The Atlantic Ocean at the last glacial maximum: 1. Objective mapping of the GLAMAP sea-surface conditions

Recent efforts of the German paleoceanographic community have resulted in a unique data set of reconstructed sea-surface temperature for the Atlantic Ocean during the Last Glacial Maximum, plus estimates for the extents of glacial sea ice. Unlike prior attempts, the contributing research groups based their data on a common definition of the Last Glacial Maximum chronozone and used the same modern reference data for calibrating the different transfer techniques. Furthermore, the number of processed sediment cores was vastly increased. Thus the new data is a significant advance not only with respect to quality, but also to quantity. We integrate these new data and provide monthly data sets of global sea-surface temperature and ice cover, objectively interpolated onto a regular 1°x1° grid, suitable for forcing or validating numerical ocean and atmosphere models. This set is compared to an existing subjective interpolation of the same base data, in part by employing an ocean circulation model. For the latter purpose, we reconstruct sea surface salinity from the new temperature data and the available oxygen isotope measurements

Evidence from U-Th dating against Northern Hemisphere forcing of the penultimate deglaciation

Milankovitch proposed that summer insolation at mid-latitudes in the Northern Hemisphere directly causes the ice-age climate cycles. This would imply that times of ice-sheet collapse should correspond to peaks in Northern Hemisphere June insolation. But the penultimate deglaciation has proved controversial because June insolation peaks 127 kyr ago whereas several records of past climate suggest that change may have occurred up to 15 kyr earlier. There is a clear signature of the penultimate deglaciation in marine oxygen-isotope records. But dating this event, which is significantly before the 14C age range, has not been possible. Here we date the penultimate deglaciation in a record from the Bahamas using a new U-Th isochron technique. After the necessary corrections for alpha-recoil mobility of 234U and 230Th and a small age correction for sediment mixing, the midpoint age for the penultimate deglaciation is determined to be 135 +/- 2.5 kyr ago. This age is consistent with some coral-based sea-level estimates, but it is difficult to reconcile with June Northern Hemisphere insolation as the trigger for the ice-age cycles. Potential alternative driving mechanisms for the ice-age cycles that are consistent with such an early date for the penultimate deglaciation are either the variability of the tropical ocean-atmosphere system or changes in atmospheric CO2 concentration controlled by a process in the Southern Hemisphere

Radiocarbon Ages Constraints on the Origin and Shedding of Bank-Top Sediment in the Bahamas during the Holocene

Great quantities of fine-sized aragonite needles are produced in the shallow waters that cover the tops of the Bahama Banks and then exported to the bank margins where they accumulate with shells of pelagic organisms. To better understand these processes, we investigated Holocene-aged sediments in a core from the southwestern margin of Little Bahama Bank. The aragonite content of the sediments, δ18O of planktonic foraminifera shells, and radiocarbon ages of aragonite-rich <63 μm sediments and coexisting planktonic foraminifera shells were determined. Sediment deposition was rapid overall, and a significant increase in deposition rate occurred 3,500-4,000 years ago, shortly after rising sea level flooded the bank top with seawater and caused a dramatic increase in the shallow water area where aragonite production occurred. During the latest Holocene when high deposition rates minimize effects of bioturbation, aragonite-rich <63 μm sediments are 400-600 years older than coexisting foraminifera. This difference indicates the net age of aragonite when it was exported from the bank top. It is consistent with expectations of the "hip-hop'n" model (Morse et al. in Geochimica et Cosmochimica Acta 67: 2819-2826, 2003) whereby aragonite needles on the bank top, formed initially by biologic or other processes, continue to grow for hundreds of years via precipitation of epitaxial carbonate cement from seawater. Earlier in the Holocene, when sea level was lower and the top of Little Bahama Bank was subaerially exposed, the deposition rate and aragonite content of the sediments were less, and the aragonite-rich <63 μm sediments are about 1,000 years younger than coexisting foraminifera. This age difference can be explained by downward mixing of latest-Holocene <63 μm material into older early-Holocene sediments. © 2011 Springer Science+Business Media B.V

Direct U-Th dating of marine sediments from the two most recent interglacial periods

