Refining C. wuellerstorfi and H. elegans Mg/Ca temperature calibrations

Funding for this research was provided by the Marie Curie Action Plan, Seventh Framework Program (Grant no. 237922) and BMBF Grants 03G0217A (MAJA) and 03G0185A (VITAL).We present core top Mg/Ca, Sr/Ca and δ18O measurements for Cibicidoides wuellerstorfi and Hoeglundina elegans from the Timor Sea and the Makassar Strait, which span a bottom water temperature (BWT) range from 2 to 18 °C. In both species Mg/Ca ratios are positively and significantly correlated with BWT, while δ18O measurements are significantly anti-correlated with BWT. Comparison of calcification temperatures derived from Mg/Ca ratios and δ18O measurements yield comparable results and closely match CTD-measured temperatures. We integrate our results with previously published data sets from the Atlantic, Indian and Pacific Oceans and provide temperature calibration equations over the temperature range from 2 to 12 °C for H. elegans and from 0 to 10 °C for C. wuellerstorfi. We found geographical differences in the relation of benthic Mg/Ca ratios to BWT: C. wuellerstorfi shows Mg/Ca sensitivity to BWT of 19% increase in Mg/Ca per °C for the Atlantic Ocean and of 15% per °C for the Indian and Pacific Oceans. H. elegans shows Mg/Ca sensitivity to BWT of 16% increase in Mg/Ca per °C for the Atlantic Ocean and of 14% per °C for the Indian and Pacific Oceans. C. wuellerstorfi Sr/Ca variability appears to be mainly driven by carbonate ion saturation, whereas H. elegans Sr/Ca variability is closely linked to BWT in the compiled data sets from the Atlantic, Indian and Pacific Oceans.PostprintPeer reviewe

Effect of boiling on the acidity of hydrothermal solutions

Natural seawater and H2O-NaCl solutions were equilibrated along the two-phase (liquid-vapor) curves between 150 and 390 degrees C to re-examine the effect of liquid-vapor phase separation on element fractionations between saline brines and vapor phases. The experimental setup allows vapor and brine to be sampled separately at in situ P-T conditions. Experimental vapor pressure is constrained by temperature and the electrolyte concentration of the brines. With increasing temperature, dissolved electrolytes react with increasing temperature to hydroxides and HCl, in both seawater and H2O-NaCl solutions. The extent of hydrolysis is more pronounced in seawater than in H2O-NaCl solutions because seawater contains, in addition to NaCl, a range of other electrolytes capable of hydrolysis. Associated HCl has a great affinity to fractionate to the vapor phase when phase separation occurs. At 365 degrees C, halite-saturated vapor phases have a pH(25) (pH after condensation to 25 degrees C) of 1.8 (seawater) and 2.8 (H2O-NaCl brines). Our data suggest that boiling of hydrothermal solutions followed by mixing of vapor condensates with seawater can impose pH(25) values as acidic as the most acidic natural hydrothermal solutions vented on the ocean floor

Holocene evolution of the North Atlantic subsurface transport

Previous studies suggested that short-term freshening events in the subpolar gyre can be counterbalanced by advection of saline waters from the subtropical gyre and thus stabilize the Atlantic Meridional Overturning Circulation (AMOC). However, little is known about the inter-gyre transport pathways. Here, we infer changes in surface and subsurface transport between the subtropical and polar North Atlantic during the last 11 000 years, by combining new temperature and salinity reconstructions obtained from combined delta O-18 and Mg/Ca measurements on surface and subsurface dwelling foraminifera with published foraminiferal abundance data from the subtropical North Atlantic, and with salinity and temperature data from the tropical and subpolar North Atlantic. This compilation implies an overall stable subtropical warm surface water transport since 10 ka BP. In contrast, subsurface warm water transport started at about 8 ka but still with subsurface heat storage in the subtropical gyre. The full strength of intergyre exchange was probably reached only after the onset of northward transport of warm saline subsurface waters at about 7 ka BP, associated with the onset of the modern AMOC mode. A critical evaluation of different potential forcing mechanisms leads to the assumption that freshwater supply from the Laurentide Ice Sheet was the main control on subtropical to subpolar ocean transport at surface and subsurface levels

