Damien Cardinal

Sea ice diatom contributions to Holocene nutrient utilization in East Antarctica

Combined high-resolution Holocene δ30Sidiat and δ13Cdiat paleorecords are presented from the Seasonal Ice Zone, East Antarctica. Both data sets reflect periods of increased nutrient utilization by diatoms during the Hypsithermal period (circa 7800 to 3500u2009calendar years (cal years) B.P.), coincident with a higher abundance of open water diatom species (Fragilariopsis kerguelensis), increased biogenic silica productivity (%BSi), and higher regional summer temperatures. The Neoglacial period (after circa 3500u2009cal years B.P.) is reflected by an increase in sea ice indicative species (Fragilariopsis curta and Fragilariopsis cylindrus, up to 50%) along with a decrease in %BSi and δ13Cdiat (< −18‰ to −23‰). However, over this period, δ30Sidiat data show an increasing trend, to some of the highest values in the Holocene record (average of +0.43‰). Competing hypotheses are discussed to account for the decoupling trend in utilization proxies including iron fertilization, species-dependent fractionation effects, and diatom habitats. Based on mass balance calculations, we highlight that diatom species derived from the semi-enclosed sea ice environment may have a confounding effect upon δ30Sidowncore compositions of the seasonal sea ice zone. A diatom composition, with approximately 28% of biogenic silica derived from the sea ice environment (diat-SI) can account for the increased average composition of δ30Sidiat during the Neoglacial. These data highlight the significant role sea ice diatoms can play with relation to their export in sediment records, which has implications on productivity reconstructions from the seasonal ice zone.

GEOTRACES Intercalibration of the Stable Silicon Isotope Composition of Dissolved Silicic Acid in Seawater

The first inter-calibration study of the stable silicon isotope composition of dissolved silicic acid in seawater, δ30Si(OH)4, is presented as a contribution to the international GEOTRACES program. Eleven laboratories from seven countries analyzed two seawater samples from the north Pacific subtropical gyre (Station ALOHA) collected at 300 m and at 1000 m water depth. Sampling depths were chosen to obtain samples with a relatively low (9 μmol L-1, 300 m) and a relatively high (113 μmol L-1, 1000 m) silicic acid concentration as sample preparation differs for low- and high- concentration samples. Data for the 1000m water sample were not normally distributed so the median is used to represent the central tendency for the two samples. Median δ30Si(OH)4 values of +1.66 ‰ for the low-concentration sample and +1.25 ‰ for the high-concentration sample were obtained. Agreement among laboratories is overall considered very good; however, small but statistically significant differences among the mean isotope values obtained by different laboratories were detected likely reflecting interlaboratory differences in chemical preparation including pre-concentration and purification methods together with different volumes of seawater volume analyzed, and the use of different mass spectrometers including the Neptune MC-ICP-MS (Thermo Fisher™, Germany), the Nu Plasma MC-ICP-MS (Nu Instruments™, Wrexham, UK), and the Finnigan™ (now Thermo Fisher™, Germany) MAT 252 IRMS. Future studies analyzing δ30Si(OH)4 in seawater should also analyze and report values for these same two reference waters in order to facilitate comparison of data generated among and within laboratories over time.

Seasonal variations, origin, and fate of settling diatoms in the Southern Ocean tracked by silicon isotope records in deep sediment traps

The Southern Ocean plays a pivotal role in the control of atmospheric CO 2 levels, via both physical and biological sequestration processes. The biological carbon transfer to the ocean interior is tightly coupled to the availability of other elements, especially iron as a trace-limiting nutrient and dissolved silicon as the mineral substrate that allows diatoms to dominate primary production. Importantly, variations in the silicon cycling are large but not well understood. Here we use δ 30 Si measurements to track seasonal flows of silica to the deep sea, as captured by sediment trap time series, for the three major zones (Antarctic, AZ; Polar Frontal, PFZ; and Sub-Antarctic, SAZ) of the open Southern Ocean. Variations in the exported flux of biogenic silica (BSi) and its δ 30 Si composition reveal a range of insights, including that (i) the sinking rate of BSi exceeds 200u2009mu2009d −1 in summer in the AZ yet decreases to very low values in winter that allow particles to remain in the water column through to the following spring, (ii) occasional vertical mixing events affect the δ 30 Si composition of exported BSi in both the SAZ and AZ, and (iii) the δ 30 Si signature of diatoms is well conserved through the water column despite strong BSi and particulate organic carbon (POC) attenuation at depth and is closely linked to the Si consumption in surface waters. With the strong coupling observed between BSi and POC fluxes in PFZ and AZ, these data provide new constraints for application to biogeochemical models of seasonal controls on production and export.

Isotopic model of oceanic silicon cycling: The Kerguelen Plateau case study

A box model is presented describing the time evolution for the three stable Si isotopes (or total concentration and natural isotopic compositions), both in the dissolved and biogenic pools. Temporal variations are controlled by uptake, dissolution (both with isotopic fractionation), settling/export and mixing/advection (without isotopic fractionation). The basic building blocks of the model are combined to form a setup for the Kerguelen Plateau where distinct ‘‘plateau’’ and ‘‘out-plateau’’ areas exist and where measurements were made at the end of the growth season (early 2005, KEOPS cruise: Kerguelen Ocean and Plateau compared Study). In addition, we distinguished between surface (0–100 m) and subsurface (100–400 m) water. This resulted in a model composed of eight compartments, each containing three variables (the three Si isotopes) whose time evolution can be modelled. The model does not assume steady state, and can therefore be used to simulate transient events like blooms. We applied the model to simulate the 2004–2005 growth season. The model parameterisations were kept as simple as possible. Still, the KEOPS measurements were satisfactorily reproduced and estimates of instantaneous and seasonally integrated fluxes compared well with previous literature. & 2012 Elsevier Ltd. All rights reserved.