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Changes in ocean circulation have been proposed as a trigger mechanism for the large coupled climate and carbon cycle perturbations at the Paleocene-Eocene Thermal Maximum (PETM, ca. 55 Ma). An abrupt warming of oceanic intermediate waters could have initiated the thermal destabilization of sediment-hosted methane gas hydrates and potentially triggered sediment slumps and slides. In an ensemble of fully coupled atmosphere-ocean general circulation model (AOGCM) simulations of the late Paleocene and early Eocene, we identify such a circulation-driven enhanced intermediate-water warming. Critically, we find an approximate twofold amplification of Atlantic intermediate-water warming when C[O.sub.2] levels are doubled from 2x to 4x preindustrial C[O.sub.2] compared to when they are doubled from 1x to 2x. This warming is largely focused on the equatorial and South Atlantic and is driven by a significant reduction in deep-water formation from the Southern Ocean. This scenario is consistent with altered PETM circulation patterns inferred from benthic carbon isotope data and the intensity of deep-sea carbonate dissolution in the South Atlantic. The linkage between intermediate-water warming and gas hydrate destabilization could provide an important feedback in the establishment of peak PETM warmth. doi: 10.1130/G31184.1

The evidence based on the pulsed climate variability hypothesis which confirms the association of highly variable climate with evolutionary innovation during tectonic time period is reviewed. These findings on climate or tectonic changes help to understand the human evolutionary tree and pathway.

It has been suggested that the release of clathrates rather than expansion of wetlands is the primary cause of the rapid increases observed in the ice-core atmospheric methane record during the Pleistocene. Because submarine sediment failures can involve as much as 5000 Gt of sediment and have the capacity to release vast quantities of methane hydrates, one of the major tests of the clathrate gun hypothesis is determining whether the periods of enhanced continental-slope failure and atmospheric methane correlate. To test the clathrate gun hypothesis, we have collated published dates for submarine sediment failures in the North Atlantic sector and correlated them with climatic change for the past 45 k.y. More than 70% by volume of continental-slope failures during the past 45 k.y. was displaced in two periods, between 15 and 13 ka and between 11 and 8 ka. Both these intervals correlate with rising sea level and peaks in the methane record during the Bolling-Allerod and Preboreal periods. These data support the clathrate gun hypothesis for glacial-interglacial transitions. The data do not, however, support the clathrate gun hypothesis for glacial millennial-scale climate cycles, because the occurrence of sediment failures correlates with Heinrich events, i.e., lows in sea level and atmospheric methane. A secondary use of this data set is the insight into the possible cause of continental-slope failures. Glacial-period slope failures occur mainly in the low latitudes and are associated with lowering sea level. This finding suggests that reduced hydrostatic pressure and the associated destabilization of gas hydrates may be the primary cause. The Bolling-Allerod sediment failures were predominantly low latitude, suggesting an early tropical response to deglaciation, e.g., enhanced precipitation and sediment load to the continental shelf or warming of intermediate waters. In contrast, sediment failures during the Preboreal period and the majority of the Holocene occurred in the high latitudes, suggesting either isostatic rebound-related earthquake activity or reduced hydrostatic pressure caused by isostatic rebound, causing destabilization of gas hydrates. Keywords: clathrates, gas hydrates, continental slope failure, Heinrich events, atmospheric methane, sea level.

Pore waters from very rapidly deposited continental margin sediments may provide a record of changes in the isotopic and chemical composition of seawater over time. Measurements made on pore waters extracted from sediments drilled at five sites during Ocean Drilling Program Leg 155 on the Amazon Fan show systematic increases in (delta)(super 18)O with depth from approximately 0.1% in surface sediments to approximately 0.4% in glacial-age sediments from below about 40 m below sea floor. This relatively small glacial-interglacial difference reflects both the overall increase in (beta)(super 18)O of seawater during the last glacial and a change in bottom-water source to the area from North Atlantic Deep Water at present to relatively (super 18)O depleted Antarctic Bottom Water during the last glacial. When combined with ((delta)(super 18)O analyses of benthic foraminifers, the pore-water data indicate a glacial-interglacial temperature change of about 4 degrees C.

Sea-surface temperature (SST) and salinity (SSS) records obtained from two deep-sea cores have been used to reconstruct meridional gradients in the mid-latitude North Atlantic over the past 40 k.y. During the buildup of ice sheets toward the last glacial maximum, SST and SSS gradients between 40 degrees and 50 degrees N increased by 6-8 degrees C and approximately 2%, respectively. This trend represents a two-fold increase in surface-water gradients between 40 and 20 ka and strongly over-prints the signal of high-frequency climatic oscillations. We attribute the gradual intensification of meridional SST and SSS gradients to a precession-driven increase in advection within the glacial North Atlantic Current resulting from changes in low-latitude insolation and seasonality. A strongly zonal ocean circulation pattern, maintained by greater advection and enhanced heat storage within the subtropical ocean at the last glacial maximum, persisted until ca.20 ka, when a decrease in glacial SST gradients, coincident with a reduction in low-latitude climate forcing, marked the initiation of the last deglaciation.

Paleoceanographic proxy data and ocean general circulation models have been combined to investigate the response of the North Atlantic Ocean to Heinrich-type meltwater episodes. Because of the uncertain. ties over the origin of the Heinrich events, three different scenarios have been modeled: (1) a Heinrich event caused by a dramatic increase in the iceberg discharge from the Laurentide ice sheet or Labrador ice shelf, (2) a Heinrich event driven by enhanced iceberg discharge from the Barents shelf, which was transported into the northern North Atlantic, and (3) a meltwater episode that was confined to the Nordic Seas. The ocean regional and global circulation models cannot distinguish between the Laurentide and Barents shelf Heinrich event scenarios because both led to a complete cessation of the deep-water thermohaline conveyor belt. The meltwater episode confined to the Nordic Seas only weakened the overturning and may represent circulation changes that may occur during the Dansgaard-Oeschger events.