Radiocarbon evidence for a possible abyssal front near 3.1 km in the glacial equatorial Pacific Ocean

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 425 (2015): 93-104, doi:10.1016/j.epsl.2015.05.025.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). Most new data from both the Equatorial Pacific and the Emperor Seamounts in the northwestern Pacific come from maxima in abundance of benthic taxa because this strategy reduces the effect of bioturbation. Although there remains considerable scatter in the ventilation age estimates, on average, ventilation ages in the Equatorial Pacific were significantly greater below 3.2 km (~3080 ±1125 yrs, n=15) than in the depth interval 1.9 to 3.0 km (~1610 ± 250 yrs, n=12). When compared to the average modern seawater Δ14C profile for the North Pacific, the Equatorial Pacific glacial data suggest an abyssal front located somewhere between 3.0 and 3.2 km modern water depth. Above that depth, the data may indicate slightly better ventilation than today, and below that depth, glacial Equatorial Pacific data appear to be as old as last glacial maximum (LGM) deep water ages reported for the deep southern Atlantic. This suggests that a glacial reservoir of aged waters extended throughout the circumpolar Southern Ocean and into the Equatorial Pacific. Renewed ventilation of such a large volume of aged (and, by corollary, carbon-rich) water would help to account for the rise in atmospheric pCO2 and the fall in Δ14C as the glaciation drew to a close.This work was funded by NSF grants OCE-1031224 and OCE-0424861 to LDK and 0851391 to SJL

Radiocarbon profiles of the NW Pacific from the LGM and deglaciation : evaluating ventilation metrics and the effect of uncertain surface reservoir ages

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 30 (2015): 174–195, doi:10.1002/2014PA002649.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 radiocarbon reconstructions of past ventilation changes in the Pacific are valuable but are hampered by poor carbonate preservation, low sediment accumulation rates, bias from bioturbation, and poorly constrained past surface reservoir age. In this study, we present paired benthic-planktonic radiocarbon measurements from the Okhotsk Sea and Emperor Seamounts. We take advantage of large contemporaneous peaks in benthic abundances from the last glacial maximum, Bolling-Allerod (BA), and early Holocene to produce time slices of radiocarbon from 1 to 4 km water depth. We explore the impact of uncertain surface reservoir age and evaluate several approaches to quantifying past ocean radiocarbon distribution using our NW Pacific data and a compilation of published data from the North Pacific. Both the calendar age and the absolute value of an ocean radiocarbon estimate depend on the assumed surface reservoir age. But for a time slice from a small geographical area with radiocarbon-independent stratigraphic correlation between cores, the shape of a water column profile is independent of surface reservoir age. The NW Pacific profiles are similar in shape to the compilation profiles for the entire North Pacific, which suggests that deglacial surface reservoir age changes across the N Pacific did not diverge dramatically across the areas sampled. The Last Glacial Maximum (LGM) profile >2 km spans a wide range of values, ranging from values similar to today to lower than today. However, by the BA the profile has a similar shape to today. Ultimately, local surface reservoir ages, end-member water mass composition, and mixing ratios must each be constrained before a radiocarbon activity reconstruction can be used to confidently infer ventilation changes.Support for this project was from NSF grants 0526764, 8312240, and 9912122, and the Williams College Divisional Research Funding Committee. M.S.C. participated in the GAIN writing retreat, which was support by NSF grants 0620101 and 0620087.2015-09-1

Comparison of large and ultra-small Δ14C measurements in core top benthic foraminifera from the Okhotsk Sea

© The Arizona Board of Regents on behalf of the University of Arizona, 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Radiocarbon 57, no. 1 (2015): 123-128, doi:10.2458/azu_rc.57.18153.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 peregrina, and bulk samples (~150 individuals) of stained and unstained specimens were dated. The stained sample was about 240 14C yr younger than the unstained, and the presence of bomb 14C is inferred by comparison to water column data in the nearby open North Pacific. Using new methods, multiple measurements were also made on samples of three stained and unstained individuals (as small as 7 µg C). Results are consistent with those from the bulk samples. This suggests that similar ultra-small measurements could be made at other locations to reveal the age distribution of individuals in a sediment sample in order to assess the extent of bioturbation and the presence of bomb 14C contamination.This work was supported by NSF grant OCE- 9302960 and NSF Cooperative Agreement OCE-1239667

