Search

The sodium content of biogenic carbonates shows potential as a palaeoceanographic proxy for salinity and/or calcium concentration but the incorporation of Na+ into inorganic and biogenic calcite is poorly understood. Taxonomic and conspecific variations in the sensitivity of carbonate Na/Ca to seawater Na+/Ca2+ and salinity point to a biological influence on Na+ partitioning and/or covariations with other environmental parameters. One major unknown of the biological control during calcification is the rate of mineral precipitation, which has a strong control on trace-element partitioning in inorganic carbonate systems. We conducted inorganic CaCO3 precipitation experiments where the effect of solution composition and crystal growth rate on Na+ uptake by carbonate crystals are independently assessed. Calcite crystals were precipitated at rates varying from 10−6.5 to 10−4.5 mol/m2/s, while faster growth rate than 10−4.5 mol/m2/s resulted in the coprecipitation of aragonite and vaterite. For a given crystal growthrate, calcite Na/Ca increases by 0.22% per % increase in solution (Na +)2/Ca2+ activity ratio. However, calcite Na/Ca increases up to fivefold per order of magnitude increase in crystal growth rate, suggesting crystal growth rate and precursor phases are likely dominant controls on marine carbonate Na/Ca. We use these results in the framework of the DePaolo (2011) model for trace element uptake by calcite to assess the origin of variable (Na/Ca)foraminifer sensitivities to [Ca2+]seawater and salinity. Last, maximum mineral growth rates are estimated for a range of marine carbonatesbased on known carbonate Na/Ca and the (Na+)2/Ca2+ activity ratio of seawater. Estimated rates vary from 10 −5.6 (planktic foraminifers) to above 10−4 (sea urchins) mol/m2/s. Such high mineral growth rates imply high degrees of oversaturation with respect to calcite (10 to >100), supporting the idea that elemental partitioning and isotopic fractionation recorded in marine biogenic carbonates are controlled by kinetic rather than equilibrium exchanges.

As a recorder of the hydrological cycle and ocean circulation, salinity is one of the most wanted parameters in paleoceanography. Current paleosalinity reconstructions mostly rely on the interpretation of stable oxygen isotope signals combined with an independent paleotemperature proxy. Due to error propagation, this indirect approach is associated with large uncertainties. Recent culture studies and a Red Sea field study have shown that incorporation of Na in foraminiferal shell calcite depends on salinity, providing a potential direct proxy for salinity. However, application of a Na/Ca‐based salinity proxy requires further calibration, which should also consider settling of foraminifera through the water column and burial in the sediment. Here we compare Na/Ca in living specimens from Red Sea surface waters with specimens collected from 0‐ to 500‐m water depth and sedimentary specimens from core‐tops. This shows that Na/Ca in Globigerinoides ruber and Trilobatus sacculifer shells decrease with increasing water depth and until the sediment surface. For both species, laser‐ablation‐ICP‐Q‐MS measurements combined with electron‐probe microanalysis show that Na is enriched in the spines. Loss of spines during settling of foraminifera through the water column hence provides a mechanistic explanation for the observed Na decrease in bulk specimens with water depth. In contrast, average Mg/Ca values increase toward the seafloor in both species, coinciding with deposition of gametogenic calcite, which is enriched in Mg but has Na/Ca values similar to that in lamellar calcite. Both spine shedding and gametogenic calcite addition hence affect the average minor/trace element composition of foraminiferal calcite.

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.

Einleitung: An Tinnitus leiden viele Menschen teils intermittierend, aber auch permanent. Die Erkrankung spielt im täglichen Alltag von HNO-Ärzten eine große Rolle. Auch Patienten, die aufgrund einer hochgradigen Schwerhörigkeit ein Cochlear Implantat erhielten, leiden mitunter[for full text, please go to the a.m. URL], 82. Jahresversammlung der Deutschen Gesellschaft für Hals-Nasen-Ohren-Heilkunde, Kopf- und Hals-Chirurgie