Lloyd D Keigwin's research while affiliated with Woods Hole Oceanographic Institution and other places
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Publications (951)
A 2,000 year-long oceanographic history, in sub-centennial resolution, from a Canadian Beaufort Sea continental shelf site (60meters water depth) near the Mackenzie River outlet is reconstructed from ostracode and foraminifera faunal assemblages, shell stable isotopes (delta 18O, delta 13C) and sediment biogenic silica. The chronology of three sedi...
During the last deglaciation substantial volumes of meltwater from the decaying Laurentide Ice Sheet were supplied to the Arctic, Gulf of Mexico and North Atlantic along different drainage routes, sometimes as catastrophic flood events. These events are suggested to have impacted global climate, for example initiating the Younger Dryas cold period....
This paper summarizes paleomagnetic secular variation (PSV) and excursions recorded from MIS 9–10 (300–374 ka) shipboard measurements of sediments from the western North Atlantic Ocean at ODP Sites 1060, 1061, 1062, and 1063. We have identified 33 inclination features, 31 declination features, and 12 paleointensity features that are reproducible am...
This paper summarizes the pattern of paleomagnetic secular variation (PSV) and excursions within MIS 6–7 (130–243 ka) from the western North Atlantic Ocean – ODP Sites 1060–1063. Composite high-resolution PSV records (both directions and relative paleointensity) have been developed for each site and inter-compared. There are 65 correlatable inclina...
Paleomagnetic secular variation (PSV) and excursion data obtained across MIS 8 (243–300 ka) from the western North Atlantic Ocean—ODP Sites 1060–1063 show composite high-resolution PSV records (both directions and relative paleointensity) developed for each site and inter-compared. Two methods of chronostratigraphy allow us to date these records. F...
We have recovered four new records of the paleomagnetic field variability within Marine Isotope Stage 5 (MIS 5: 71–130 ka) from deep-sea sediment cores of the Bahama Outer Ridge, western North Atlantic Ocean. These cores contain reproducible evidence of the Blake Event and are located within 300 km of cores used by Smith and Foster (1969) to define...
An amendment to this paper has been published and can be accessed via a link at the top of the paper.
The Nd isotope composition of seawater has been used to reconstruct past changes in the contribution of different water masses to the deep ocean. In the absence of contrary information, the Nd isotope compositions of endmember water masses are usually assumed constant during the Quaternary. Here we show that the Nd isotope composition of North Atla...
The first regional model for the deglacial history of the Beaufort margin slope, in the western Arctic, is presented. The conceptual model was developed using new high-resolution CHIRP seismic reflection, multibeam bathymetry, and sediment core data acquired along the margin. This synthesis provides important constraints on sediment source and disp...
Calcareous microfossil assemblages in late Holocene sediments from the western Arctic continental shelf provide an important baseline for evaluating the impacts of today’s changing Arctic oceanography. This study compares ¹⁴C-dated late Holocene microfaunal assemblages of sediment cores SWERUS-L2-2-PC1, 2-MC4 and 2-KL1 (57 mwd), which record the la...
Millennial scale events marked by the contribution of detrital sand are recorded in North Atlantic sediments during the Last Glacial Maximum (LGM), between Heinrich events (HE) 1 and 2, and left their imprint on Laurentian Fan (LF – 43°N) sediments off eastern Canada. The LF counterpart of the well-known detrital events consist of glacial red-brick...
Paleoclimate reconstructions are only as good as their chronology. In particular, different chronological assumptions for marine sediment cores can lead to different reconstructions of ocean ventilation age and atmosphere-ocean carbon exchange history. Here we build the first high-resolution chronology that is free of the dating uncertainties commo...
A period of cooling about 13,000 years ago interrupted about 2,000 years of deglacial warming. Known as the Younger Dryas (YD), the event is thought to have resulted from a slowdown of the Atlantic meridional overturning circulation in response to a sudden flood of Laurentide Ice Sheet meltwater that reached the Nordic Seas. Oxygen isotope evidence...
The notion of a shallow northern sourced intermediate water mass is a well evidenced feature of the Atlantic circulation scheme of the Last Glacial Maximum (LGM). However, recent observations from stable carbon isotopes (δ¹³C) at the Corner Rise in the deep northwest Atlantic suggested a significant contribution of a Northern Component Water mass t...
The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that has an essential role in Earth's climate, redistributing heat and influencing the carbon cycle1, 2. The AMOC has been shown to be weakening in recent years 1 ; this decline may reflect decadal-scale variability in convection in the Labrador Sea, but short obse...
