Heiko Pälike

Centennial-scale climate cooling with a sudden cold event around 8,200 years ago

The extent of climate variability during the current interglacial period, the Holocene, is still debated. Temperature records derived from central Greenland ice cores show one significant temperature anomaly between 8,200 and 8,100 years ago, which is often attributed to a meltwater outflow into the North Atlantic Ocean and a slowdown of North Atlantic Deep Water formation—this anomaly provides an opportunity to study such processes with relevance to present-day freshening of the North Atlantic. Anomalies in climate proxy records from locations around the globe are often correlated with this sharp event in Greenland. But the anomalies in many of these records span 400 to 600 years, start from about 8,600 years ago and form part of a repeating pattern within the Holocene. More sudden climate changes around 8,200 years ago appear superimposed on this longer-term cooling. The compounded nature of the signals implies that far-field climate anomalies around 8,200 years ago cannot be used in a straightforward manner to assess the impact of a slowdown of North Atlantic Deep Water formation, and the geographical extent of the rapid cooling event 8,200 years ago remains to be determined.

Rapid stepwise onset of Antarctic glaciation and deeper calcite compensation in the Pacific Ocean.

The ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution is termed the calcite compensation depth. At present, this depth is ∼4,500 m, with some variation between and within ocean basins. The calcite compensation depth is linked to ocean acidity, which is in turn linked to atmospheric carbon dioxide concentrations and hence global climate. Geological records of changes in the calcite compensation depth show a prominent deepening of more than 1 km near the Eocene/Oligocene boundary (∼ 34 million years ago) when significant permanent ice sheets first appeared on Antarctica, but the relationship between these two events is poorly understood. Here we present ocean sediment records of calcium carbonate content as well as carbon and oxygen isotopic compositions from the tropical Pacific Ocean that cover the Eocene/Oligocene boundary. We find that the deepening of the calcite compensation depth was more rapid than previously documented and occurred in two jumps of about 40,000 years each, synchronous with the stepwise onset of Antarctic ice-sheet growth. The glaciation was initiated, after climatic preconditioning, by an interval when the Earths orbit of the Sun favoured cool summers. The changes in oxygen-isotope composition across the Eocene/Oligocene boundary are too large to be explained by Antarctic ice-sheet growth alone and must therefore also indicate contemporaneous global cooling and/or Northern Hemisphere glaciation.

The Cenozoic palaeoenvironment of the Arctic Ocean

The history of the Arctic Ocean during the Cenozoic era (0–65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm ‘greenhouse’ world, during the late Palaeocene and early Eocene epochs, to a colder ‘icehouse’ world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ∼14 Myr, we find sedimentation rates of 1–2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (∼3.2 Myr ago) and East Antarctic ice (∼14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (∼45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at ∼49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (∼55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change.

The Heartbeat of the Oligocene Climate System

A 13-million-year continuous record of Oligocene climate from the equatorial Pacific reveals a pronounced “heartbeat” in the global carbon cycle and periodicity of glaciations. This heartbeat consists of 405,000-, 127,000-, and 96,000-year eccentricity cycles and 1.2-million-year obliquity cycles in periodically recurring glacial and carbon cycle events. That climate system response to intricate orbital variations suggests a fundamental interaction of the carbon cycle, solar forcing, and glacial events. Box modeling shows that the interaction of the carbon cycle and solar forcing modulates deep ocean acidity as well as the production and burial of global biomass. The pronounced 405,000-year eccentricity cycle is amplified by the long residence time of carbon in the oceans.

Escape of methane gas from the seabed along the West Spitsbergen continental margin

More than 250 plumes of gas bubbles have been discovered emanating from the seabed of the West Spitsbergen continental margin, in a depth range of 150-400 m, at and above the present upper limit of the gas hydrate stability zone (GHSZ). Some of the plumes extend upward to within 50 m of the sea surface. The gas is predominantly methane. Warming of the northward-flowing West Spitsbergen current by 1°C over the last thirty years is likely to have increased the release of methane from the seabed by reducing the extent of the GHSZ, causing the liberation of methane from decomposing hydrate. If this process becomes widespread along Arctic continental margins, tens of Teragrams of methane per year could be released into the ocean.

Thresholds for Cenozoic bipolar glaciation.

The long-standing view of Earth’s Cenozoic glacial history calls for the first continental-scale glaciation of Antarctica in the earliest Oligocene epoch (∼33.6 million years ago), followed by the onset of northern-hemispheric glacial cycles in the late Pliocene epoch, about 31 million years later. The pivotal early Oligocene event is characterized by a rapid shift of 1.5 parts per thousand in deep-sea benthic oxygen-isotope values (Oi-1) within a few hundred thousand years, reflecting a combination of terrestrial ice growth and deep-sea cooling. The apparent absence of contemporaneous cooling in deep-sea Mg/Ca records, however, has been argued to reflect the growth of more ice than can be accommodated on Antarctica; this, combined with new evidence of continental cooling and ice-rafted debris in the Northern Hemisphere during this period, raises the possibility that Oi-1 represents a precursory bipolar glaciation. Here we test this hypothesis using an isotope-capable global climate/ice-sheet model that accommodates both the long-term decline of Cenozoic atmospheric CO2 levels and the effects of orbital forcing. We show that the CO2 threshold below which glaciation occurs in the Northern Hemisphere (∼280 p.p.m.v.) is much lower than that for Antarctica (∼750 p.p.m.v.). Therefore, the growth of ice sheets in the Northern Hemisphere immediately following Antarctic glaciation would have required rapid CO2 drawdown within the Oi-1 timeframe, to levels lower than those estimated by geochemical proxies and carbon-cycle models. Instead of bipolar glaciation, we find that Oi-1 is best explained by Antarctic glaciation alone, combined with deep-sea cooling of up to 4 °C and Antarctic ice that is less isotopically depleted (-30 to -35‰) than previously suggested. Proxy CO2 estimates remain above our model’s northern-hemispheric glaciation threshold of ∼280 p.p.m.v. until ∼25 Myr ago, but have been near or below that level ever since. This implies that episodic northern-hemispheric ice sheets have been possible some 20 million years earlier than currently assumed (although still much later than Oi-1) and could explain some of the variability in Miocene sea-level records.

