Global Spatial and Temporal Variation of Cd:P in Euphotic Zone Particulates

Abstract:

The merits of dissolved cadmium as a paleo-proxy for phosphorus has long been discussed. Dissolved ratios of Cd:P (nmol/µmol) have been shown in previous studies to be relatively constant in deep-water samples. Younger Atlantic waters typically have dissolved Cd:P of about 0.2 while older Pacific waters have a higher value of 0.36. The mid-water dissolved Cd:P differences between the Atlantic and Pacific represent an integrated signature of remineralized particulates over a span of hundreds to 1,000 years. The trends of upper ocean dissolved Cd:P suggest regional differences exist due to differences in phytoplankton uptake and particle export. Particles turn over quickly (week time scales in near surface waters – year time scales at 1000 m) and therefore direct analysis can be used to understand what causes variability in the Cd:P ratios in different phytoplankton communities under a variety of conditions. To address the drivers of inter-ocean variation, concentrations of elements, in particular Cd, Co, Zn and P were determined by Inductively Coupled Plasma Mass-Spectrometry in size-fractionated particulates collected using the Multiple Unit Large Volume in-situ Filtration System (MULVFS) from the upper 1000 m at the 34 different locations throughout the Atlantic, Pacific and Southern Oceans since 1991. The MULVFS technique simultaneously collects samples in three size fractions from thousands of liters of water: <1 μm, 1-51 μm and >51 μm. The collection sites have wide spatial and temporal distribution, allowing us to study the effects of an El Nino, upwelling, large scale in-situ Fe fertilization, anoxic conditions, a phytoplankton bloom and subsequent decline, and seasonal variation of Cd:P in particulates. Overall, Cd:P tends to be higher (typically 1-2 pmol/nmol) in particulates gathered in biologically dynamic waters, especially in High Nutrient Low Chlorophyll (HNLC) areas such as the subarctic north Pacific, equatorial Pacific, and Southern Ocean, and in Upwelling areas, and is much lower (typically <0.1 pmol/nmol) in oligotrophic regions. Here we present a global analysis exploring how differences in regional Cd:P particulates lead to the relatively constant dissolved deep water ratios. These results are the first global collection of particulate euphotic zone Cd:P ratios.