We present a synthesis of 1361 deep-sea radiocarbon data spanning the past 40 kyr and computed (for 14C-dated records) from the same calibration to atmospheric 14C. The most notable feature in our compilation is a long-term Δ14C decline in deep oceanic basins over the past 25 kyr. The Δ14C decline mirrors the drop in reconstructed atmospheric Δ14C,...
Significance
Understanding glacial ocean circulation is tied to understanding heat transport in the North Atlantic, growth and maintenance of ice sheets, and atmospheric CO 2 content. The proxy data are sparse, but, for decades, it has been thought that the North Atlantic did not produce deep water during the Last Glacial Maximum, unlike today. New...
The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true wou...
An ocean of climate impacts
Large decreases in Atlantic meridional overturning circulation accompanied every one of the cold Northern Hemispheric stadial events that occurred during the heart of the last glacial period. These events, lasting on average around 1000 years each, have long been thought to result from changes in deep ocean circulation....
We have carried out a paleomagnetic study on three deep-sea cores from the Chukchi Sea (72°N) in order to characterize the Holocene paleomagnetic secular variation (PSV) in this high-latitude region. The Chukchi Sea lies within the geomagnetic-field tangent cylinder and PSV variability in this region might be expected to have a different pattern th...
A transect of sediment cores from high-sedimentation rate locations from the Panama Bight (eastern tropical Pacific) in combination with climate model experiments provides an opportunity to improve our understanding of the role of the tropical hydrologic cycle as a potential driver of global climate change during the Holocene and Termination 1. The...
We investigate the radiocarbon ventilation age in deep equatorial Pacific sediment cores using the difference in conventional 14C age between coexisting benthic and planktonic foraminifera, and integrate those results with similar data from around the North Pacific Ocean in a reconstruction for the last glaciation (15 to 25 conventional 14C ka). Mo...
The radiocarbon activity of benthic foraminifera was investigated in surface sediments from a high deposition rate location at a depth of 1000 m in the Okhotsk Sea. Sediments were preserved and stained with Rose Bengal to identify foraminifera that contain cytoplasm. The benthic fauna at this site is dominated by large specimens of Uvigerina peregr...
We report planktonic foraminiferal fluxes (accumulation rates) and oxygen isotopes (δ18O) from a nine-month sediment trap deployment, and δ18O from three sediment cores in Jordan Basin, Gulf of Maine. The sediment trap was deployed at 150 m, about halfway to the basin floor, and samples were collected every three weeks between August 2010 and May 2...
A high-resolution diatom census coupled with other proxy data from Laurentian Fan (LF) provides a detailed description of the last deglaciation, bringing new insight to that period by revealing directly the timing of sea-ice formation and melting. Cold events Heinrich Event 1 (H1) and the Younger Dryas (YD) were multiphase events. H1 (~16.8-15.7 ca...
During the last deglaciation, the ventilation of the subarctic Pacific is hypothesized to have changed dramatically, including the rejuvenation of a poorly-ventilated abyssal water mass that filled the deep ocean, and fluctuations in the strength of North Pacific intermediate and deep water formation at millennial timescales. Foraminiferal radiocar...
In a piston core from the central Bering Sea, diatom microfossil-bound N isotopes and the concentrations of opal, biogenic barium, calcium carbonate, and organic N are measured over the last glacial/interglacial cycle. Compared to the interglacial sections of the core, the sediments of the last ice age are characterized by 3 per mil higher diatom-b...
During the Bølling-Allerød warm period of the last
deglaciation, about 14kyr ago, there was a strong and pervasive spike in
primary productivity in the North Pacific Ocean. It has been suggested
that this productivity event was caused by an influx of the
micronutrient iron from surrounding continental shelves as they were
flooded by sea-level rise....
The direct correlation between terrestrial (pollen) and marine
(planktonic δ18O) proxies from a slope core (KNR 178-2 JPC 32),
retrieved in the Cape Hatteras (35°58.58'N, 74°42.77'W, 1006 m),
provide substantial information on the Eastern North American vegetation
response to the Holocene climate and oceanographic changes of the
western North Atlan...
A strong and pervasive productivity peak has been observed in cores
around the North Pacific during the Bølling-Allerød warm
period of the last deglaciation. Recently, it has been hypothesized that
this peak may have been caused by an influx of iron from the continental
shelves as they were flooded during the deglaciation (1). Here, we
examine this...
