Planktic foraminifera test iodine to calcium ratios represent an emerging proxy method to assess subsurface seawater oxygenation states. Several core-top studies show lower planktic foraminifera I/Ca in locations with oxygen depleted subsurface waters compared to well oxygenated environments. The reasoning behind this trend is that only the oxidized species of iodine, iodate, is incorporated in foraminiferal calcite. The I/Ca of foraminiferal calcite is thought to reflect iodate contents in seawater. To test this hypothesis, we compare planktic foraminifera I/Ca ratios, obtained from plankton tows, with published and new seawater iodate concentrations from 1) the Eastern North Pacific with extensive oxygen depletion, 2) the Benguela Current System with moderately depleted oxygen concentrations, and 3) the well oxygenated North and South Atlantic. We find the lowest I/Ca ratios (0.07 µmol/mol) in planktic foraminifera retrieved from the Eastern North Pacific, and higher values for samples (up to 0.72 µmol/mol) obtained from the Benguela Current System and North and South Atlantic. The I/Ca ratios of plankton tow foraminifera from environments with well oxygenated subsurface waters, however, are an order of magnitude lower compared to core-tops from similarly well-oxygenated regions. This would suggest that planktic foraminifera gain iodine post-mortem, either when sinking through the water column, or during burial.
more »
« less
Foraminifera Iodine to Calcium Ratios: Approach and Cleaning
Planktic and benthic foraminiferal iodine (I) to calcium (Ca) molar ratios have been proposed as an exciting new proxy to assess subsurface and bottom water oxygenation in the past. Compared to trace metals, the analysis of iodine in foraminiferal calcite is more challenging, as iodine is volatile in acid solution. Here, we compare previous analyses that use tertiary amine with alternative analyses using tetramethylammonium hydroxide (TMAH) and ammonium hydroxide (NH4OH) to stabilize iodine in solution. In addition, we assess the effect of sample size and cleaning on planktic and benthic foraminiferal I/Ca. Our stabilization experiments with TMAH and NH4OH show similar trends as those using tertiary amine, giving relatively low I/Ca ratios for planktic and benthic foraminifera samples from poorly oxygenated waters, and high ratios for well‐oxygenated waters. This suggests that both alternative methods are suitable to stabilize iodine initially dissolved in acid. Samples that contain 5–10 specimens show a wide spread in I/Ca. Samples containing 20 specimens or more show more centered I/Ca values, indicating that a larger sample size is more representative of the average planktic foraminifera community. The impact of cleaning on planktic and benthic foraminifera I/Ca ratios is very similar to Mg/Ca, with the largest effect occurring during the clay removal step. The largest iodine contaminations were recorded at locations characterized by moderate to high organic carbon contents. In those circumstances, we recommend doubling the oxidative cleaning steps (4 instead of 2 repetitions) to ensure that all organic material is removed.
more » « less- PAR ID:
- 10363485
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 22
- Issue:
- 11
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract The iodine to calcium ratio in carbonate (I/Ca) has been widely used to indicate ocean oxygenation level in the past. Given the volatility of iodine, I/Ca has been measured in alkaline solutions in previous studies. However, this limits the application of I/Ca with other element/Ca (El/Ca) proxies at the same time and in the same foraminifera because other El/Ca data are preferably analyzed in acidic solutions. This study assesses the reliability of I/Ca measurements in acidic solutions measured with other El/Ca as well as the effects of different sample pre‐treatments on measured foraminiferal I/Ca. Our results show that when samples are measured within hours of prepaparation, the pH of the final solution has an insignificant effect on I/Ca measurements of a carbonate reference material JCp‐1 and a multi‐element standard solution, consistent with the slow kinetics of iodine volatilization. We find, however, that low pH possibly reduces the measured I/Ca in foraminiferal tests in some samples. Our experiments also suggest a resolvable effect of reductive cleaning, yielding lower foraminiferal I/Ca compared to without reductive cleaning. The HNO3concentration used to dissolve foraminiferal shells has a negligible effect. Despite the different solution pHs and cleaning and dissolving methods, our core top planktic I/Ca data are able to differentiate well‐oxygenated from oxygen‐depleted waters in the upper ocean, and after correcting for cleaning effect, our data are generally consistent with the published studies that analyzed I/Ca without reductive cleaning and in basic solutions. This study shows that measurements of I/Ca within hours of sample dissolutions yield reliable planktic I/Ca data, while also allowing the acquisition of other El/Ca values for paleoceanographic studies.
