Beschreibung: Marine calcifying eukaryotic phytoplankton (coccolithophores) is a major contributor to the pelagic production of CaCO3 and plays an important role in the biogeochemical cycles of C, Ca and other divalent cations present in the crystal structure of calcite. The geochemical signature of coccolithophore calcite is used as palaeoproxy to reconstruct past environmental conditions and to understand the underlying physiological mechanisms (vital effects) and precipitation kinetics. Here, we present the stable Sr isotope fractionation between seawater and calcite (Δ88/86Sr) of laboratory cultured coccolithophores in individual dependence of temperature and seawater carbonate chemistry. Coccolithophores were cultured within a temperature and a pCO2 range from 10 to 25°C and from 175 to 1,240 μatm, respectively. Both environmental drivers induced a significant linear increase in coccolith stable Sr isotope fractionation. The temperature correlation at constant pCO2 for Emiliania huxleyi and Coccolithus braarudii is expressed as Δ88/86Sr = −7.611 × 10−3 T + 0.0061. The relation of Δ88/86Sr to pCO2 was tested in Emiliania huxleyi at 10 and 20°C and resulted in Δ88/86Sr = −5.394 × 10−5 pCO2 – 0.0920 and Δ88/86Sr = −5.742 × 10−5 pCO2 – 0.1351, respectively. No consistent relationship was found between coccolith Δ88/86Sr and cellular physiology impeding a direct application of fossil coccolith Δ88/86Sr as coccolithophore productivity proxy. An overall significant correlation was detected between the elemental distribution coefficient (DSr) and Δ88/86Sr similar to inorganic calcite with a physiologically induced offset. Our observations indicate (i) that temperature and pCO2 induce specific effects on coccolith Δ88/86Sr values and (ii) that strontium elemental ratios and stable isotope fractionation are mainly controlled by precipitation kinetics when embedded into the crystal lattice and subject to vital effects during the transmembrane transport from seawater to the site of calcification. These results provide an important step to develop a coccolith Δ88/86Sr palaeoproxy complementing the existing toolbox of palaeoceanography.

Beschreibung: Precisely quantifying the current climate-related sea level change requires accurate knowledge of long-term geological processes known as Glacial Isostatic Adjustments (GIA). Although the major postglacial melting phase is likely to have ended ∼6–4 ka BP (before present), GIA is still significantly affecting the present-day vertical position of the mean sea surface and the sea bottom. Here we present empirical rsl (relative sea level) data based on U/Th dated fossil corals from reef platforms of the Society Islands, French Polynesia, together with the corresponding GIA-modeling. Fossil coral data constrain the timing and amplitude of rsl variations after the Holocene sea level maximum (HSLM). Upon correction for isostatic island subsidence, we find that local rsl was at least ∼1.5 ± 0.4 m higher than present at ∼5.4 ka. Later, minor amplitude variations occurred until ∼2 ka, when the rsl started dropping to its present position with a rate of ∼0.4 mm/yr. The data match with predicted rsl curves based on global ice-sheet chronologies confirming the role of GIA-induced ocean siphoning effect throughout the mid to late Holocene. A long lasting Late Holocene highstand superimposed with second-order amplitudinal fluctuations as seen from our data suggest that the theoretical predicted timing of rsl change can still be refined pending future calibration.

Beschreibung: The universally known subsidence theory of Darwin, based on Bora Bora as a model, was developed without information from the subsurface. To evaluate the influence of environmental factors on reef development, two traverses with three cores, each on the barrier and the fringing reefs of Bora Bora, were drilled and 34 uranium-series dates obtained and subsequently analysed. Sea-level rise and, to a lesser degree, subsidence were crucial for Holocene reef development in that they have created accommodation space and controlled reef architecture. Antecedent topography played a role as well, because the Holocene barrier reef is located on a Pleistocene barrier reef forming a topographic high. The pedestal of the fringing reef was Pleistocene soil and basalt. Barrier and fringing reefs developed contemporaneously during the Holocene. The occurrence of five coralgal assemblages indicates an upcore increase in wave energy. Age–depth plots suggest that barrier and fringing reefs have prograded during the Holocene. The Holocene fringing reef is up to 20 m thick and comprises coralgal and microbial reef sections and abundant unconsolidated sediment. Fringing reef growth started 8780 ± 50 yr bp; accretion rates average 5·65 m kyr−1. The barrier reef consists of 〉30 m thick Holocene coralgal and microbial successions. Holocene barrier-reef growth began 10 030 ± 50 yr bp and accretion rates average 6·15 m kyr−1. The underlying Pleistocene reef formed 116 900 ± 1100 yr bp, i.e. during marine isotope stage 5e. Based on Pleistocene age, depth and coralgal palaeobathymetry, the subsidence rate of Bora Bora was estimated to be 0·05 to 0·14 m kyr−1. In addition to subsidence, reef development on shorter timescales like in the late Pleistocene and Holocene has been driven by glacioeustatic sea-level changes causing alternations of periods of flooding and subaerial exposure. Comparisons with other oceanic barrier-reef systems in Tahiti and Mayotte exhibit more differences than similarities.