Sediment flux and recent paleoclimate in Jordan Basin, Gulf of Maine

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Continental Shelf Research 96 (2015): 45-55, doi:10.1016/j.csr.2015.01.008.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 2011. The planktonic foraminiferal fauna in the trap is dominated by Neogloboquadrina incompta that reached a maximum flux in the second half of October. Oxygen isotope ratios on that species indicate that on average during the collecting period it lived in the surface mixed layer, when compared to predicted values based on data from a nearby hydrographic buoy from the same period. New large diameter piston cores from Jordan Basin are 25 and 28 m long. Marine hemipelagic sediments are 25 m thick, and the sharp contact with underlying red deglacial sediments is bracketed by two radiocarbon dates on bivalves that indicate ice-free conditions began 16,900 calibrated years ago. Radiocarbon dating of foraminifera indicates that the basin floor sediments (270-290 m) accumulated at >3 m/kyr during the Holocene, whereas rates were about one tenth that on the basin slope (230 m). In principle, Jordan Basin sediments have the potential to provide time series with interannual resolution. Our results indicate the Holocene is marked by ~2°C variability in SST, and the coldest events of the 20th century, during the mid 1960s and mid 1920s, appear to be recorded in the uppermost 50 cm of the seafloor.Cruise 198 of R/V Knorr was supported by the Grayce B. Kerr Fund

Deglacial floods in the Beaufort Sea preceded Younger Dryas cooling

Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Nature Geosciencevolume 11 (2018): 599-604, doi:10.1038/s41561-018-0169-6.The Younger Dryas cooling at ~13 ka, after 2 kyr of postglacial warming, is a century-old climate problem. The Younger Dryas is thought to have resulted from a slow-down of the Atlantic meridional overturning circulation in response to a sudden flood of Laurentide Ice Sheet meltwater that reached the Nordic Seas. Although there is no oxygen isotope evidence in planktonic foraminifera from the open western North Atlantic for a local source of meltwater from the Gulf of St. Lawrence where it was predicted, we report here that the eastern Beaufort Sea contains the long-sought signal of 18O-depleted water. Beginning at ~12.94 ± 0.15 ka, oxygen isotopes in planktonic foraminifera from two sediment cores as well as sediment and seismic data indicate a flood of melt water, ice and sediment to the Arctic via Mackenzie River that lasted about 700 years. The minimum in oxygen isotope ratios lasted ~130 years. The floodwater would have travelled north along the Canadian Archipelago, and through Fram Strait to the Nordic Seas where freshening and freezing near sites of deepwater formation would have suppressed convection, and caused the Younger Dryas cooling by reducing the meridional overturningThis research was funded by NSF grants ARC 1204045 to L.D.K., and ARC 1203944 to N.W.D

North Atlantic Late Miocene Stable-Isotope Stratigraphy, Biostratigraphy, and Magnetostratigraphy