-
none. (Ed.)Foraminifera are single celled organisms that have tests that are composed of calcium carbonate or detrital materials. The assemblages of foraminifera have been influenced by their immediate environment which depict the influence and results of man’s activities and other natural processes that occur in the environment. These environmental changes include salinity, pH, hydrocarbon pollution and organic matter. With these factors, paleoenvironmental interpretations are made by identifying the different patterns in the foraminifera communities. Variations in oxygen concentrations at the sediment-water interface have a significant impact on benthic foraminiferal assemblages and morphologic properties. This is seen in the vertical distribution of foraminifera in response to factors such as food, pore water, and oxygen. This study documents foraminiferal ecology and abundances across an oxygen transect off the coast of San Diego. Available oxygen ranges from >1.0ml/l are considered oxic; O2 values from 0.1 - 1.0ml/l will be considered dysoxic and O2 values <0.10ml/l will be considered anoxic. Previous work in this region has suggested that sediment grain size, rather than oxygen availability, may have as much of an impact on foraminiferal assemblages. These observations were made based on the fact that Cibicidoides wuellerstorfi, an epibenthic foraminifera preferring elevated substrates in well-oxygenated environments, were found in greater abundances at areas with coarser grained materials despite low available oxygen. C. wuellerstorfi has also been found to have I/Ca and test porosity (size and abundance of pores on the surface of the test) which correlate to the available oxygen in bottom waters at the time of test formation. Not only will this study document foraminiferal assemblages and abundances across an oxygen transect, but C. wuellerstorfi from key oxygen environments will be examined under SEM and used in porosity and I/Ca analyses which will contribute to the development of a quantitative oxygen proxy. The development of this quantitative oxygen proxy is essential because despite oxygen being one of the primary variables influencing major geochemical and faunal responses within the world’s ocean, no clear proxy currently exists in paleoceanographic reconstructions.more » « less
-
more » « less
Abstract. The close association between planktic foraminiferal assemblages and local hydrography make foraminifera invaluable proxies for environmental conditions. Modern foraminiferal seasonality is important for interpreting fossil distributions and shell geochemistry as paleoclimate proxies. Understanding this seasonality in an active upwelling area is also critical for anticipating which species may be vulnerable to future changes in upwelling intensity and ocean acidification. Two years (2012–2014) of plankton tows, along with conductivity–temperature–depth profiles and carbonate chemistry measurements taken along the north-central California shelf, offer new insights into the seasonal dynamics of planktic foraminifera in a seasonal coastal upwelling regime. This study finds an upwelling affinity for Neogloboquadrina pachyderma as well as a seasonal and upwelling associated alternation between dominance of N. pachyderma and Neogloboquadrina incompta, consistent with previous observations. Globigerina bulloides, however, shows a strong affinity for non-upwelled waters, in contrast to findings in Southern California where the species is often associated with upwelling. We also find an apparent lunar periodicity in the abundances of all species and document the presence of foraminifera even at very low saturation states of calcite.
-
To use planktic foraminiferal tests as paleoproxy substrates, it is necessary to delineate environmental versus biological controls on trace element incorporation. Here we utilize laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to explore interspecies, chamber-to-chamber, and intratest trace element (i.e., Mg, Na, Sr, Ba, Mn, Zn) variability in thickly-calcified specimens of the polar and subpolar planktic foraminifera Neogloboquadrina incompta, N. pachyderma, and Turborotalita quinqueloba collected from plankton tows in the Northern California Current. Among the study taxa, test Mg/Ca, Na/Ca, and Sr/Ca are likely dominantly controlled by depth habitat. The neogloboquadrinids record higher Ba/Ca and Mn/Ca, and also show positive covariance between these elements, possibly due to calcifying in an oxygen-depleted marine snow microhabitat. Trace elements are found to be more enriched in the lamellar calcite than the outer chamber wall dominated by gametogenic crust. The data presented herein provide insight into potential vital effects, paleoproxy considerations, ontogeny, and biomineralization processes.more » « less
