Kevin W. Burton

Actual timing of neodymium isotopic variations recorded by FeMn crusts in the western North Atlantic

Hydrogenetic ferromanganese (FeMn) crusts from the western North Atlantic record variations in the Nd and Pb isotopic composition of Cenozoic deep water preserved during their growth. The timing and cause of the most striking change have been the subject of debate. Some have proposed that the shift took place after 4 Ma in response to the closure of the Panama gateway. Others have argued that the major change in isotope composition occurred as early as 8 Ma. This study presents high-resolution Nd isotope records for crusts previously dated using 10Be/9Be chronology. These data confirm that the shifts in Nd occurred after 4 Ma, consistent with a likely relationship with the closure of the Central American Isthmus and intensification of Northern Hemisphere Glaciation, and in accordance with changes seen in other physical and chemical records. These results illustrate the need for both a robust chronological framework and high-resolution records before a reliable paleoceanographic interpretation can be made of the variations recorded by FeMn crusts.

Changes in erosion and ocean circulation recorded in the Hf isotopic compositions of North Atlantic and Indian Ocean ferromanganese crusts

Abstract High-resolution Hf isotopic records are presented for hydrogenetic Fe–Mn crusts from the North Atlantic and Indian Oceans. BM1969 from the western North Atlantic has previously been shown to record systematically decreasing Nd isotopic compositions from about 60 to ∼4 Ma, at which time both show a rapid decrease to unradiogenic Nd composition, thought to be related to the increasing influence of NADW or glaciation in the northern hemisphere. During the Oligocene, North Atlantic Hf became progressively less radiogenic until in the mid-Miocene (∼15 Ma) it reached +1. It then shifted gradually back to an ϵHf value of +3 at 4 Ma, since when it has decreased rapidly to about −1 at the present day. The observed shifts in the Hf isotopic composition were probably caused by variation in intensity of erosion as glaciation progressed in the northern hemisphere. Ferromanganese crusts SS663 and 109D are from about 5500 m depth in the Indian Ocean and are now separated by ∼2300 km across the Mid-Indian Ridge. They display similar trends in Hf isotopic composition from 20 to 5 Ma, with the more northern crust having a composition that is consistently more radiogenic (by ∼2 ϵHf units). Paradoxically, during the last 20 Ma the Hf isotopic compositions of the two crusts have converged despite increased separation and subsidence relative to the ridge. A correlatable negative excursion at ∼5 Ma in the two records may reflect a short-term increase in erosion caused by the activation of the Himalayan main central thrust. Changes to unradiogenic Hf in the central Indian Ocean after 5 Ma may alternatively have been caused by the expanding influence of NADW into the Mid-Indian Basin via circum-Antarctic deep water or a reduction of Pacific flow through the Indonesian gateway. In either case, these results illustrate the utility of the Hf isotope system as a tracer of paleoceanographic changes, capable of responding to subtle changes in erosional regime not readily resolved using other isotope systems.

Using (234U/238U) to assess diffusion rates of isotope tracers in ferromanganese crusts

Abstract Ferromanganese crusts record seawater isotope history for several elements useful for paleoceanography. Interpreting crust isotope profiles, however, requires an assessment of the rate of diffusion of each element within the crust. We address this issue using U- and Th-isotope profiles, coupled with concentration measurements of some trace elements whose isotopes are often measured in crusts. A depth profile of (230Thxs/234U) for a North Atlantic crust (Alv539) yields a growth rate of 3.05 mm/Myr, consistent with an existing 10Be/9Be record. But the growth rate implied by a (234U/238U) profile in the same crust is ≈6 times faster due to diffusion of U in the crust. The difference between the observed (234U/238U) profile and that expected from (230Thxs/234U) enables the effective diffusivity of U to be calculated as 1.2×10−6 cm2/year. Recalculated literature data indicate that other crusts have diffusivities which range down to 4.7×10−8 cm2/yr. This variation is to be expected given the range of porosity observed for ferromanganese crusts. Concentration measurements of various elements in the surface layer of the crust provide relative distribution coefficients for each element between seawater and crust. These distribution coefficients enable the effective diffusivity of each element to be calculated by comparison with the diffusivity of U. Such diffusivities are assessed for crust Alv539 and give a first indication of the expected preservation of isotope signals in ferromanganese crusts. Li, Os, and Sr have diffusivities of 2×10−4, 3×10−5 and 2×10−5 cm2/year respectively and diffusion is expected to be sufficiently fast that crusts will not accurately preserve past seawater compositions. Th, Nd, Pb and Be have diffusivities less than 10−9 cm2/year and are highly immobile in crusts, a reassuring result for Th and Be crust chronologies and for Nd- and Pb-isotope tracing of past water masses using crusts. Hf has a diffusivity of 3×10−8 in Alv539 — fast enough to perturb but not destroy its isotope history. For studies of Hf in this and other crusts, diffusion should therefore be considered. Measuring U-isotope profiles provides a potential way of assessing such diffusion for Hf or for other elements with relatively low distribution coefficients into ferromanganese crusts.