A KNOWLEDGE of the age of marine sediments is necessary to determine the timing of events and rates of processes in the marine realm, and the relationships among marine and other climatically sensitive records. The establishment of an accurate chronology for Pleistocene marine sediments beyond the range of radiocarbon dating (approximately the past 45 kyr) has therefore been a goal o f palaeoceanographers for decades. Early attempts based on measurements of the radionuclides 230Th and 231Pa were beset with problems, and subsequent studies focused on tying fluctuations in marine sediment oxygen-isotope records to events such as the formation of coral reef terraces and changes in the Earth's magnetic polarity, and tuning the resultant chronologies to the Earth's orbitally driven insolation variations. But these chronologies (especially the age and duration of the last interglacial period) have been challenged by several studies, raising questions about the fundamental cause of Pleistocene climate fluctuations. Here we report the direct U-Th dating of aragonite-rich marine sediments from the Bahamas, and present an accurately dated marine oxygen-isotope record for the last two interglacials. We obtain dates of 120-127 kyr BP for the last interglacial and 189-190 kyr BP for the late stage 7 interglacial. These dates are in accord with the general theory of orbitally forced climate fluctuations and demonstrate the potential of our direct-dating approach for developing an absolute chronology for the Pleistocene marine oxygen-isotope record

U-Th dating of carbonate platform and slope sediments

Absolute chronology of marine sediment beyond the 14C age range provides a test for models of climate change and has many other applications. U-Th techniques have been used for such chronology by dating corals, but extending these techniques to marine sediment is complicated by the presence of significant initial 230Th-both in detrital material and scavenged from seawater. In this study, we investigate four methods of solving the initial 230Th problem for a particular type of marine sediment-the aragonite-rich sediments of carbonate platforms and slopes. Bulk sediment U-Th analyses can be corrected for initial Th to yield ages with ≈2 to 3 kyr precision for highstand periods when sediment aragonite contents are particularly high. Uncertainty on the corrections causes inadequate precision for sediment from other periods, however. Removal of scavenged Th before analysis would enable a dramatic increase in this precision but has not proved successful despite a range of chemical leach approaches. Using heavy liquids to separate the various carbonate minerals found in Bahamas sediment enables an isochron approach to correct for initial Th, but the presence of initial Th from two sources requires correction or removal of one source of initial Th before the other is deconvolved by the isochron. Quantitative removal of detrital material before isochron analysis proves a successful approach. Such isochron data demonstrate that, although sediment remains closed to U-Th on a centimetre scale, nuclides are moved from grain to grain by α-recoil. Such intergrain exchange is expected to be observed in all sediments containing mineral grains with different U concentrations. Measured 234U/238U allows the recoil movement to be corrected and results in isochron ages with precision sometimes as low as 3 kyr. The accuracy of this approach has been proved by dating samples within the 14C age range. Sediments spanning the penultimate deglaciation have been also dated. After a small correction for bioturbation, the age for this event is found to be 135.2 ± 3.5 ka. This date is ≈8 kyr before the peak in northern hemisphere insolation and suggests that deglaciation is initiated by a mechanism in the southern hemisphere or tropics. This isochron approach shows considerable promise for dating of sediments older than this event, which will provide further information about the timing and mechanism of global climate change. Copyright © 2001 Elsevier Science Ltd

Variation in bioturbation with water depth on marine slopes: a study on the Little Bahamas Bank

