L. J. de Nooijer

Salinity controls on Na incorporation in Red Sea planktonic foraminifera

Whereas several well-established proxies are available for reconstructing past temperatures, salinity remains challenging to assess. Reconstructions based on the combination of (in)organic temperature proxies and foraminiferal stable oxygen isotopes result in relatively large uncertainties, which may be reduced by application of a direct salinity proxy. Cultured benthic and planktonic foraminifera showed that Na incorporation in foraminiferal shell calcite provides a potential independent proxy for salinity. Here we present the first field calibration of such a potential proxy. Living planktonic foraminiferal specimens from the Red Sea surface waters were collected and analyzed for their Na/Ca content using laser ablation quadrupole inductively coupled plasma mass spectrometry. Using the Red Sea as a natural laboratory, the calibration covers a broad range of salinities over a steep gradient within the same water mass. For both Globigerinoides ruber and Globigerinoides sacculifer calcite Na/Ca increases with salinity, albeit with a relatively large intraspecimen and interspecimen variability. The field-based calibration is similar for both species from a salinity of ~36.8 up to ~39.6, while values for G. sacculifer deviate from this trend in the northernmost transect. It is hypothesized that the foraminifera in the northernmost part of the Red Sea are (partly) expatriated and hence should be excluded from the Na/Ca-salinity calibration. Incorporation of Na in foraminiferal calcite therefore provides a potential proxy for salinity, although species-specific calibrations are still required and more research on the effect of temperature is needed.

The long‐term impact of magnesium in seawater on foraminiferal mineralogy: Mechanism and consequences

Foraminifera are unicellular protists, primarily known for their calcium carbonate shells that provide an extensive fossil record. This record, ranging from Cambrian to present shows both major shifts and gradual changes in the relative occurrence of taxa producing different polymorphs of carbonate. Here we present evidence for coupling between shifts in calcite- versus aragonite-producing species and periods with, respectively, low and high seawater Mg/Ca throughout the Phanerozoic. During periods when seawater Mg/Ca is <2 mol/mol, low-Mg calcite-producing species dominate the foraminiferal community. Vice versa, high-Mg calcite- and aragonite-producing species are more abundant during periods with relatively high seawater Mg/Ca. This alteration in dominance of the phase precipitated is due to selective recovery of groups producing the favorable polymorph after shifts from calcite to aragonite seas. In addition, relatively high extinction rates of species producing the mineral phase not favored by the seawater Mg/Ca of that time may be responsible for this alteration. These results imply that the current high seawater Mg/Ca will, in the long term, favor prevalence of high-Mg and aragonite-producing foraminifera over calcite-producing taxa, possibly shifting the balance toward a community in which calcite production is less dominant.

A Saltier Glacial Mediterranean Outflow: A saltier Glacial Mediterranean Outflow

AbstractThe state of Atlantic Meridional Overturning Circulation (AMOC) is influenced by both thestrength and the location of the Mediterranean Outflow Water (MOW) plume in the Gulf of Cadiz. Toevaluate the influence of MOW on AMOC over deglaciations, precise and accurate salinity and temperaturereconstructions are needed. For this purpose, we measured Mg/Ca and clumped isotopes of several benthicforaminiferal species at Integrated Ocean Drilling Program Site U1390 in the Gulf of Cadiz. The clumpedisotope results of Cibicidoides pachyderma, Uvigerina mediterranea, and Pyrgo spp. are consistent betweenspecies and record no significant difference in Last Glacial Maximum to Holocene deep water temperature.Over the deglaciation, the Mg/Ca-based temperatures derived from U. mediterranea indicate three periods ofMOW absence at Site U1390. Mg/Ca-based temperatures of Hoeglundina elegans and C. pachyderma are onaverage 6°C too cold when compared to the present core-top temperature, which we explain by a carbonateion effect on these epibenthic species related to the high alkalinity of the MOW. Combining deep watertemperature estimates with the benthic oxygen isotope data and considering different relationshipsbetween seawater oxygen isotopes and salinity, we infer a salinity decrease of MOW by three to eight unitsover the deglaciation and four units during Sapropel 1, accounting for the global δ18O depletion due to thedecrease in ice volume. Our findings confirm that the Mediterranean Sea accumulates excess salt during aglacial low stand and suggest that this salt surged into the Atlantic over the deglaciation, presumably duringHeinrich Stadial 1.Plain Language Summary The Gulf Stream is slowing down because of the meltdown of theGreenland ice sheet. In the past, such a slowdown often resulted in a brief but quite extreme climatecooling in the Northern Hemisphere. Fortunately, the Gulf Stream would eventually speed up again forreasons that remain poorly understood. It is thought that the exchange of water between the Atlantic Oceanand the Mediterranean Sea through the Strait of Gibraltar plays an important role in bringing the Gulf Streamback to speed. In order to test this idea, we need to know the strength of the Atlantic-Mediterraneanexchange during times at which the Gulf Stream slowed down. Little shell-like organisms called benthicforaminifera, which live at the bottom of the ocean, record information about the properties of the water inwhich they grow within their shells. By analyzing a set of foraminifera living at a location close to the Strait ofGibraltar, we infer that it is indeed likely that the Atlantic-Mediterranean exchange changed significantlyduring a slowdown of the Gulf Stream. It is questionable whether or not the Gibraltar exchange will alsointensify due to the current melting of ice.back to speed. In order to test this idea, we need to know the strength of the Atlantic-Mediterraneanexchange during times at which the Gulf Stream slowed down. Little shell-like organisms called benthicforaminifera, which live at the bottom of the ocean, record information about the properties of the water inwhich they grow within their shells. By analyzing a set of foraminifera living at a location close to the Strait ofGibraltar, we infer that it is indeed likely that the Atlantic-Mediterranean exchange changed significantlyduring a slowdown of the Gulf Stream. It is questionable whether or not the Gibraltar exchange will alsointensify due to the current melting of ice.

The Impacts of Seawater Mg/Ca and Temperature on Element Incorporation in Benthic Foraminiferal Calcite

On geological timescales, oceanic [Mg2+] and [Ca2+] vary with changing rates of weathering, seafloor spreading and dolomite formation. Accurate reconstruction of the ratio between [Mg2+] and [Ca2+] in seawater (Mg/Casw), may potentially be reconstructed using foraminiferal Mg/Ca ratios. Since both temperature and seawater Mg/Ca impact foraminiferal Mg/Ca, successful reconstruction of Mg/Casw requires quantification of both these parameters independently on foraminiferal Mg/Ca, as well as their combined effect on Mg-incorporation. Here we present the combined and isolated impacts of temperature and Mg/Casw on Mg incorporation in two model species, the benthic hyaline (i.e., perforate) foraminifer Elphidium crispum and porcelaneous (i.e., miliolid) foraminifer Quinqueloculina sp. using controlled growth experiments. Specimens of these two species were kept at four different temperatures (ranging from 10 to 27°C) and three Mg/Casws (3.4, 6.4 and 8.5 mol/mol), resulting in 12 experimental conditions. Newly grown calcite was analyzed for a number of elements (Na, Mg and Sr) by laser ablation-ICP-MS. Results show that although the Mg/Ca varied by more than an order of magnitude between species, the sensitivity of Mg incorporation with respect to temperature appeared not to be influenced by Mg/Casw. By extension, these results may also help improving accuracy in the reconstruction of past Mg/Casw based on foraminifera with contrasting Mg/Ca.