Petrogenesis of plagiogranites from the Troodos Ophiolite Complex, Cyprus

Small volumes of felsic melt, commonly known as oceanic plagiogranites, appear as melt pockets or dikes within the gabbroic section and the sheeted dikes root zone of the oceanic crust. Plagiogranites from the Troodos Ophiolite Complex on Cyprus are among the best exposures of felsic rocks that are embedded in a complete section of obducted oceanic lithosphere. Nevertheless, their exact petrogenesis is still a matter of debate, largely due to limited high-quality trace element and radiogenic isotope data. Previously proposed models for Troodos plagiogranites have included both low-pressure dehydration melting and fractional crystallisation at deeper levels of the oceanic crust. To evaluate both models, oceanic plagiogranites from the Troodos Ophiolite Complex were analysed in this study for their major and trace elements, and for the first time also for Hf-Nd-Sr isotope compositions. The trace element measurements also include for the first time high-precision measurements of high-field-strength element (HFSE) abundances that now permit to better unravel the petrogenesis of the Troodos plagiogranites and their possible mantle sources. In general, the Troodos plagiogranites exhibit a narrow range of Nb/Ta and Zr/Hf that overlap the compositions of mid-ocean ridge basalts (MORB). In line with earlier studies, three compositional groups can be identified: two groups formed by either fractional crystallisation or combined fractional crystallisation and wall rock assimilation, and one group derived from partial melting of slightly altered oceanic crust. The majority of the Troodos plagiogranites (Main Group) are the product of extensive fractional crystallisation of ambient arc-tholeiitic mafic melts. A second group of plagiogranites (Spilia Group) is generated by fractional crystallisation of boninitic precursor melts and the assimilation of arc-tholeiitic crustal material. Variable HFSE concentrations and diagnostic Hf-Nd isotope signatures that are unique to both suites allow discriminating between the two parental melts and fractionation processes. A small group of plagiogranites (Zoopigi Group) is interpreted to derive from partial melting in the conductive layer of active magma chamber lenses (AML). Elevated Nb/Ta, Zr/Hf, and light rare-earth element (LREE) enrichments in these rocks are in support of this model. Collectively, our data suggest that low-pressure fractional crystallisation (also in combination with assimilation of wall rocks) might be the predominant process controlling the formation of felsic rocks on Cyprus, whereas dehydration melting appears to be less important. If compared to Archean tonalitic-trondhjemitic-granodioritic suites (TTGs), compositions of plagiogranites from Cyprus mirror shallow-level processes in thin oceanic crust, which is illustrated by their narrow, MORB-like range of HFSE ratios and their distinct enrichment in heavy rare-earth elements (HREE) that distinguishes them from the Archean TTGs

Geochemical evolution of the Rabaul volcanic complex, Papua New Guinea - Insights from HFSE, Sr-Nd-Hf, and Fe isotopes

