ALBERT

Description: Measurements of delta 13C, delta 15N, and C/N on diatom-bound organic matter were made over the Holocene and Last Glacial Maximum (LGM) from 3 piston cores in the Southern Ocean, one from each of the three sectors. The site in the Scotia Sea differed considerably from the other two sites by having markedly lower delta 13C, more variable delta 15N and C/N ratios, and a sedimentary diatom assemblage that was never dominated by Fragilariopsis kerguelensis. Although environmental parameters certainly have a strong impact on the isotope ratios, delta 13C is also correlated to the proportion of F. kerguelensis in the 3 cores investigate here (r**2 = 0.8). Extreme values of delta 13C, delta 15N, and C/N at the Last Glacial Maximum were also related to the abundance of winter stages of Eucampia antarctica. These results suggest that diatom specific isotope records should be interpreted in conjunction with information on the species composition of the samples.

Description: The goal of this work has been to examine the influence of upper ocean food web structure and functioning on both the natural and artificially enhanced sequestration of carbon within the ocean. Data obtained in the mesocosm experiment run in the Bay of Hopavågen in August 2012 are used to assess the extent to which organic matter produced within four different food webs is retained in the upper ocean food web versus remineralized back to carbon dioxide and inorganic nutrients (ammonium, dissolved silicon, phosphate) versus exported from the system in the form of rapidly sinking particles. The experiment was carried out in a set of 12 mesocosms covering, in triplicate, 2 different phytoplankton communities (diatom versus non-diatom) exposed to 2 different zooplankton communities (-copepod and +copepod). These starting conditions were established by first filling the bags, roughly simultaneously, with seawater from the Bay of Hopavågen. Mesozooplankton were then removed to the most complete extent possible immediately removed from half of the mesocosms through repeated vertical hauls of a plankton net (200 µm mesh). Nitrate and phosphate was added to half mesocosms daily to promote the growth of non-siliceous phytoplankton (e.g. dinoflagellates or coccolithophores). To the other half of the mesocosms, nitrate, phosphate, and silicate were added to promote the growth of diatoms. Material was allowed to settle and the two distinct phytoplankton populations were allowed to develop for 4 days, after which copepods collected from the Bay of Hopavågen were added back to the half of the N+P mesocosms and to the half of the N+P+Si mesocosms from which mesozooplankton had not been removed at the beginning. This yielded a set of four initial starting conditions (N+P-copepods, N+P+copepods, N+P+Si-copepods, and N+P+Si+copepods). In the primary mesocosms, samples for a set of core parameters were taken every time the mesocosms were sampled. Samples for particulates (PIC, BSi, POC, PON) were collected on GF/F or 0.4 µm polycarbonate.

Description: This dataset contains stable Si isotope values from radiolarian tests and sponge spicules analyzed on MC-ICP-MS and SIMS. These are in association with a study aiming at investigating the changes to the Si cycle during the Paleocene-Eocene Thermal Maximum from siliceous microfossils from ODP hole 1051B.

Description: An 18 million year record of the Ca isotopic composition (d44/42Ca) of planktonic foraminiferans from ODP site 925, in the Atlantic, on the Ceara Rise, provides the opportunity for critical analysis of Ca isotope-based reconstructions of the Ca cycle. ?44/42Ca in this record averages +0.37+/-0.05 (1 sigma SD) and ranges from +0.21‰ to +0.52‰. The record is a good match to previously published Neogene Ca isotope records based on foraminiferans, but is not similar to the record based on bulk carbonates, which has values that are as much as 0.25‰ lower. Bulk carbonate and planktonic foraminiferans from core tops differ slightly in their d44/42Ca (i.e., by 0.06+/-0.06‰ (n = 5)), while the difference between bulk carbonate and foraminiferan values further back in time is markedly larger, leaving open the question of the cause of the difference. Modeling the global Ca cycle from downcore variations in d44/42Ca by assuming fixed values for the isotopic composition of weathering inputs (d44/42Ca_w) and for isotope fractionation associated with the production of carbonate sediments (D_sed) results in unrealistically large variations in the total mass of Ca2+ in the oceans over the Neogene. Alternatively, variations of +/-0.05‰ in the Ca isotope composition of weathering inputs or in the extent of fractionation of Ca isotopes during calcareous sediment formation could entirely account for variations in the Ca isotopic composition of marine carbonates. Ca isotope fractionation during continental weathering, such as has been recently observed, could easily result in variations in d44/42Ca_w of a few tenths of permil. Likewise a difference in the fractionation factors associated with aragonite versus calcite formation could drive shifts in D_sed of tenths of permil with shifts in the relative output of calcite and aragonite from the ocean. Until better constraints on variations in d44/42Ca_w and D_sed have been established, modeling the Ca2+ content of seawater from Ca isotope curves should be approached cautiously.