Abstract: Columbia River megafloods occurred repeatedly during the last deglaciation, but the impacts of this fresh water on Pacific hydrography are largely unknown. To reconstruct changes in ocean circulation during this period, we used a numerical model to simulate the flow trajectory of Columbia River megafloods and compiled records of sea surface temperature, paleo-salinity, and deep-water radiocarbon from marine sediment cores in the Northeast Pacific. The North Pacific sea surface cooled and freshened during the early deglacial (19.0-16.5 ka) and Younger Dryas (12.9-11.7 ka) intervals, coincident with the appearance of subsurface water masses depleted in radiocarbon relative to the sea surface. We infer that Pacific meltwater fluxes contributed to net Northern Hemisphere cooling prior to North Atlantic Heinrich Events, and again during the Younger Dryas stadial. Abrupt warming in the Northeast Pacific similarly contributed to hemispheric warming during the Bølling and Holocene transitions. These findings underscore the importance of changes in North Pacific freshwater fluxes and circulation in deglacial climate events.

Abstract: Founding populations of the first Americans likely occupied parts of Beringia during the Last Glacial Maximum (LGM). The timing, pathways, and modes of their southward transit remain unknown, but blockage of the interior route by North American ice sheets between ~26 and 14 cal kyr BP (ka) favors a coastal route during this period. Using models and paleoceanographic data from the North Pacific, we identify climatically favorable intervals when humans could have plausibly traversed the Cordilleran coastal corridor during the terminal Pleistocene. Model simulations suggest that northward coastal currents strengthened during the LGM and at times of enhanced freshwater input, making southward transit by boat more difficult. Repeated Cordilleran glacial-calving events would have further challenged coastal transit on land and at sea. Following these events, ice-free coastal areas opened and seasonal sea ice was present along the Alaskan margin until at least 15 ka. Given evidence for humans south of the ice sheets by 16 ka and possibly earlier, we posit that early people may have taken advantage of winter sea ice that connected islands and coastal refugia. Marine ice-edge habitats offer a rich food supply and traversing coastal sea ice could have mitigated the difficulty of traveling southward in watercraft or on land over glaciers. We identify 24.5 to 22 ka and 16.4 to 14.8 ka as environmentally favorable time periods for coastal migration, when climate conditions provided both winter sea ice and ice-free summer conditions that facilitated year-round marine resource diversity and multiple modes of mobility along the North Pacific coast.

Abstract: Global comparison of bottom‐water and benthic foraminifera ( Cibicides ) carbon isotope (δ 13 C) data Cibicides δ 13 C Cib reflect sea water dissolved inorganic carbon δ 13 C DIC Carbonate ion and pressure effects impact δ 13 C Cib

Abstract: 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.

Abstract: We examine the utility of the uranium (U) content of planktonic foraminifera tests as an indicator of past changes in seawater U content. The U/Ca ratio in foraminifera from Atlantic and Caribbean cores is constant in the Holocene and decreases by ∼25% during the last glacial period. Magnesium/calcium (Mg/Ca) ratios of the same samples show similar trends. While the timing of the U/Ca changes appears to be associated with glacial‐interglacial changes, the magnitude of the change is too large to be caused by variations in the extent of anoxic or suboxic sediments or by changes in riverine input. We assume that the same process produced changes in both U/Ca and Mg/Ca ratios because of a strong correlation between the two ratios. Partial dissolution of the calcite is ruled out, because we observe the same changes in well‐preserved cores from basins with opposite dissolution histories. We also reject exchange between foraminiferal and pore water U because of the oxic depositional environment of both cores and because of the consistency in the U/Ca trends from cores in different parts of the ocean. We suggest that the observed foraminiferal U/Ca and Mg/Ca trends may be the result of a temperature effect on the incorporation of these metals. If this is true, it introduces the possibility of a new paleotemperature indicator but complicates the use of U/Ca ratios in planktonic foraminifera tests as an indicator of past seawater U changes.

Abstract: Northeast Pacific benthic foraminiferal δ 18 O and δ 13 reveal repeated millennial‐scale events of strong deep‐sea ventilation (associated with nutrient depletion and/or high gas exchange) during stadial (cool, high ice volume) episodes from 10 to 60 ka, opposite the pattern in the deep North Atlantic. Two climate mechanisms may explain this pattern. North Pacific surface waters, chilled by atmospheric transmission from a cold North Atlantic and made saltier by reduced freshwater vapor transports, could have ventilated the deep Pacific from above. Alternatively, faster turnover of Pacific bottom and mid‐depth waters, driven by Southern Ocean winds, may have compensated for suppressed North Atlantic Deep Water production during stadial intervals. During the Younger Dryas event (∼11.6–13.0 cal ka), ventilation of the deep NE Pacific (∼2700 m) lagged that in the Santa Barbara Basin (∼450 m) by 〉 500 years, suggesting that the NE Pacific was first ventilated at intermediate depth from above and then at greater depth from below. This apparent lag may reflect the adjustment time of global thermohaline circulation.