Beschreibung: Holocene reef growth, present date bioerosion .and recorded carbonate production were studied in the fringing reef at Aqaba, Red Sea. Water depth, wave impact as well as nutrient availability were considered. The carbonate production was measured for several coral samples. Samples of Porites-colonies were collected from several depths and sites near the Marine Science Station at Aqaba. Growth rate depends on water depth, size and age of colonies. Within the coral optimum of water depth growth rates vary between 5 and 16 mm/yr. Coral carbonate production was calculated on the base of annual growth increments and skeletal density using transects from shallow subtidal down to 40 m water depth. High resolution stable isotope data were measured to prove the origin of growth increments. Long-term trends of sea surface temperature and carbon isotope shift (1800-today) fit to the known global deviations. Bioerosion rates were determined using standard dead coral substrates exposed in different water depths and environmental settings. Rates vary between 0.6 and 1.4 kg/m2yr. Sediment export evaluated by means of simple sediment traps ranges between 0.3 and 0.7 kg/m2yr. Gross carbonate production, mainly built up by scleractinian corals, amounts to ca. 1.57 kg/m2yr. Bioerosion alters approx. 1.3 kg/m2yr of hard substrates into sediment. Sediment export is estimated to be ca. 0.4-0.6 kg/m2yr. Thus a net production of ca. 0.7 to 0.9 kg/m2yr should remain in the present reef, which is proved by the recorded carbonate production (reef drillings). Net production preserved in the reef can be given with ca. 800 kg/m2kyr (=0.8 kg/m2yr).

Beschreibung: The modern history of North Atlantic sea surface temperature shows variability coinciding with changes in air temperature and rainfall over the Northern Hemisphere. There is a debate about this variability and, in particular, whether it is internal to the ocean‐atmosphere system or is forced by external factors (natural and anthropogenic). Here we present a temperature record, obtained using the Sr/Ca ratio measured in a skeleton of a sclerosponge, that shows agreement with the instrumental record over the past 150 years as well as multidecadal temperature variability over the last 600 years. Comparison with climate simulations of the last millennium shows that large cooling events recorded, in the sclerosponge, are consistent with natural (primarily volcanic activity) and anthropogenic forcings. There are, however, multidecadal periods not connected to current estimates of external forcing over the last millennium allowing for alternative explanations, such as internally driven changes in ocean and atmospheric circulation.

Beschreibung: A proposal is made to standardise the reporting of Ca isotope data to the δ44Ca/40Ca notation (or δ44Ca/42Ca) and to adopt NIST SRM 915a as the reference standard.

Beschreibung: We report delta Ca-44/40((SRM 915a)) values for eight fused MPI-DING glasses and the respective original powders, six USGS igneous rock reference materials, the U-Th disequilibria reference material TML, IAEA-CO1 (Carrara marble) and several igneous rocks (komatiites and carbonatites). Sample selection was guided by three considerations: (1) to address the need for information values on reference materials that are widely available in support of interlaboratory comparison studies; (2) support the development of in situ laser ablation and ion microprobe techniques, which require isotopically homogenous reference samples for ablation; and (3) provide Ca isotope values on a wider range of igneous and metamorphic rock types than is currently available in the scientific literature. Calcium isotope ratios were measured by thermal ionisation mass spectrometry in two laboratories (IFM-GEOMAR and Saskatchewan Isotope Laboratory) using Ca-43/Ca-48- and Ca-42/Ca-43-double spike techniques and reported relative to the calcium carbonate reference material NIST SRM 915a. The measurement uncertainty in both laboratories was better than 0.2 parts per thousand at the 95% confidence level. The impact of different preparation methods on the delta Ca-44/40((SRM 915a)) values was found to be negligible. Except for ML3-B, the original powders and the respective MPI-DING glasses showed identical delta Ca-44/40((SRM 915a)) values; therefore, possible variations in the Ca isotope compositions resulting from the fusion process are excluded. Individual analyses of different glass fragments indicated that the glasses are well homogenised on the mm scale with respect to Ca. The range of delta Ca-44/40((SRM 915a)) values in the igneous rocks studied was larger than previously observed, mostly owing to the inclusion of ultramafic rocks from ophiolite sections. In particular, the dunite DTS-1 (1.49 +/- 0.06 parts per thousand) and the peridotite PCC-1 (1.14 +/- 0.07 parts per thousand) are enriched in Ca-44 relative to volcanic rocks (0.8 +/- 0.1 parts per thousand). The Carrara marble (1.32 +/- 0.06 parts per thousand) was also found to be enriched in Ca-44 relative to the values of assumed precursor carbonates (〈 0.8 parts per thousand). These findings suggest that the isotopes of Ca are susceptible to fractionation at high temperatures by, as yet, unidentified igneous and metamorphic processes.