As a critical component of the cryosphere, sea ice helps regulate albedo
and therefore the overall radiative balance of the Earth. Over the last
30 years, satellite data suggests that polar sea ice has been rapidly
declining and continues to do so at an unprecedented rate. In order to
contextualize recent changes in sea ice and their causes, new me...
We reconstructed subsurface (˜200-400 m) ocean temperature and
sea-ice cover in the Canada Basin, western Arctic Ocean from
foraminiferal δ18O, ostracode Mg/Ca ratios, and
dinocyst assemblages from two sediment core records covering the last
8000 years. Results show mean temperature varied from -1 to
0.5°C and -0.5 to 1.5°C at 203 and 369 m water d...
High-resolution diatom species counts describe major deglacial and
Holocene oceanographic changes over the Laurentian Fan. Diatoms provide
direct evidences for temperature and salinity changes and attest the
presence of ice. No diatom paleoceanographic reconstruction is available
for this location yet, although the parameters they trace are known t...
There is controversy over the role of marine methane hydrates in atmospheric methane concentrations and climate change during the last glacial period. In this study of two sediment cores from the southeast Bering Sea (700 m and 1467 m water depth), we identify multiple episodes during the last glacial period of intense methane flux reaching the sea...
Western subtropical North Atlantic oceanic and atmospheric circulations
connect tropical and subpolar climates. Variations in these circulations
can generate regional climate anomalies that are not reflected in
Northern Hemisphere averages. Assessing the significance of
anthropogenic climate change at regional scales requires proxy records
that all...
Western subtropical North Atlantic oceanic and atmospheric circulations connect tropical and subpolar climates. Variations in these circulations can generate regional climate anomalies that are not reflected in Northern Hemisphere averages. Assessing the significance of anthropogenic climate change at regional scales requires proxy records that all...
here is controversy over the role of marine methane hydrates in atmospheric methane concentrations and climate change during the last glacial period. In this study of two sediment cores from the southeast Bering Sea (700 m and 1467 m water depth), we identify multiple episodes during the last glacial period of intense methane flux reaching the seaf...
Satellite observations of declining Arctic sea-ice coverage underscore the importance of understanding Arctic climate variability on decadal to centennial timescales. A majority of marine proxy records from Arctic sediment cores, however, do not resolve sub-millennial-scale changes due to low sedimentation rates (< 1 cm kyr-1) in central Arctic Oce...
Climate-dependent chemical weathering trends have a strong impact on the dissolved Pb isotopic composition of continental runoff during glacial terminations, so that this tracer can be used to reconstruct the impact of the North American deglaciation on aspects of freshwater runoff and ocean chemistry. Here we present authigenic Fe–Mn oxyhydroxide-...
In piston cores from the open subarctic Pacific and the Okhotsk Sea, diatom-bound δ15N (δ15Ndb), biogenic opal, calcium carbonate, and barium were measured from coretop to the previous glacial maximum (MIS 6). Glacial intervals are generally characterized by high δ15Ndb (∼8‰) and low productivity, whereas interglacial intervals have a lower δ15Ndb...
The interpretation of sea-surface paleosalinity reconstructions from the eastern tropical Pacific in terms of Atlantic-to-Pacific moisture transport is revisited. It is argued that the use of modern analogs of interannual climate variability may help to reconcile seemingly contradictory results from paleosalinity reconstructions at different locati...
Heinrich event 1 (H1) is a climate event resulting from the release into the North Atlantic of a huge volume of sea ice and icebergs from the northern hemisphere ice sheets. We present here high-resolution diatom records from the Bermuda Rise (Sargasso Sea) and the Laurentian Fan (South of Newfoundland) to assess its impacts on North Atlantic surfa...
The Depths of the Changes
Over the course of the past glacial cycle, there have been two major types of rapid, large climate warming events: shorter-lived warm intervals lasting on the order of 1000 years and the last glacial-interglacial transition. Although both involved dramatic changes in large-scale ocean circulation, the extent to which those...
Understanding changes in ocean circulation during the last deglaciation is crucial to unraveling the dynamics of glacial-interglacial and millennial climate shifts. We used neodymium isotope measurements on postdepositional iron-manganese oxide coatings precipitated on planktonic foraminifera to reconstruct changes in the bottom water source of the...
Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 24 (2009): PA4212, doi:10.1029/2008PA001727. Most seafloor sediments are dated with radiocarbon, and the sediment is assumed to...
Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 24 (2009): PA4101, doi:10.1029/2008PA001729. Diatom assemblages document surface hydrographic changes over the Bermuda Rise. Be...
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 286 (2009): 546-555, doi:10.1016/j.epsl.2009.07.020. During the Last Glacial Maximum much of North Ame...
Advances in the our knowledge of past changes in ocean circulation have been made using Nd isotopes, as a proxy for changes in water mass structure [1], and 231Pa/230Th ratios as a proxy for the rate of ocean circulation [2]. Despite the potential power of these proxies when applied together, no records have yet been published of both proxies measu...
Most seafloor sediments are dated with radiocarbon, and the sediment is assumed to be zero-age (modern) when the signal of atmospheric testing of nuclear weapons is present (Fraction modern (Fm) > 1). Using a simple mass balance, we show that even with Fm > 1, half of the planktonic foraminifera at the seafloor can be centuries old, because of biot...
The Atlantic-to-Pacific water vapor transport across Central America has been postulated to play an important role in the overturning circulation of the Atlantic Ocean (Schmittner et al., 2000). Paleoceanographic data suggest that the delta18Osw, and presumably salinity, of western Pacific surface waters has decreased over the course of the Holocen...
During the last deglaciation (ca. 21–10 ka), freshening of the North Atlantic surface likely caused reductions in Atlantic meridional overturning circulation (AMOC); the mechanisms related to AMOC recovery remain poorly understood. Here we present three new deglacial surface temperature and δ18Oseawater (δ18Osw) reconstructions from the western sub...
Radiocarbon is a useful tracer of the air-sea partitioning of CO2, as well as ocean circulation. Reconstructing the distribution of radiocarbon of the past ocean can place constraints on the mechanisms that cause changes in the carbon cycle, ocean circulation and climate. In a collection of marine sediment cores from the Okhotsk Sea and Emperor Sea...
Diatom assemblages document the surface hydrographic changes over the
Bermuda Rise between 19.2 and 14.5 ka BP. Subtropical warm water diatom
species and Chaetoceros resting spores dominate the assemblages, as
would be expected from more productive regions in the North Atlantic.
Beginning 16.9 ka BP and lasting until 14.6 ka BP, brackish diatoms ar...
Andrews, J. T., Austin, W. E. N., Bergsten, H. & Jennings, A. E. (eds), 1996: Late Quaternary Paleoceanography of the North Atlantic Margins
Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 23 (2008): PA1101, doi:10.1029/2007PA001500. Models indicate that a complete shutdown of deep and intermediate water production...
A high-resolution authigenic Nd isotope record has been extracted from the Fe–Mn oxyhydroxide fraction of drift sediments along the Blake Ridge in the North Atlantic. These sediments facilitate reconstruction of the timing and extent of major hydrographic changes in the western North Atlantic since the Last Glacial Maximum (LGM). This is one of the...
Models indicate that a complete shutdown of deep and intermediate water production is a possible consequence of extreme climate conditions in the northern North Atlantic, and the high ratio of 231Pa to 230Th on Bermuda Rise is evidence that this might have happened ?17 ka during Heinrich event 1 (H1). However, new radiocarbon data from bivalves tha...
Detailed Fe grain provenance for a 19.9 meter long piston core
(HLY02-JPC16) with about 17.5 m of Holocene sediment provides a
sub-century scale resolution of the Arctic Oscillation (AO). The
presence of Fe grains matched to Russian shelves fluctuates throughout
this core, located 125 km north of Alaska in 1300 m water depth. High
amounts of these...
The tropical Pacific plays a central role in the climate system by providing large diabatic heating that drives the global atmospheric circulation. Quantifying the role of the tropics in late Pleistocene climate change has been hampered by the paucity of paleoclimate records from this region and the lack of realistic transient climate model simulat...
Using CHIRP subbottom profiling across the Chukchi shelf, offshore NW Alaska, we observed a large incised valley that measures tens of kilometers in width. The valley appears to have been repeatedly excavated during sea level lowering; however, the two most recent incisions appear to have been downcut during the last sea level rise, suggesting an i...
Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 22 (2007): PA3102, doi:10.1029/2007PA001420. A simple ocean/atmosphere feedback may reduce the amplitude of climate variability...
In a piston core from the central Bering Sea, diatom microfossil-bound N isotopes and the concentrations of Opal, biogenic barium, calcium carbonate, and organic N are measured over the last glacial/interglacial cycle. Compared to the interglacial sections of the core, the sediments of the last ice age are characterized by 3‰ higher diatom-bound δ1...