Abstract: The global precipitation EOF1 shows a more complex spatial response than the global temperature EOF1, whereas the initial increase in the associated PC1 significantly lags the initial increase in the global temperature PC1 and exhibits greater millennial-scale structure than seen in the global temperature PC1 ( Fig. P1 ). Insofar as precipitation increases should accompany a warming planet, the approximately 2-ky lag between the initial increase in temperature and precipitation may reflect one or more mechanisms that affect low-latitude hydrology, including the impact of Oldest Dryas cooling, a nonlinear response to Northern Hemisphere forcing by insolation and glacial boundary conditions, or interhemispheric latent heat transports. This response may then have been modulated by subsequent millennial-scale changes in the AMOC and its attendant effects on African and Asian monsoon systems and the position of the Intertropical Convergence Zone and North American storm tracks. In contrast, the global temperature PC2 is remarkably similar to a North Atlantic Pa/Th record ( r 2  = 0.86) ( Fig. P1 ) that is interpreted as a kinematic proxy for the strength of the AMOC ( 3 ). Similar millennial-scale variability is identified in several other proxies of intermediate- and deep-ocean circulation, identifying a strong coupling between SSTs and ocean circulation. The large reduction in the AMOC during the Oldest Dryas can be explained as a response to the freshwater forcing associated with the 19-ka meltwater pulse from Northern Hemisphere ice sheets, Heinrich event 1, and routing events along the southern Laurentide Ice Sheet margin, whereas the reduction during the Younger Dryas was likely caused by freshwater routing through the St. Lawrence River and Heinrich event 0. The sustained strength of the AMOC following meltwater pulse 1a supports arguments for a large contribution of this event from Antarctica . With EOF2 accounting for only 13% of deglacial global climate variability, we conclude that the direct global impact of AMOC variations was small in comparison to other processes operating during the last deglaciation. Our analysis indicates that the superposition of two orthogonal modes explains much of the variability (64–100%) in regional and global climate during the last deglaciation ( Fig. P1 ). The nearly uniform spatial pattern of the global temperature EOF1 and the large magnitude of the temperature principal component 1 (PC1) variance indicate that this mode reflects the global warming of the last deglaciation. Given the large global forcing of greenhouse gases (GHGs) ( 2 ), the strong correlation between PC1 and the combined GHG forcing ( r 2  = 0.97) ( Fig. P1 ) supports arguments that GHGs were a major driver of global warming. Fig. P1. ( A ) Comparison of the global temperature PC1 (blue line, with confidence intervals showing results of jackknifing procedure for 68% and 95% of records removed) with record of atmospheric CO 2 from European Project for Ice Coring in Antarctica Dome C ice core (red line with age uncertainty) ( 4 ) on revised timescale from ref.  5 . ( B ) Comparison of the global temperature PC2 (blue line, with confidence intervals showing results of jackknifing procedure for 68% and 95% of records removed) with Pa/Th record (a proxy for Atlantic meridional overturning circulation) ( 3 ) (green and purple symbols). Also shown are freshwater fluxes from ice-sheet meltwater, Heinrich events, and routing events. ( C ) Comparison of the global precipitation PC1 (blue line) with record of methane (green line) and radiative forcing from greenhouse gases (red line). OD, Oldest Dryas; BA, Bølling—Allerød; YD, Younger Dryas; MWP, meltwater pulse. We used empirical orthogonal functions (EOFs) to provide an objective characterization of the temporal and spatial patterns of the leading modes of global surface climate variability for the 20- to 11-ka interval as derived from 166 published proxy records. In addition to characterizing sea surface temperature (SST) variability, we also characterize variability in regional and global continental temperature and precipitation, as well as derive a composite of global temperature variability. The low concentrations of atmospheric CO 2 during the LGM are thought to have been caused by greater storage of carbon in the deep ocean through stratification of the Southern Ocean ( 1 ). Release of the sequestered carbon may have occurred due to deep Southern Ocean overturning induced by enhanced wind-driven upwelling and sea-ice retreat associated with times of Antarctic warming, coincident with the Oldest and Younger Dryas cold events in the north. Several proxies identify large changes in the volume and circulation of the major water masses that fill the deep ocean. During the LGM, there was a marked division in the Atlantic, Indian, and Pacific oceans separating shallower, nutrient-poor intermediate water from more nutrient-rich deep water. In the North Atlantic, Antarctic Bottom Water expanded northward and upward at the expense of North Atlantic Deep Water (NADW), while both water masses maintained a vigorous circulation. In the southwest Pacific and the Arabian Sea, there was an increased influence of Antarctic Intermediate Water (AAIW). During the subsequent deglaciation, there was a net decrease of the Atlantic meridional overturning circulation (AMOC) below LGM strength during the Oldest Dryas, renewed production of NADW at the start of the Bølling–Allerød, followed by a subsequent decrease during the Younger Dryas. In the southwest Pacific and the Arabian Sea, the influence of AAIW further increased during the Oldest Dryas, decreased again during the Bølling–Allerød, and subsequently increased during the Younger Dryas. In contrast, intermediate-depth sites in the southeast Pacific suggest greatest expansion of AAIW during the LGM, followed by stepwise reduction between 17 and 11 ka. Deciphering the evolution of global climate from the end of the Last Glacial Maximum (LGM) approximately 19 ka to the early Holocene 11 ka presents an outstanding opportunity for understanding the transient response of Earth’s climate system to external and internal forcings. During this interval of global warming, virtually every component of the climate system underwent large-scale change, sometimes at extraordinary rates, as the world emerged from the grips of the last ice age. This dramatic time of global change was triggered by changes in insolation, with associated changes in ice sheets, greenhouse gas concentrations, and other amplifying feedbacks that produced distinctive regional and global responses. In addition, there were several episodes of large and rapid sea-level rise and abrupt climate change that produced regional climate signals superposed on those associated with global warming. Considerable ice-sheet melting and sea-level rise occurred after 11 ka, but otherwise the world had entered the current interglaciation with near-pre-Industrial greenhouse gas concentrations and relatively stable climates. Here we summarize a major effort by the paleoclimate research community to characterize these changes through the development of well-dated, high-resolution records of the deep and intermediate ocean as well as surface climate.