TOC, carbonate, opal composition and trace element concentration from ODP Holes 175-1082A and 175-1084A

Distinctive light-dark color cycles in sediment beneath the Benguela Current Upwelling System indicate repetitive alternations in sediment delivery and deposition. Geochemical proxies for paleoproductivity and for depositional conditions were employed to investigate the paleoceanographic processes involved in creating these cycles in three mid-Pleistocene intervals from ODP Sites 1082 and 1084. Concentrations of total organic carbon (TOC) vary between 3.5 and 17.1%. Concentrations of CaCO3 vary inversely to TOC and Al, which suggests that both carbonate dissolution and terrigenous dilution contribute to the light-dark cycles. Opal concentrations are independent of both TOC and CaCO3, therefore eliminating diatom production and lateral transport of shelf material as causes of the light-dark cycles. d13Corg and d15Ntot values do not vary across light-dark sediment intervals, implying that the extent of relative nutrient utilization did not change. The stable d15Ntot values represent a balanced change in nitrate supply and export production and therefore indicate that productivity was elevated during deposition of the TOC-rich layers. Parallel changes in concentrations of indicator trace elements and TOC imply that changes in organic matter delivery influenced geochemical processes on the seafloor by controlling consumption of pore water oxygen. Cu, Ni, and Zn are enriched in the darker sediment as a consequence of greater organic matter delivery. Redox-sensitive metals vary due to loss (Mn and Ba) or enrichment (Mo) under reducing conditions created by TOC oxidation. Organic matter delivery impacts subsequent geochemical changes such as carbonate dissolution, sulfate reduction and the concentration of metals. Thus, export production is considered ultimately responsible for the generation of the color cycles.

Time series of zooplankton abundance at station L4 in the English Channel from 1988 to 2011

Ongoing zooplankton research at the Plymouth Marine Laboratory has established a time series of zooplankton species since 1988 at L4, a coastal station off Plymouth. Samples were collected by vertical net hauls (WP2 net, mesh 200 µm; UNESCO 1968) from the sea floor (approximately 50 m) to the surface and stored in 4% formalin. Much of the zooplankton analysis has been to the level of "major taxonomic groups" only, and a number of different analysts have participated over the years. The level of expertise has generally been consistent, but the user should be aware that levels of taxonomic discrimination may vary during the course of the dataset. The dominant calanoid copepods are generally well discriminated to species throughout. Calanus has not been routinely examined for species determination, the assumption being that the local population is entirely composed of Calanus helgolandicus. In certain years there has been a particular interest in Temora stylifera, Centropages cherchiae and other species reflected in the dataset. The lack of records in other previous years does not necessarily reflect species absence. We view it as essential for all users of L4 plankton data to establish and maintain contact with the nominated current data originators as well as fully consulting the metadata. While not impinging on free data access, this ensures that this large, species-rich but slightly complex species database is being used in the correct way, and any potential issues with the data are clarified. Furthermore, a proper dialogue with these local experts on the time series will enable where appropriate the most recent sampling timepoints to be used. The data can be downloaded from BODC or from doi:10.1594/PANGAEA.778092 as files for each year by searching for "L4 zooplankton". The most comprehensive dataset is the version downloadable directly from this page. The entire set of zooplankton samples is stored at the Plymouth Marine Laboratory in buffered formalin, and may be available for further taxonomic analysis on request.

Calcium isotope ratios of pore fluids and solid phase of organic rich sediments

Pore fluid calcium isotope, calcium concentration and strontium concentration data are used to measure the rates of diagenetic dissolution and precipitation of calcite in deep-sea sediments containing abundant clay and organic material. This type of study of deep-sea sediment diagenesis provides unique information about the ultra-slow chemical reactions that occur in natural marine sediments that affect global geochemical cycles and the preservation of paleo-environmental information in carbonate fossils. For this study, calcium isotope ratios (d44/40Ca) of pore fluid calcium from Ocean Drilling Program (ODP) Sites 984 (North Atlantic) and 1082 (off the coast of West Africa) were measured to augment available pore fluid measurements of calcium and strontium concentration. Both study sites have high sedimentation rates and support quantitative sulfate reduction, methanogenesis and anaerobic methane oxidation. The pattern of change of d44/40Ca of pore fluid calcium versus depth at Sites 984 and 1082 differs markedly from that of previously studied deep-sea Sites like 590B and 807, which are composed of nearly pure carbonate sediment. In the 984 and 1082 pore fluids, d44/40Ca remains elevated near seawater values deep in the sediments, rather than shifting rapidly toward the d44/40Ca of carbonate solids. This observation indicates that the rate of calcite dissolution is far lower than at previously studied carbonate-rich sites. The data are fit using a numerical model, as well as more approximate analytical models, to estimate the rates of carbonate dissolution and precipitation and the relationship of these rates to the abundance of clay and organic material. Our models give mutually consistent results and indicate that calcite dissolution rates at Sites 984 and 1082 are roughly two orders of magnitude lower than at previously studied carbonate-rich sites, and the rate correlates with the abundance of clay. Our calculated rates are conservative for these sites (the actual rates could be significantly slower) because other processes that impact the calcium isotope composition of sedimentary pore fluid have not been included. The results provide direct geochemical evidence for the anecdotal observation that the best-preserved carbonate fossils are often found in clay or organic-rich sedimentary horizons. The results also suggest that the presence of clay minerals has a strong passivating effect on the surfaces of biogenic carbonate minerals, slowing dissolution dramatically even in relation to the already-slow rates typical of carbonate-rich sediments.