Major Cation, Carbon System and Trace Element Chemistry in Pore Waters from a Depth Transect of Cores on the Iberian Margin: Implications for Paleoproxies.

A significant body of work exists on the chemistry of pore waters from DSDP and ODP drilling cores (e.g. Gieskes 1975; Sayles 1981) showing large gradients in sea salt cations and anions interpreted in terms of diagenetic reactions such as the formation of Mg-rich clays and dolomite formation (Higgins and Schrag, 2010). Another class of diagenetic reactions involves the breakdown of organic matter and trace element behaviour (Froelich et al., 1979). The translation of chemical gradients into fluxes requires estimates of pore water chemistry across the sea water - sediment surface boundary. Additionally, the use of the chemistry of benthic foraminiferal calcite for seawater paleochemistry requires estimation of the chemistry of pore waters which may differ from that of bottom seawater because of diagenetic reactions.

In this work we have collected multi core samples from 10 core sites on cruise RRS James Cook JC089 on the southwest Iberian continental margin. Pore waters were extracted from the core surface and at 1 cm depth intervals down core (typically to ~40 cm depth) using Rhizon samplers and analysed for Alkalinity, DIC, ∂¹³C and Na, K, Mg, Ca, Li, Mn, Fe, Ba, B, Sr by atomic emission spectrophotometry as well as O₂ penetration and pH by microelectrodes. This has allowed us to inspect chemical behavior at the bottom water - sediment interface. Some examples of results are a large gradient in ∂¹³C of DIC, the similarity of zero O₂ penetration followed by an increase in Mn concentration and then decrease to zero, the similarity of Li to Mn and, in contrast to much DSDP/ODP work, Ca²⁺ and Mg²⁺both decrease with depth in pore waters near the sediment surface.

References: Gieskes J.M. Annu. Rev. Earth Planet. Sci. 3, 433 (1975). Sayles F. L. Geochim. Cosmochim. Acta45, 1061 (1981). Higgins J.A. and D.P. Schrag. Geochim. Cosmochim. Acta.74, 5039 (2010). Froelich, P.N., et al., Geochim. Cosmochim. Acta. 43, 1075 (1979).