A Cenozoic record of the equatorial Pacific carbonate compensation depth

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

Oligocene climate dynamics

A planktonic and benthic foraminiferal stable isotope stratigraphy of the Oligocene equatorial Pacific (Ocean Drilling Program, Site 1218) was generated at 6 kyr resolution between magnetochrons C9n and C11n.2n (~26.4–30 Ma on a newly developed astronomically calibrated timescale). Our data allow a detailed examination of Oligocene paleoceanography, the evolution of the early cryosphere, and the influence of orbital forcing on glacioeustatic sea level variations. Spectral analysis reveals power and coherency for obliquity (40 kyr period) and eccentricity (~110, 405 kyr) orbital bands, with an additional strong imprint of the eccentricity and 1.2 Myr obliquity amplitude cycle, driving ice sheet oscillations in the Southern Hemisphere. Planktonic and benthic foraminifera ?18O are used to constrain the magnitude and timing of major fluctuations in ice volume and global sea level change. Glacial episodes, related to obliquity and eccentricity variations, occurred at 29.16, 27.91, and 26.76 Ma, corresponding to glacioeustatic sea level fluctuations of 50–65 m. Alteration of high-latitude temperatures and Antarctic ice volume had a significant impact on the global carbon burial and equatorial productivity, as cyclic variations are also recorded in the carbon isotope signal of planktonic and benthic foraminifera, the water column carbon isotope gradient, and estimated percent carbonate of bulk sediment. We also investigate the implications of a close correspondence between oxygen and carbon isotope events and long-term amplitude envelope extrema in astronomical calculations during the Oligocene, and develop a new naming scheme for stable isotope events, on the basis of the 405 kyr eccentricity cycle count.

Age model and core‐seismic integration for the Cenozoic Arctic Coring Expedition sediments from the Lomonosov Ridge

Cenozoic biostratigraphic, cosmogenic isotope, magnetostratigraphic, and cyclostratigraphic data derived from Integrated Ocean Drilling Program Expedition 302, the Arctic Coring Expedition (ACEX), are merged into a coherent age model. This age model has low resolution because of poor core recovery, limited availability of biostratigraphic information, and the complex nature of the magnetostratigraphic record. One 2.2 Ma long hiatus occurs in the late Miocene; another spans 26 Ma (18.2–44.4 Ma). The average sedimentation rate in the recovered Cenozoic sediments is about 15 m/Ma. Core-seismic correlation links the ACEX sediments to the reflection seismic stratigraphy of line AWI-91090, on which the ACEX sites were drilled. This seismostratigraphy can be correlated over wide geographic areas in the central Arctic Ocean, implying that the ACEX age model can be extended well beyond the drill sites.

Eocene global warming events driven by ventilation of oceanic dissolved organic carbon

‘Hyperthermals’ are intervals of rapid, pronounced global warming known from six episodes within the Palaeocene and Eocene epochs (∼65–34 million years (Myr) ago). The most extreme hyperthermal was the ∼170 thousand year (kyr) interval of 5–7 °C global warming during the Palaeocene–Eocene Thermal Maximum (PETM, 56 Myr ago). The PETM is widely attributed to massive release of greenhouse gases from buried sedimentary carbon reservoirs, and other, comparatively modest, hyperthermals have also been linked to the release of sedimentary carbon. Here we show, using new 2.4-Myr-long Eocene deep ocean records, that the comparatively modest hyperthermals are much more numerous than previously documented, paced by the eccentricity of Earth’s orbit and have shorter durations (∼40 kyr) and more rapid recovery phases than the PETM. These findings point to the operation of fundamentally different forcing and feedback mechanisms than for the PETM, involving redistribution of carbon among Earth’s readily exchangeable surface reservoirs rather than carbon exhumation from, and subsequent burial back into, the sedimentary reservoir. Specifically, we interpret our records to indicate repeated, large-scale releases of dissolved organic carbon (at least 1,600 gigatonnes) from the ocean by ventilation (strengthened oxidation) of the ocean interior. The rapid recovery of the carbon cycle following each Eocene hyperthermal strongly suggests that carbon was re-sequestered by the ocean, rather than the much slower process of silicate rock weathering proposed for the PETM. Our findings suggest that these pronounced climate warming events were driven not by repeated releases of carbon from buried sedimentary sources, but, rather, by patterns of surficial carbon redistribution familiar from younger intervals of Earth history.