Age determination with strontium isotopes on sediment core CRP-1 from the Ross Sea, Antarctica

Strontium isotope stratigraphy was used to date five discrete horizons within CRP-1. Early and late Quaternary (0.87-1.3 Ma and 0-0.67 Ma respectively) age sediments overlie a major sequence boundary at 43.15 meters below sea floor (mbsf). This hiatus is estimated to account for ~16 m.y. of missing section. Early Miocene (16.6-~20.8-25 Ma) age deposits below this boundary are in turn cut by multiple erosion surface representing hiatus is of between 0.2 and 1.2 m.y. Estimated minimum sedimentation rates range between 0.9 and 2.8 cm/k.y. in the Quaternary, and 1.5 and 6.4 cm/ky in the lower Miocene.

Isotope analysis, age calculation and calcite concentration from IODP Hole 310-M0005D

We present uranium-thoriumchronology for a 102 mcore through a Pleistocene reef at Tahiti (French Polynesia) sampled during IODP Expedition 310 "Tahiti Sea Level". We employ total and partial dissolution procedures on the older coral samples to investigate the diagenetic overprint of the uranium-thoriumsystem. Although alteration of the U-Th system cannot be robustly corrected, diagenetic trends in the U-Th data, combined with sea level and subsidence constraints for the growth of the corals enables the age of critical samples to be constrained to marine isotope stage 9. We use the ages of the corals, together with d18O based sea-level histories, to provide maximum constraints on possible paleo water-depths. These depth constraints are then compared to independent depth estimates based on algal and foraminiferal assemblages, microbioerosion patterns, and sedimentary facies, confirming the accuracy of these paleo water-depth estimates. We also use the fact that corals could not have grown above sea level to place amaximumconstraint on the subsidence rate of Tahiti to be 0.39 m ka**-1,with the most likely rate being close to the existing minimum estimate of 0.25m ka**-1.

Age models of sediment cores from the Southern Ocean

Recent geochemical models invoke ocean alkalinity changes, particularly in the surface Southern Ocean, to explain glacial age pCO2 reduction. In such models, alkalinity increases in glacial periods are driven by reductions in North Atlantic Deep Water (NADW) supply, which lead to increases in deep-water nutrients and dissolution of carbonate sediments, and to increased alkalinity of Circumpolar Deep Water upwelling in the surface Southern Ocean. We use cores from the Southeast Indian Ridge and from the deep Cape Basin in the South Atlantic to show that carbonate dissolution was enhanced during glacial stages in areas now bathed by Circumpolar Deep Water. This suggests that deep Southern Ocean carbonate ion concentrations were lower in glacial stages than in interglacials, rather than higher as suggested by the polar alkalinity model [Broecker and Peng, 1989, doi:10.1029/GB001i001p00015]. Our observations show that changes in Southern Ocean CaCO3 preservation are coherent with changes in the relative flux of NADW, suggesting that Southern Ocean carbonate chemistry is closely linked to changes in deepwater circulation. The pattern of enhanced dissolution in glacials is consistent with a reduction in the supply of nutrient-depleted water (NADW) to the Southern Ocean and with an increase of nutrients in deep water masses. Carbonate mass accumulation rates on the Southeast Indian Ridge (3200-3800 m), and in relatively shallow cores (<3000 m) from the Kerguelen Plateau and the South Pacific were significantly reduced during glacial stages, by about 50%. The reduced carbonate mass accumulation rates and enhanced dissolution during glacials may be partly due to decreases in CaCO3:Corg flux ratios, acting as another mechanism which would raise the alkalinity of Southern Ocean surface waters. The polar alkalinity model assumes that the ratio of organic carbon to carbonate production on surface alkalinity is constant. Even if overall productivity in the Southern Ocean were held constant, a decrease in the CaCO3:Corg ratio would result in increased alkalinity and reduced pCO2 in Southern Ocean surface waters during glacials. This ecologically driven surface alkalinity change may enhance deepwater-mediated changes in alkalinity, and amplify rapid changes in pCO2.

