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Quaternary magnetic and oxygen isotope stratigraphy in diatom-rich sediments of the southern Gardar Drift (IODP Site U1304, North Atlantic)
Get PDFThe sediment sequence from Integrated Ocean Drilling Program (IODP) Site U1304 (53°03.40?N, 33°31.78?W; water depth, 3024 m) on the southern Gardar Drift, North Atlantic Ocean, covers the last ?1.8 Myr with a mean sedimentation rate of ?15 cm/kyr. At Site U1304, paleomagnetic directional and relative paleointensity (RPI) records have been generated for the last ?1.5 Myr, and benthic oxygen isotope data for the last ?1 Myr. The age model for Site U1304 was established by matching ?18O and RPI data to calibrated reference records. Prominent intervals of diatom mats at Site U1304 are associated with the latter stages of interglacial marine isotope stages (MIS) 9, 11, 13, 15, 21, 27 and 35, and with the weak glacial of MIS 14. The deposition of diatom mats, and associated diluted magnetic mineral concentrations, leads to weak magnetizations and susceptibilities in these intervals. Comparison of histograms of paleomagnetic directions and RPI from diatom-rich sediments and from surrounding silty clays indicates that, although results from diatom-rich sediments are more scattered, the occurrence of diatom mats does not appreciably distort the paleomagnetic directional and RPI records. Site U1304 sediments record the Matuyama/Brunhes boundary (?772 ka), the Jaramillo Subchron (?993–1071 ka) and the Cobb Mountain Subchron (?1193–1219 ka), as well as three apparent geomagnetic excursions in the Matuyama Chron, interpreted as the “Kamikatsura/Santa Rosa”, “Punaruu”, and “Gardar” excursions, at ?888 ka, ?1124 ka, and ?1463 ka, respectively. The Site U1304 RPI record can be correlated with the PISO-1500 RPI stack and with other high-resolution RPI records from the North Atlantic Ocean. Wavelet analyses performed on the Site U1304 RPI record and a new high-resolution North Atlantic paleointensity stack for the last 1500 kyr (HINAPIS-1500), comprising Ocean Drilling Program (ODP) Sites 983, 984 and IODP Sites U1306 and U1304, did not reveal significant orbital power
Climate-induced variability in Mediterranean outflow to the North Atlantic Ocean during the late Pleistocene
No full textMediterranean Outflow Water (MOW) adds salt and density to open ocean intermediate waters and is therefore an important motor of Atlantic meridional overturning circulation (AMOC) and climate variability. However, the variability in strength and depth of MOW on geological timescales is poorly documented. Here we present new detailed records, with excellent age control, of MOW variability from 416 ka to present from rapidly accumulated marine sediments recovered from the West Iberian Margin during Integrated Ocean Drilling Program (IODP) Expedition 339. Our records of x-ray fluorescence (XRF), physical grain size and palaeocurrent information from the anisotropy of magnetic susceptibility (AMS) indicate (i) a close relationship between the orientation of principle AMS axes and glacial-interglacial cycles and (ii) two distinct regimes of MOW behaviour over the last ~416 kyrs in grain size and AMS variability at orbital (mainly precessional) and suborbital timescales. Between marine isotope stages (MIS) 10 and MIS 4, MOW was focused at a generally shallow depth on the West Iberian Margin, and changes in MOW strength were strongly paced by precession. A transition interval occurred during MIS 5 and 4, when MOW deepened and millennial-scale variability in strength flow strength was superimposed on orbitally paced change. During MIS 11 and from MIS 3 to present, MOW was deeply focused and millennial-scale variability dominated. We infer that late Pleistocene variability in MOW strength and depth were strongly climate- influenced and that changes in circum-Mediterranean rainfall climate were likely a primary control
Surface and deep-water hydrography on Gardar Drift (Iceland Basin) during the last interglacial period
