Ocean nutrient proxy records over periods as short as 500 yr, this deep water variability did not promote a direct link between climate variability in the high latitudes of the two hemispheres on millennial timescales. In particular, the proxy records indicate that, on average, northern hemisphere climate fluctuations lagged those of the southern hemisphere by 1500 yr.

The oxygen isotopic composition of seawater during the Last Glacial Maximum

High-resolution oxygen and hydrogen isotope measurements were made on pore fluids from deep-sea sediments from sites in the North and South Atlantic. The data provide direct measurements of changes in the isotopic composition of bottom waters during the Last Glacial Maximum (LGM). Results from Ocean Drilling Program (ODP) Site 981 in the North Atlantic, currently bathed in North Atlantic Deep Water (NADW) reproduces previous results from the Ceara and Bermuda Rises, constraining the glacial–interglacial change in δ^(18)O of the deep Atlantic to be 0.7–0.8‰. Results from Site 984, which is located north of Site 981 and at a shallower water depth, yield a similar value (0.8‰), providing insight into the properties of Glacial North Atlantic Intermediate Water (GNAIW). Sites from ODP Leg 177 in the South Atlantic span the modern boundary between northern and southern sources of deep water. Data from the northern site (1088) yield a similar result to sites in the tropical and North Atlantic (0.7‰). At the southern site (1093), located south of the polar front, the change is substantially larger (1.1‰), representing the change in δ^(18)O of southern source waters since the LGM. These results confirm previous estimates that the global average change in δ^(18)O of seawater is 1.0±0.1‰. Hydrogen isotopes measured on pore fluids from three sites are consistent with the oxygen isotopes from these locations, giving further support to these results. At all sites studied, the temperature of the deep ocean during the LGM, calculated by combining the pore fluid results with oxygen isotope data from benthic foraminifera, was within 1°C of the freezing point of seawater.

Glacial-Interglacial Changes in Moisture Sources for Greenland: Influences on the Ice Core Record of Climate

Large, abrupt shifts in the l8O/16O ratio found in Greenland ice must reflect real features of the climate system variability. These isotopic shifts can be viewed as a result of air temperature fluctuations, but determination of the cause of the changes—the most crucial issue for future climate concerns—requires a detailed understanding of the controls on isotopes in precipitation. Results from general circulation model experiments suggest that the sources of Greenland precipitation varied with different climate states, allowing dynamic atmospheric mechanisms for influencing the ice core isotope shifts.

Highly Variable El Niño–Southern Oscillation Throughout the Holocene

ENSO Variability The El Niño–Southern Oscillation (ENSO) is the most energetic, quasiperiodic climate oscillation in the world—every few years warming large expanses of the surface equatorial Pacific Ocean surface and impacting temperatures and rainfall patterns across the globe. A pressing question, in the context of global warming, is whether ENSO might be affected by the rising atmospheric temperatures caused by anthropogenic greenhouse gas emissions. Climate models do not agree on the answer to this question, but one place to look for data about how global temperatures might influence ENSO is the record of past ENSO variability. Cobb et al. (p. 67) present a record of ENSO variability spanning the past 7000 years, in an attempt better to define its response to insolation forcing over this same period. The findings reveal high variability in ENSO behavior that has no clear dependence on insolation, which implies that a link to warming, if it exists, may be difficult to detect. Coral records show that the El Niño–Southern Oscillation may be less sensitive to past climate forcing than previously thought. The El Niño–Southern Oscillation (ENSO) drives large changes in global climate patterns from year to year, yet its sensitivity to continued anthropogenic greenhouse forcing is uncertain. We analyzed fossil coral reconstructions of ENSO spanning the past 7000 years from the Northern Line Islands, located in the center of action for ENSO. The corals document highly variable ENSO activity, with no evidence for a systematic trend in ENSO variance, which is contrary to some models that exhibit a response to insolation forcing over this same period. Twentieth-century ENSO variance is significantly higher than average fossil coral ENSO variance but is not unprecedented. Our results suggest that forced changes in ENSO, whether natural or anthropogenic, may be difficult to detect against a background of large internal variability.

Evaluating climate indices and their geochemical proxies measured in corals

Abstract. Standard ocean/climate indices such as the Niño-3 sea surface temperature (SST) index, based on sparse instrumental data, and atmospheric indices such as the Southern Oscillation Index (SOI), may now be substituted and/or extended by coral-based indices. Several elements or their isotopes are incorporated in coral aragonitic skeletons at predictable concentrations, some of which are temperature or salinity dependent. The availability of century-old corals, at key oceanographic sites, permits the establishment of a network of proxy climate indices.