The last glacial period was punctuated by millennial-scale climate events which appear to have been global in impact. The most extensive documentation of these events is in the North Atlantic region, but relatively little is known of how the Pacific responded. We constructed high-resolution climate records from 50 to 20 ky ago in a piston core from...
We have assessed temperature and carbonate ion effects on elemental ratios in benthic foraminifera using core top samples from Atlantic (Cape Hatteras Continental Shelf (CHCS) and Norwegian Sea) and Pacific (Indonesia, Hawaii, and New Zealand) depth transects. Our previous studies, based on comparing samples from Little Bahama Banks (LBB), Hawaii,...
There is considerable interest in the approx. 400 permil decrease in atmospheric radiocarbon activity since the last glacial maximum because it is too large to be explained by production rate changes alone. In a few papers, Broecker and colleagues have concluded that some large part of the D14C decrease is most likely accounted for by changes in de...
Three new sediment cores from the Chukchi Sea preserve a record of local paleoenvironment, sedimentation, and flooding of the Chukchi Shelf (˜-50 m) by glacial-eustatic sea-level rise. Radiocarbon dates on foraminifera provide the first marine evidence that the sea invaded Hope Valley (southern Chukchi Sea, -53 m) as early as 12 ka. The lack of sig...
Detailed Fe grain provenance for a 19.9 meter long piston core (HLY02-JPC16) with about 17.5 m of Holocene sediment provides the highest resolution yet for the ice-rafted detritus history in the Beaufort Sea, Arctic Ocean. The presence of Fe grains matched to Russian shelves throughout this core, located 125 km north of Alaska in 1300 m water depth...
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 r...
Various lines of evidence, including the Nd isotope composition of
Southern Atlantic seawater as reconstructed from authigenic
ferromanganese coatings in pelagic sediments from the Cape Basin,
suggest that the strength of the thermohaline overturning circulation in
the North Atlantic was significantly reduced during the Last Glacial
Maximum (LGM) a...
There are multiple negative excursions in planktonic and benthic foraminifer delta13C in a core from 1467m in the southeast Bering Sea. These excursions occur episodically during the last glacial period, and may coincide with Dansgaard-Oeschger (D-O) events. Measured foraminifer delta13C during the excursions is as low as -140/00 and are probably t...
Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 20 (2005): PA4011, doi:10.1029/2005PA001150. Monthly samples of stratified plankton tows taken from the slope waters off Cape C...
In the equatorial west Pacific and the tropics of northwestern South America, extreme phases of the El Niño-Southern Oscillation (ENSO) are associated with a coherent pattern of hydrological anomalies. While El Niño leads to reduced rainfall and river runoff, La Niña causes extreme maxima in precipitation and river discharge to the Pacific and Atla...
Millennial-scale oscillations in climate-sensitive geochemical proxies are seen throughout Marine Isotope Stage 3 (MIS3, ~30 to 60 ka BP) in marine and ice-core records, with variability nearly as large as that seen on the last deglaciation. Nutrient proxies, such as delta13C, have been widely used to reconstruct water mass reorganizations associat...
Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B. V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 52 (2005): 2163-2173, doi:10.1016/j.dsr2.2005.07.004. The lithology of deglacial...
[1] Stable isotope, trace metal, alkenone paleothermometry, and radiocarbon methods have been applied to sediment cores in the western subpolar North Atlantic between Hudson Strait and Cape Hatteras to reveal the history of climate in that region over the past 11 kyr. We focus on cores from the Laurentian Fan, which is known to have rapid and conti...
We have recovered two new high-resolution paleomagnetic records of the Laschamp Excursion (∼41,000 calendar years B.P.) from deep-sea sediments of the western North Atlantic Ocean. The records document that the Laschamp Excursion was characterized locally by (1) declination changes of ±120°, (2) inclination changes of more than 140°, (3) ∼1200-year...
Glacial freshwater discharge to the Atlantic Ocean during deglaciation may have inhibited oceanic thermohaline circulation, and is often postulated to have driven climatic fluctuations. Yet attributing meltwater-discharge events to particular climate oscillations is problematic, because the location, timing, and amount of meltwater discharge are of...
As a result of the growing use of multiple geochemical proxies to reconstruct ocean and climate changes in the past, there is an increasing need to establish temporal relationships between proxies derived from the same marine sediment record and ideally from the same core sections. Coupled proxy records of surface ocean properties, such as those ba...