The Rabaul volcanic complex, Papua New Guinea, is among the few calderas worldwide with ongoing volcanic activity. Its volcanic lithologies vary from mafic lavas from outer caldera cones to differentiated lavas within. In this study, representative lavas have been analysed for their major and trace element concentrations and radiogenic Sr-Nd-Hf isotope compositions to study the geochemical evolution of the magmatic system. Stable Fe isotopes and concentrations of high field strength elements (HFSE) complement the analyses as novel tools to assess the effect of high-temperature fractional crystallisation during ongoing differentiation. Major element systematics reveal a typical fractional crystallisation sequence of olivine, pyroxene, and plagioclase as the critical process controlling the magmatic evolution. A distinct increase of Zr/Hf from the basaltic (older) outer caldera lavas (similar to 39) to the dacitic (younger) inner caldera lavas (similar to 41-44) can be explained by fractionation of clinopyroxene and amphibole. Ratios of Nb/Ta tend to decrease with an increasing degree of differentiation, consistent with fractional crystallisation of amphibole but not clinopyroxene. Additional fractionation of Ti-magnetite and rather oxidising conditions are further supported by the Fe isotope compositions in the inner caldera lavas (delta Fe-57 from +0.03 to +0.22 parts per thousand, +/- 0.04 parts per thousand, 2 SD). The high Nb/Ta in more primitive outer caldera samples are coupled with increasing rare earth elements (REE) abundances, and slab melt-like subduction components can explain high Sr/Y and Gd-N/Yb-N in the magma sources. Complementary enrichments in fluid-mobile trace elements indicate that slab dehydration controlled the sub-arc enrichment of the inner caldera volcanism. Coupled Hf-Nd isotope compositions reveal the presence of the Indian-Australian mantle domain beneath Rabaul and a temporal trend towards sediment melt components overprinting inner caldera lavas. In conclusion, geochemical features show a temporal evolution controlled by (i) variable influence of partial slab melts vs slab fluids and (ii) a change in fractional crystallisation patterns from solely olivine and pyroxene-controlled to increasingly titano-magnetite and amphibole-controlled fractionation

Coral Sr/Ca records provide realistic representation of eastern Indian Ocean cooling during extreme positive Indian Ocean Dipole events

Extreme positive Indian Ocean Dipole (pIOD) events are amplified by non-linear ocean-atmosphere interactions and are characterized by pronounced cooling in the eastern equatorial Indian Ocean. These non-linear feedbacks are not adequately represented in historical products of sea surface temperatures that underestimate the magnitude of extreme pIOD events. Here, we present a sea surface temperature (SST) reconstruction based on monthly coral Sr/Ca ratios measured in two coral cores from Enggano Island (Indonesia), that lies in the eastern pole of the IOD. The coral SST reconstruction extends from 1930 to 2008 and captures the magnitude of cooling during extreme pIOD events as shown in recent satellite and reanalysis data of SST that include ocean dynamics. The corals indicate that the 1961 pIOD event was at least as severe as the 1997 event, while the 1963 pIOD was more comparable to the 2006 event. The magnitude 1967 pIOD is difficult to assess at present due to poor replication between coral cores, and may be comparable to either 1997 or 2006. Cooling during the 1972 pIOD was short-lived and followed by pronounced warming, as seen in the moderate pIOD event of 1982. A combination of coral SST reconstructions and an extension of new reanalysis products of SST to historical time scales could help to better assess the severity and impact of past pIOD events such as the ones seen in the 1960s

Refining <i>C. wuellerstorfi</i> and <i>H. elegans </i>Mg/Ca temperature calibrations

We present core top Mg/Ca, Sr/Ca and δ18O measurements for Cibicidoides wuellerstorfi and Hoeglundina elegans from the Timor Sea and the Makassar Strait, which span a bottom water temperature (BWT) range from 2 to 18 °C. In both species Mg/Ca ratios are positively and significantly correlated with BWT, while δ18O measurements are significantly anti-correlated with BWT. Comparison of calcification temperatures derived from Mg/Ca ratios and δ18O measurements yield comparable results and closely match CTD-measured temperatures. We integrate our results with previously published data sets from the Atlantic, Indian and Pacific Oceans and provide temperature calibration equations over the temperature range from 2 to 12 °C for H. elegans and from 0 to 10 °C for C. wuellerstorfi. We found geographical differences in the relation of benthic Mg/Ca ratios to BWT: C. wuellerstorfi shows Mg/Ca sensitivity to BWT of 19% increase in Mg/Ca per °C for the Atlantic Ocean and of 15% per °C for the Indian and Pacific Oceans. H. elegans shows Mg/Ca sensitivity to BWT of 16% increase in Mg/Ca per °C for the Atlantic Ocean and of 14% per °C for the Indian and Pacific Oceans. C. wuellerstorfi Sr/Ca variability appears to be mainly driven by carbonate ion saturation, whereas H. elegans Sr/Ca variability is closely linked to BWT in the compiled data sets from the Atlantic, Indian and Pacific Oceans