No full textChanges in surface and deep-water hydrography were inferred from variations in stable isotopes and sortable silt mean grain size, respectively, on the southern Gardar Drift in the subpolar North Atlantic. The bathymetric ?13C gradient during the penultimate glaciation was similar to the last glaciation with high- ?13C Glacial North Atlantic Intermediate water above ? 2000 m, and low- ?13C water derived from the Southern Ocean below. During Termination II, low-?13C water was present throughout the water column with minimum values at intermediate depths (? 1500–2000 m) and below 3000 m. This pattern continued well into the early part of the Last Interglacial (LIG) period. Sortable silt mean size at 3275 m suggests that deep-water circulation on Gardar Drift was relatively weak during the earliest part of the LIG (128 to 124.5 ka) when planktonic ?18O was at a minimum, reflecting warming and/or reduced salinity. We suggest that low- ?13C water and slow current speed on Gardar Drift during the early part of the LIG was related to increased melt water fluxes to the Nordic Seas during peak boreal summer insolation, which decreased the flux and/or density of overflow to the North Atlantic. The resumption of the typical interglacial pattern of strong, well-ventilated Iceland Scotland Overflow Water was delayed until ? 124 ka. These changes may have affected Atlantic Meridional Overturning Circulation
Relative paleointensity (RPI) and age control in Quaternary sediment drifts off the Antarctic Peninsula
Get PDFLack of foraminiferal carbonate in marine sediments deposited at high latitudes results in traditional oxygen isotope stratigraphy not playing a central role in Quaternary age control for a large portion of the globe. This limitation has affected the interpretation of Quaternary sediment drifts off the Antarctic Peninsula in a region critical for documenting past instability of the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula Ice Sheet (APIS). Here we use piston cores recovered from these sediment drifts in 2015 during cruise JR298 of the RRS James Clark Ross to test the usefulness for age control of relative paleointensity (RPI) data augmented by scant δ 18 O data. Thermomagnetic and magnetic hysteresis data, as well as isothermal remanent magnetization (IRM) acquisition curves, indicate the presence of prevalent magnetite and subordinate oxidized magnetite (“maghemite”) in the cored sediments. The magnetite is likely detrital. Maghemite is an authigenic mineral, associated with surface oxidation of magnetite grains, which occurs preferentially in the oxic zone of the uppermost sediments, and buried oxic zones deposited during prior interglacial climate stages. Low concentrations of labile organic matter apparently led to arrested pore-water sulfate reduction explaining oxic zone burial and downcore survival of the reactive maghemite coatings. At some sites, maghemitization has a debilitating effect on RPI proxies whereas at other sites maghemite is less evident and RPI proxies can be adequately matched to the RPI reference template. Published RPI data at ODP Site 1101, located on Drift 4, can be adequately correlated to contemporary RPI templates, probably as a result of disappearance (dissolution) of maghemite at sediment depths >∼10 m
Age model, d18O, declination, inclination, MAD and RPI from ODP Site 172-1063, Bermuda Rise
No full textAn age model for the Brunhes Chron for Ocean Drilling Program (ODP) Site 1063 (Bermuda Rise) is based on the tandem correlation of oxygen isotope (from G. ruber) and relative paleointensity data to calibrated reference templates. Four intervals in the Brunhes Chron where component inclinations are negative for both u-channel samples and discrete samples are correlated to the following magnetic excursions with Site 1063 ages in brackets: Laschamp (41 ka), Blake (116 ka), Iceland Basin (190 ka), Pringle Falls (239 ka). These ages are consistent with current age estimates for these excursions, other than for "Pringle Falls" which has an apparent age older than a current estimate by ~20-30 kyrs. For each of these excursions (termed Category 1 excursions), virtual geomagnetic poles (VGPs) reach high southerly latitudes implying paired polarity reversals of the Earth's main dipole in a brief time span (<2 kyr in each case) that is several times shorter than the observed duration of long-lived polarity transitions. Several intervals of low component inclinations, that are low and negative in one case, are observed both in u-channel and discrete samples at ~318 ka (MIS 9), ~412 ka (MIS 11) and in the 500-600 ka interval (MIS 14-15). These Category 2 excursions may constitute inadequately recorded excursions, or high amplitude secular variation
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