Reconstructing the paleoceanography of intermediate-depth waters is dependent on sedimentary records preserved on marine slopes. But knowledge of bioturbation processes in such slope environments is much poorer than for the deep sea. In this study, 210Pb profiles were measured for the upper ≃ 20 cm of sediment from five box cores taken on the slopes of the Bahamas in water depths ranging from 400 to 1500 m. These were compared with data from the Bahamas bank tops and from elsewhere to assess the rate and depth of mixing in the slope environment. Excess 210Pb inventories are too high to explain by water-column decay of 226Ra and are derived largely from atmospheric fallout. Down-core 210Pb(xs) profiles show an upper zone where 210Pb(xs) activities decrease exponentially with depth suggesting uniform mixing, and a lower zone with patchy 210Pb values reflecting isolated burrows. In contrast to bank-top sediment which shows 210Pb(xs) mixing to depths of ≃ 40 cm and mixing rates of > 1000 cm2/kyr, mixing of 210Pb on the slopes penetrates to only ≃ 8 cm with mixing rates of < 100 cm2/kyr. Some systematic variation of mixing parameters with water depth are seen on the slopes but these are subtle compared to the dramatic change between bank-top and slope. This pattern is also seen in literature data from regions with relatively slow sedimentation rates and is fit reasonably well by the relationship: mixing rate = 6330(depth(-1.65)) [Soetaert, K., Herman, P.M.J., Middelburg, J.J., Heip, C., deStigter, H.S., van Weering, T.C.E., Epping, E., Helder, W., 1996. Modelling 210Pb-derived mixing activity in ocean margin sediments: Diffusive versus nonlocal mixing. J. Mar. Res. 54, 1207-1227]. A simple model is used to assess the effects of mixing on typical Bahamian slope sediments which have deposition rates and compositions which vary dramatically in response to sea-level change. Proxy records such as δ18O are expected to be displaced by up to 0.8 kyr in bulk sediment. And individual sediment constituents may be moved by up to 2 kyr relative to one another. These mixing effects are reasonably small and may be safely ignored for some studies. But for high-resolution records, or studies where precise age control is important, bioturbation of slope sediments should be considered

U-Th dating of marine isotope stage 7 in Bahamas slope sediments

In order to understand the driving forces for Pleistocene climate change more fully we need to compare the timing of climate events with their possible forcing. In contrast to the last interglacial (marine isotope stage (MIS) 5) the timing of the penultimate interglacial (MIS 7) is poorly constrained. This study constrains its timing and structure by precise U-Th dating of high-resolution δ18O records from aragonite-rich Bahamian slope sediments of ODP Leg 166 (Sites 1008 and 1009). The major glacial-interglacial cycles in δ18O are distinct within these cores and some MIS 7 substages can be identified. These sediments are well suited for U-Th dating because they have uranium concentrations of up to 12 ppm and very low initial 230Th contributions with most samples showing 230Th/232Th activity ratio of > 75. U and Th concentrations and isotope ratios were measured by thermal ionisation mass spectrometry and multiple collector inductively coupled plasma mass spectrometry, with the latter providing dramatically better precision. Twenty-nine of the 41 samples measured have a δ234U value close to modern seawater suggesting that they have experienced little diagenesis. Ages from 27 of the 41 samples were deemed reliable on the basis of both their U and their Th isotope ratios. Ages generally increase with depth, although we see a repeated section of stratigraphy in one core. Extrapolation of constant sedimentation rate through each substage suggests that the peak of MIS 7e lasted from ~237 to 228 ka and that 7c began at 215 ka. This timing is consistent with existing low precision radiometric dates from speleothem deposits. The beginning of both these substages appears to be slightly later than in orbitally tuned timescales. The end of MIS 7 is complex, but also appears to be somewhat later than is suggested by orbitally tuned timescales, although this event is not particularly well defined in these cores. © 2002 Elsevier Science B.V. All rights reserved

Fluid flow through carbonate platforms: constraints from U-234/U-238 and Cl- in Bahamas pore-waters

The geometry, timing, and rate of fluid-flow through carbonate margins and platforms is not well constrained. In this study, we use U concentrations and isotope ratios measured on small volumes of pore-water from Bahamas slope sediment, coupled with existing chlorinity data, to place constraints on the fluid-flow in this region and, by implication, other carbonate platforms. These data also allow an assessment of the behaviour of U isotopes in an unusually well constrained water-rock system. We report pore-water U concentrations which are controlled by dissolution of high-U organic material at shallow depths in the sediment and by reduction of U to its insoluble 4+ state at greater depths. The dominant process influencing pore-water (234U/238U) is alpha recoil. In Holocene sediments, the increase of pore-water (234U/238U) due to recoil provides an estimate of the horizontal flow rate of 11 cm/year, but with considerable uncertainty. At depths in the sediment where conditions are reducing, features in the U concentration and (234U/238U) profiles are offset from one another which constrains the effective diffusivity for U in these sediments to be ~1-2 x 10-8 cm2 s-1. At depths between the Holocene and these reducing sediments, pore-water (234U/238U) values are unusually low due to a recent increase in the dissolution rate of grain surfaces. This suggests a strengthening of fluid flow, probably due to the flooding of the banks at the last deglaciation and the re-initiation of thermally-driven venting of fluid on the bank top and accompanying recharge on the slopes. Interpretation of existing chlorinity data, in the light of this change in flow rate, constrain the recent horizontal flow rate to be 10.6 (±3.4) cm/year. Estimates of flow rate from (234U/238U) and C1- are therefore in agreement and suggest flow rates close to those predicted by thermally-driven models of fluid flow. This agreement supports the idea that flow within the Bahamas Banks is mostly thermally driven and suggests that flow rates on the order of 10 cm/year are typical for carbonate platforms where such flow occurs