Mineralogy and age determination of sediment core D11957P

The Tore Seamount is a circular, volcano-like feature 100 km in diameter with its summit at 2200 m water depth and a small, 5000 m deep basin in its interior. It is situated approximately 300 km west of Lisbon and is surrounded by deep abyssal plains. This site with a standard pelagic stratigraphy is the southernmost point where the so-called Heinrich events have so far been recorded. A succession of alternating interglacial/glacial periods reveals a stratigraphic record back to the beginning of isotopic stage 7 (225 kyr). Climatic changes are identifiable by coherent variations in colour, carbonate content and distribution of ice-rafted detritus in the carbonate-free fraction. Inputs of ice-rafted quartz are well defined. Characteristics in common with other sites showing Heinrich layers include a high terrigenous to biogenic ratio, a dramatic decrease in the accumulation rate of foraminifera shells, an increase in dolomite abundance and the occurrence of polar foraminiferal species indicating southwards penetration of cold waters which lead us to consider a wider southeastern extent of the North Atlantic ice-rafted detritus belt than hitherto. If the presently accepted position of the Polar Front is maintained, icebergs must have been swept southwards from the southern boundary of the pack ice in a current merging into the ancestral Canary Current, bringing ice-rafted material to the Tore Seamount. The coincidence of reddish-feldspar, probably derived from the northern Appalachian Triassic red facies, with the transparent quartz suggests at least a partial Labrador source for all the Heinrich layers here, including HL 3. In comparison to other sites in the entire North Atlantic, two exceptions stand out: the absence of HL 5 and the low detritus to biogenics ratio for HL 3. The simultaneous occurrence of these two types of ice-rafted minerals is a new piece in the puzzle of the origin of Heinrich layers.

Age model and Nd isotope ratios of DSDO Hole 43-386 and ODP Hole 210-1276A

Modern thermohaline circulation plays a role in latitudinal heat transport and in deep-ocean ventilation, yet ocean circulation may have functioned differently during past periods of extreme warmth, such as the Cretaceous. The Late Cretaceous (100-65 Ma) was an important period in the evolution of the North Atlantic Ocean, characterized by opening ocean gateways, long-term climatic cooling and the cessation of intermittent periods of anoxia (oceanic anoxic events, OAEs). However, how these phenomena relate to deep-water circulation is unclear. We use a proxy for deep-water mass composition (neodymium isotopes; e-Nd) to show that, at North Atlantic ODP Site 1276, deep waters shifted in the early Campanian (~78-83 Ma) from e-Nd values of ~-7 to values of ~-9, consistent with a change in the style of deep-ocean circulation but >10 Myr after a change in bottom water oxygenation conditions. A similar, but more poorly dated, trend exists in e-Nd data from DSDP Site 386. The Campanian e-Nd transition observed in the North Atlantic records is also seen in the South Atlantic and proto-Indian Ocean, implying a widespread and synchronous change in deep-ocean circulation. Although a unique explanation does not exist for the change at present, we favor an interpretation that invokes Late Cretaceous climatic cooling as a driver for the formation of Southern Component Water, which flowed northward from the Southern Ocean and into the North Atlantic and proto-Indian Oceans.

Potassium and argon concentrations and age determination of ODP Site 125-782 and 125-786

K-Ar whole-rock ages have been obtained for 30 samples from Sites 782 and 786, Ocean Drilling Program Leg 125 in the Izu-Bonin (Ogasawara) forearc region. They form a trimodal spread of ages between 9 Ma and 44 Ma and are, with a few exceptions, consistent with the inferred lithostratigraphy. The ages have been interpreted in terms of at least two distinct episodes of magmatic and/or hydrothermal activity. A group of ten samples, including the lava flows, gave an isochron age of 41.3 ± 0.5 Ma (middle-late Eocene). This is thought to represent the age of the principal magmatic development of the volcanic forearc basement, and is comparable to published ages on equivalent rocks from other parts of the forearc basement high (e.g., the Ogasawara Islands). It may be significant that this age is slightly younger than the timing of major plate reorganization in the Western Pacific at about 43 Ma. This was followed by a minor episode of intrusive magmatism at 34.6 ± 0.7 Ma (early Oligocene) which appears to have reset the ages of some of the earlier units. This event probably corresponds to the initiation of rifting of the "proto-arc" to form the Parece Vela Basin. Boninitic samples were erupted during both episodes of magmatism, the earlier being of low-Ca boninite type and the later being of medium- and high-Ca types. It is also possible that a third episode of intrusive magmatism affected the Izu-Bonin forearc region at both Sites 782 and 786 at about 17 Ma. This would be consistent with magmatic activity elsewhere in the region during the Miocene, associated with the end of active spreading in the Parece Vela Basin and the start of arc activity in the West Mariana Ridge.