Biogenic opal in Southern Ocean sediments over the last 450,000 years : implications for surface water chemistry and circulation

We present records of biogenic opal percentage and burial rate in 12 piston cores from the Atlantic and Indian sectors of the Southern Ocean. These records provide a detailed, quantitative description of changing patterns of opal deposition over the last 450 kyr. The striking regional coherence of these records suggests that dissolution in the deep sea and sediment pore waters does not obscure the surface productivity signal, and therefore these opal time series can be used in combination with other surface water tracers to make inferences about the chemistry and circulation of the Southern Ocean under different global climate conditions. Three broad depositional patterns can be distinguished. Northernmost records (39°–42°S latitude) are characterized by enhanced opal burial during glacial periods and strong 41 kyr periodicity. Records from cores just north of the present Antarctic Polar Front (46°–49°S) show even larger increases in opal burial rate during glacial intervals, but have variance concentrated in the 100 and 23 kyr bands. Southernmost records (51°–55°S) are completely out of phase with those to the north, with greatly reduced opal burial rates during glacial periods. Taken as a whole, the opal records show no evidence for the increased total Antarctic productivity predicted by recent geochemical models of atmospheric CO2 variability. The areal expansion of Southern Ocean sea ice over the present zone of high siliceous productivity provides one plausible explanation for the glacial-interglacial opal patterns. The excess silica not taken up in this zone during glacial periods would contribute to greater nutrient availability and thus higher productivity in the subantarctic region. However, local circulation changes may act to modify this basic signal, possibly accounting for the observed differences in the opal variance spectra.

Origin of Global Meltwater Pulses

The fact that frequencies measured in climate records are the same as those predicted by the astronomical theory of climate change is undisputed (Hays, Imbrie & Shackleton 1976). However, the mechanisms by which these small changes in seasonal insolation are amplified into glacial cycles remain a fundamental mystery of the Earth’s climate system. The Barbados postglacial sea-level record is sufficiently detailed to resolve, for the first time, the rates as well as the magnitude of continental ice melting (Fairbanks 1989, 1990) (Fig 30.1A). The Barbados meltwater discharge curve is not smooth but pulsed, with peaks at 12,000 14C years1 and 9500 14C years (Fig 30.1B). Sea level rose more than 24 m during each of these pulses, with annual rates of sea-level rise exceeding 3 cm/yr. These enormous pulses must mark the ice-sheet response to a change in one or more of the climate amplifiers (eg, greenhouse gases and oceanic heat transports). The suspected amplifiers have different time constants and different regional sensitivities. Therefore, the discovery of both the pulsed deglaciation itself and the geographic origin of the pulses may help pinpoint the factors responsible for the timing of the large sea-level change associated with the last deglaciation, as well as the cause of previous “terminations” which recur every 100,000 14C years during the late Pleistocene Epoch (Broecker 1984).

Changes in the mode of Southern Ocean circulation over the last glacial cycle revealed by foraminiferal stable isotopic variability

Benthic foraminiferal oxygen and carbon isotopic records from Southern Ocean sediment cores show that during the last glacial period, the South Atlantic sector of the deep Southern Ocean filled to roughly 2500 m with water uniformly low in δ13C, resulting in the appearance of a strong mid-depth nutricline similar to those observed in glacial northern oceans. Concomitantly, deep water isotopic gradients developed between the Pacific and Atlantic sectors of the Southern Ocean; the δ13C of benthic foraminifera in Pacific sediments remained significantly higher than those in the Atlantic during the glacial episode. These two observations help to define the extent of what has become known as the ‘Southern Ocean low δ13C problem’. One explanation for this glacial distribution of δ13C calls upon surface productivity overprints or changes in the microhabitat of benthic foraminifera to lower glacial age δ13C values. We show here, however, that glacial–interglacial δ13C shifts are similarly large everywhere in the deep South Atlantic, regardless of productivity regime or sedimentary environment. Furthermore, the degree of isotopic decoupling between the Atlantic and Pacific basins is proportional to the magnitude of δ13C change in the Atlantic on all time scales. Thus, we conclude that the profoundly altered distribution of δ13C in the glacial Southern Ocean is most likely the result of deep ocean circulation changes. While the characteristics of the Southern Ocean δ13C records clearly point to reduced North Atlantic Deep Water input during glacial periods, the basinal differences suggest that the mode of Southern Ocean deep water formation must have been altered as well.

Glacial to Interglacial Changes in the Isotopic Gradients of Southern Ocean Surface Water

The Atlantic sector of the Southern Ocean is a region characterized by intense horizontal gradients in surface water properties, including the stable isotopic content. These gradients are climatically sensitive and can be exploited for paleoceanographic purposes, provided there is a means of recording their variability. Here we show that core top values of δ 18O and δ 13C in the planktic foraminiferal species Neoghboquadrina pachyderma l.c. follow those expected for calcite precipitated in equilibrium with surface waters. The entire equator-to-pole gradient in δ 18Ocalcite for surface waters is roughly 6‰, and the δ 18O of core top N. pachyderma records fully half of this gradient, increasing by 3‰ over a latitudinal range of 41° – 60°S. Meanwhile, the geographic pattern of δ 13O of core top N. pachyderma is similar to observed trends in δ 13C of surface ΣCO2. Highest values are recorded near the present Antarctic Polar Front (APF), where gas exchange rates are the highest; lower values occur both to the south, in the Weddell Gyre, and to the north, near the Subtropical Convergence. Thus, the isotopic composition of N. pachyderma from Quaternary sediments may serve as an effective tracer of the paleochemistry of Southern Ocean surface waters.