Upper Miocene foraminiferal nannofossil ooze and chalk from DSDP Hole 552A in the northeast Atlantic Ocean have been closely sampled for biostratigraphic, paleomagnetic, and stable-isotopic studies. Sampling at 10-cm intervals resulted in an uppermost Miocene isotope stratigraphy with a 1000- to 3000-yr. resolution. Covariance in benthic (Planulina wuellerstorfi) and planktonic (Globigerina bulloides) foraminiferal δ^O¸ records is taken as evidence for variability in continental ice volume. Our best estimate is that glacial maxima occurred at ~ 5.0 and ~ 5.5 Ma and lasted no more than 20,000 yrs. These events probably lowered sea level by 60 m below the latest Miocene average. There is little oxygen-isotope evidence, however, for a prolonged glaciation during the last 2 m.y. of the late Miocene. High- and low-frequency variability in the δ^13C record of foraminifers is useful for correlation among North Atlantic DSDP Sites 408, 410, 522, 610, and 611, and for correlation with sites in other oceans. Similar δ^13C changes are seen in P. wuellerstorfi and G. bulloides, but the amplitude of the signal is always greater in G. bulloides. Variability in δ^13C common to both species probably reflects variability in the δ^13C of total CO2 in seawater. Major long-term features in the δ^13C record include a latest Miocene maximum (P. wuellerstorfi = 1.5‰) in paleomagnetic Chron 7, an abrupt decrease in δ^13C at -6.2 Ma, and a slight increase at -5.5 Ma. The decrease in δ^13C at -6.2 Ma, which has been paleomagnetically dated only twice before, occurs in the upper reversed part of Chronozone 6 at Holes 552A and 611C, in excellent agreement with earlier studies. Cycles in δ^13C with a period of ~ 104 yrs. are interpreted as changes in seawater chemistry, which may have resulted from orbitally induced variability in continental biomass. Samples of P. wuellerstorfi younger than 6 Ma from throughout the North Atlantic have δ^13C near 1‰, on average ~ 1‰ greater than samples of the same age in the Pacific Ocean. Thus, there is no evidence for cessation of North Atlantic Deep Water production resulting from the Messinian "salinity crisis." Biostratigraphic results indicate continuous sedimentation during the late Miocene after about -6.5 Ma at Hole 552A. Nannofossil biostratigraphy is complicated by the scarcity of low-latitude marker species, but middle and late Miocene Zones NN7 through NN11 are recognized. A hiatus is present at -6.5 Ma, on the basis of simultaneous first occurrences of Amaurolithusprimus, Amaurolithus delicatus, Amaurolithus amplificus, and Scyphosphaera globulata. The frequency and duration of older hiatuses increase downsection in Hole 552A, as suggested by calcareous nannofossil biostratigraphy and magnetostratigraphy. Paleomagnetic results at Hole 552A indicate a systematic pattern of inclination changes. Chronozone 6 was readily identified because of its characteristic nannoflora (sequential occurrences of species assigned to the genus Amaurolithus) and the δ^13C decrease in foraminifers, but its lower reversed interval is condensed. Only the lower normal interval of Chronozone 5 was recognized at Hole 552A; the upper normal interval and the lowest Gilbert sediment are not recognized, owing to low intensity of magnetization and to coring disturbance. Interpreting magnetic reversals below Chronozone 6 was difficult because of hiatuses, but a lower normally magnetized interval is probably Chronozone 7. Correlation between DSDP Hole 552A and other North Atlantic sites is demonstrated using coiling direction changes in the planktonic foraminifer Neogloboquadrina. At most sites this genus changed its coiling preference from dominantly right to dominantly left during the late Miocene. At Hole 552A this event probably occurred about 7 m.y. ago. At the same time, P. wuellerstorfi had maximum δ^13C values. A similar δ^13C maximum and coiling change occurred together in Chron 7 at Hole 611C, and at Hole 610E. In sediment younger than -5.5 Ma, the coiling of small Neogloboquadrina species is random, but the larger species N. atlantica retains preferential left coiling

Cenozoic paleoceanography 1986: An introduction

New developments in Cenozoic paleoceanography include the application of climate models and atmospheric general circulation models to questions of climate reconstruction, the refinement of conceptual models for interpretation of the carbon isotope record in terms of carbon mass balance, paleocirculation, paleoproductivity, and the regional mapping of paleoceanographic events by acoustic stratigraphy. Sea level change emerges as a master variable to which changes in the ocean environment must be traced in many cases, and tests of the onlap-offlap paradigm therefore are of crucial importance

North Atlantic ocean circulation and abrupt climate change during the last glaciation

The most recent ice age was characterized by rapid and hemispherically asynchronous climate oscillations, whose origin remains unresolved. Variations in oceanic meridional heat transport may contribute to these repeated climate changes, which were most pronounced during marine isotope stage 3, the glacial interval 25 thousand to 60 thousand years ago. We examined climate and ocean circulation proxies throughout this interval at high resolution in a deep North Atlantic sediment core, combining the kinematic tracer protactinium/thorium (Pa/Th) with the deep water-mass tracer, epibenthic δ13C. These indicators suggest reduced Atlantic overturning circulation during every cool northern stadial, with the greatest reductions during episodic Hudson Strait iceberg discharges, while sharp northern warming followed reinvigorated overturning. These results provide direct evidence for the ocean’s persistent, central role in abrupt glacial climate change