U-Th dating and diagenesis of Pleistocene highstand sediments from the Bahamas slope

We have performed U-Th isotope analyses on pure aragonite samples from the upper sections of Leg 166 cores to assign each aragonite-rich sediment package to the correct sea-level highstand. The uppermost sediment package from each of the four sites investigated (Sites 1003, 1005, 1006, and 1007) yielded a Holocene U-Th age. Sediment packages from deeper in the cores have suffered diagenesis. This diagenesis consists of significant U loss (up to 40%) in the site nearest the platform (Site 1005), slight U gain in sites further from the platform, and continuous loss of pure 234U caused by alpha recoil at all sites. The difference in diagenesis between the sites can be explained by the different fluid-flow histories they have experienced. Site 1005 is sufficiently close to the platform to have probably experienced a change in flow direction whenever the banks have flooded or become exposed. Other sites have probably experienced continuous flow into the sediment. Although diagenesis prevents assignment of accurate ages, it is sufficiently systematic that it can be corrected for and each aragonite-rich package assigned to a unique highstand interval. Site 1005 has sediment packages from highstands associated with marine isotope Stages 1, 5, 7, 9, and 11. Site 1006 is similar, except that the Stage 7 highstand is missing, at least in Hole 1006A. Site 1003 has sediment only from Stage 1 and 11 highstands within the U-Th age range. And Site 1007 has sediment only from the stage 1 highstand. This information will allow the construction of better age models for these sites. No high-aragonite sediments are seen for Stage 3 or Substages 5a and 5c. Unless rather unusual erosion has occurred, this indicates that the banks did not flood during these periods. If true, this would require the sea level for Substages 5a and 5c to have remained at least ~ 10 m lower than today

Glacial-to-Holocene sedimentation on the western slope of Great Bahama Bank

Thick, late Quaternary sediment sections were recovered at several sites on the leeward slope of Great Bahama Bank during Leg 166 of the Ocean Drilling Program. These sections have paleoceanographic records with potentially high temporal resolution. To make an initial assessment of the records corresponding to the Holocene highstand of sea level, we have identified and dated the sediments from the four upper slope sites (1004, 1005, 1008, and 1009) that were deposited during the period of time which spans the last glaciation through the Holocene. Age identifications are based upon the abundances of the Globorotalia menardii complex of planktonic foraminifera, the stable oxygen isotopic ratios of bulk sediment and the planktonic foraminifera Globogerinoides ruber, and AMS C-14 dating of bulk sediment. Comparison of these data with the sediment lithologic and geoacoustic properties shows that consistent stratigraphic relationships exist at each site: The uppermost interval of aragonite-rich sediments corresponds to the Holocene highstand of sea level (i.e. oxygen isotope stage 1) and these sediments are underlain by a relatively thin interval of aragonite-poor, partially lithified sediments which corresponds to the last glaciation when sea level was significantly lower than today (i.e. oxygen isotope stages 2-4). The Leg 166 upper slope sites possess carbonate accumulation and paleoceanographic proxy records with very high temporal resolution, with Sites 1004, 1008, and 1009 appearing to have the greatest stratigraphic integrity. Comparison of core and high-resolution seismic profile data establishes the Holocene nature of the uppermost seismic unit in the stratigraphic package of the western slope of Great Bahama Bank. © 2002 Elsevier Science B.V. All rights reserved