Strontium and neodymium isotope data for barite separates and pore waters, and isotope and concentration data for foraminifera and fish teeth from DSDP Leg 85 sediments

Strontium and neodymium isotopic data are reported for barite samples chemically separated from Late Miocene to Pliocene sediments from the eastern equatorial Pacific. At a site within a region of very high productivity close to the equator, 87Sr/86Sr ratios in the barite separates are indistinguishable from those of foraminifera and fish teeth from the same samples. However, at two sites north of the productivity maximum barite separates have slightly, but consistently lower (averaging 0.000062) ratios than the coexisting phases, although values still fall within the total range of published values for the contemporaneous seawater strontium isotope curve. We examine possible causes for this offset including recrystallization of the foraminifera, fish teeth or barite, the presence of non-barite contaminants, or incorporation of older, reworked deep-sea barite; the inclusion of a small amount of hydrothermal barite in the sediments seems most consistent with our data, although there are difficulties associated with adequate production and transportation of this phase. Barite is unlikely to replace calcite as a preferred tracer of seawater strontium isotopes in carbonate-rich sediments, but may prove a useful substitute in cases where calcite is rare or strongly affected by diagenesis. In contrast to the case for strontium, neodymium isotopic ratios in the barite separates are far from expected values for contemporary seawater, and appear to be dominated by an (unobserved) eolian component with high neodymium concentration and low 143Nd/144Nd. These results suggest that the true potential of barite as an indicator of paleocean neodymium isotopic ratios and REE patterns will be realized only when a more selective separation procedure is developed.

South Pacific dissolved Nd isotope compositions, rare earth element and nutrient concentrations from SONNE cruise SO213

Despite its enormous extent and importance for global climate, the South Pacific has been poorly investigated in comparison to other regions with respect to chemical oceanography. Here we present the first detailed analysis of dissolved radiogenic Nd isotopes (epsilon-Nd) and rare earth elements (REEs) in intermediate and deep waters of the mid-latitude (~40°S) South Pacific along a meridional transect between South America and New Zealand. The goal of our study is to gain better insight into the distribution and mixing of water masses in the South Pacific and to evaluate the validity of Nd isotopes as a water mass tracer in this remote region of the ocean. The results demonstrate that biogeochemical cycling (scavenging processes in the Eastern Equatorial Pacific) and release of LREEs from the sediment clearly influence the distribution of the dissolved REE concentrations at certain locations. Nevertheless, the Nd isotope signatures clearly trace water masses including AAIW (Antarctic Intermediate Water) (average epsilon-Nd = -8.2 ± 0.3), LCDW (Lower Circumpolar Deep Water) (average epsilon-Nd = -8.3 ± 0.3), NPDW (North Pacific Deep Water) (average epsilon-Nd = -5.9 ± 0.3), and the remnants of NADW (North Atlantic Deep Water) (average epsilon-Nd = -9.7 ± 0.3). Filtered water samples taken from the sediment-water interface under the deep western boundary current off New Zealand suggest that boundary exchange processes are limited at this location and highlight the spatial and temporal variability of this process. These data will serve as a basis for the paleoceanographic application of Nd isotopes in the South Pacific.

Biogeochemical investigation of cold seep sediments in the Eastern Mediterranean Sea

Cold seep ecosystems are highly productive, fragmented ecosystems of the deep-sea floor. They form worldwide where methane reaches the surface seafloor, and are characterized by rich chemosynthetic communities fueled by the microbial utilization of hydrocarbons. Here we investigated with in situ (benthic chamber, microprofiler) and ex situ (pore water constituents, turnover rates of sulfate and methane, prokaryote abundance) techniques reduced sites from three different seep ecosystems in the Eastern Mediterranean deep-sea. At all three cold seep systems, the Amon Mud Volcano, Amsterdam Mud Volcano and the Nile Deep Sea Fan Pockmark area, we observed and sampled patches of highly reduced, methane-seeping sulfidic sediments which were separated by tens to hundreds of (kilo)meters with non-reduced oxygenated seafloor areas. All investigated seep sites were characterized by gassy, sulfidic sediments of blackish color, of which some were overgrown with thiotrophic bacterial mats. Fluxes of methane and oxygen, as well as sulfate reduction rates varied between the different sites.

Neodymium isotope signature of Holocene corals

The neodymium isotopic composition of marine precipitates is increasingly recognized as a powerful tool for identifying changes in ocean circulation and mixing on million year to millennial timescales. Unlike nutrient proxies such as ?13C or Cd/Ca, Nd isotopes are not thought to be altered in any significant way by biological processes, and thus they can serve as a quasi-conservative water mass tracer. However, the application of Nd isotopes in understanding the role of thermohaline circulation in rapid climate change is currently hindered by the lack of direct constraints on the signature of the North Atlantic end-member through time. Here we present the first results of Nd isotopes measured in U-Th-dated deep-sea corals from the New England seamounts in the northwest Atlantic Ocean. Our data are consistent with the conclusion that the Nd isotopic composition of North Atlantic deep and intermediate water has remained nearly constant through the last glacial cycle. The results address long-standing concerns that there may have been significant changes in the Nd isotopic composition of the North Atlantic end-member during this interval and substantiate the applicability of this novel tracer on millennial timescales for paleoceanography research.

Images of a groundwater flow system in Nicaragua

Conceptualization of groundwater flow systems is necessary for water resources planning. Geophysical, hydrochemical and isotopic characterization methods were used to investigate the groundwater flow system of a multi-layer fractured sedimentary aquifer along the coastline in Southwestern Nicaragua. A geologic survey was performed along the 46 km2 catchment. Electrical resistivity tomography (ERT) was applied along a 4.4 km transect parallel to the main river channel to identify fractures and determine aquifer geometry. Additionally, three cross sections in the lower catchment and two in hillslopes of the upper part of the catchment were surveyed using ERT. Stable water isotopes, chloride and silica were analyzed for springs, river, wells and piezometers samples during the dry and wet season of 2012. Indication of moisture recycling was found although the identification of the source areas needs further investigation. The upper-middle catchment area is formed by fractured shale/limestone on top of compact sandstone. The lower catchment area is comprised of an alluvial unit of about 15 m thickness overlaying a fractured shale unit. Two major groundwater flow systems were identified: one deep in the shale unit, recharged in the upper-middle catchment area; and one shallow, flowing in the alluvium unit and recharged locally in the lower catchment area. Recharged precipitation displaces older groundwater along the catchment, in a piston flow mechanism. Geophysical methods in combination with hydrochemical and isotopic tracers provide information over different scales and resolutions, which allow an integrated analysis of groundwater flow systems. This approach provides integrated surface and subsurface information where remoteness, accessibility, and costs prohibit installation of groundwater monitoring networks.