Water chemistry reveals a significant decline in coral calcification rates in the southern Red Sea

Experimental and field evidence support the assumption that global warming and ocean acidification is decreasing rates of calcification in the oceans. Local measurements of coral growth rates in reefs from various locations have suggested a decline of ~6-10% per decade since the late 1990’s. Here we show by measuring open water strontium-to-alkalinity ratios along the Red Sea that the net contribution of hermatypic corals to the CaCO3 budget of the southern and central Red Sea declined by ~100% between 1998 and 2015 and remained low between 2015 and 2018. Measured differences in total-alkalinity of the Red Sea surface water indicate a 26±16% decline in total CaCO3 deposition rates along the basin. These findings suggest that coral reefs of the southern Red Sea are under severe stress and demonstrate the strength of geochemical measurements as cost-effective indicators for calcification trends on regional scales.Blavatnik fellowship to Z

Calcium isotope fractionation during microbially induced carbonate mineral precipitation

We report the calcium isotope fractionation during the microbially-induced precipitation of calcium carbonate minerals in pure cultures of the marine sulfate-reducing bacterium Desulfovibrio bizertensis. These data are used to explore how the calcium isotope fractionation factor during microbially-induced carbonate mineral precipitation differs from the better-constrained calcium isotope fractionation factors during biogenic or abiotic carbonate mineral precipitation. Bacterial growth was then modulated with antibiotics, and the evolution of δ44Ca in solution was monitored under different microbial growth rates. The faster the microbial growth rate, the larger the calcium isotope fractionation during carbonate mineral precipitation, ranging from Δ44Ca(s-f) between -1.07‰ and -0.48‰. The reported calcium isotope fractionation can help us understand the link between calcium isotope fractionation and microbial metabolism in carbonate minerals precipitated during sedimentary diagenesis.The work was supported by ERC 307582 StG (CARBONSINK) to AVT and NERC 700 NE/R013519/1 to HJ

The calcium isotopic composition of carbonate hardground cements: A new record of changes in ocean chemistry?

Reconstructing changes in the calcium isotopic composition (δ44Ca) of the ocean over Earth history has been challenging. This difficulty is due to the large range of calcium isotope fractionation factors during mineral precipitation and the potential for overwriting the initial δ44Ca of minerals during shallow marine diagenesis. We present a new δ44Ca record measured in carbonate hardground cements, an inorganic carbonate-mineral precipitate that rapidly forms at or near the sediment-water interface. The range in the δ44Ca for any particular carbonate hardground cements is between 0.05 and 0.56‰. In some cases, the progressive increase in the δ44Ca during precipitation can be observed, consistent with precipitation in a ‘closed-system’. Our data show an average calcium isotope fractionation during carbonate hardground cement precipitation that is −0.57 ± 0.27‰, similar to the calcium isotope fractionation factor for inorganic calcite precipitates in previous laboratory and modelling studies, and closer to what is considered a kinetic end member calcium isotope fractionation than growth at equilibrium. This is consistent with the rapid carbonate mineral precipitation expected for carbonate hardground cements. Our δ44Ca record over the Phanerozoic is similar to other calcium-bearing mineral records over the same time interval, with average δ44Ca becoming lower going back in time by about 0.5 to 0.7‰. Our results add further support for the evolution of seawater δ44Ca over time, and we discuss the possible causes of these changes with suggestions for future studies

Coupled measurements of δ¹⁸O and δD of hydration water and salinity of fluid inclusions in gypsum from the Messinian Yesares Member, Sorbas Basin (SE Spain)

We studied one cycle (Cycle 6) of gypsum-marl deposition from the Messinian Yesares Member in Sorbas Basin, Spain. The objective was to reconstruct the changing environment of deposition and its relation to astronomically-forced climate change. The δ1818O and δD of gypsum hydration water (CaSO4•2H2O) and salinity of fluid inclusions were measured in the same samples to test if they record the composition of the mother fluid from which gypsum was precipitated. Water isotopes are highly correlated with fluid inclusion salinity suggesting the hydration water has not exchanged after formation. The relatively low water isotope values and fluid inclusion salinities indicate a significant influence of meteoric water, whereas δ34S, δ18OSO4 26 and 87Sr/86Sr support a dominant marine origin for the gypsum deposits. The discrepancy between water and elemental isotope signatures can be reconciled if meteoric water dissolved previously deposited marine sulfates supplying calcium and sulfate ions to the basin which maintained gypsum saturation. This recycling process accounts for the marine δ34S, δ18OSO4 and 87Sr/86Sr signatures, whereas the low δ18O and δD values of gypsum hydration water and fluid inclusion salinities reflect the influence of freshwater. The cyclic deposition of gypsum and marl in the Yesares Member has previously been interpreted to reflect changing climate related to Earth’s precession cycle. We demonstrate that the δ18O, δD and salinity of the parent brine increased from low values at the base of Cycle 6 to a maximum in the massive gypsum palisade, and decreased again to lower values in the supercones at the top of the cycle. This pattern, together with changes in mineralogy (calcite-dolomite-gypsum), is consistent with a precession-driven change in climate with wettest conditions (summer insolation maxima) associated with the base of the calcium carbonate marls and driest conditions (summer insolation minima) during formation of the gypsum palisade.The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement n. 339694 (Water Isotopes of Hydrated Minerals) to D. A. Hodell.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.epsl.2015.07.07

Modelling the Effects of Non-Steady State Transport Dynamics on the Sulfur and Oxygen Isotope Composition of Sulfate in Sedimentary Pore Fluids

We present the results of an isotope-enabled reactive transport model of a sediment column undergoing active microbial sulfate reduction to explore the response of the sulfur and oxygen isotopic composition of sulfate under perturbations to steady state. In particular, we test how perturbations to steady state influence the cross plot of δ34S and δ18O for sulfate. The slope of the apparent linear phase (SALP) in the cross plot of δ34S and δ18O for sulfate has been used to infer the mechanism, or metabolic rate, of microbial metabolism, making it important that we understand how transient changes might influence this slope. Tested perturbations include changes in boundary conditions and changes in the rate of microbial sulfate reduction in the sediment. Our results suggest that perturbations to steady state influence the pore fluid concentration of sulfate and the δ34S and δ18O of sulfate but have a minimal effect on SALP. Furthermore, we demonstrate that a constant advective flux in the sediment column has no measurable effect on SALP. We conclude that changes in the SALP after a perturbation are not analytically resolvable after the first 5% of the total equilibration time. This suggests that in sedimentary environments the SALP can be interpreted in terms of microbial metabolism and not in terms of environmental parameters.</jats:p

The Production and Fate of Volatile Organosulfur Compounds in Sulfidic and Ferruginous Sediment

Volatile organic sulfur compounds (VOSCs) link the atmospheric, marine, and terrestrial sulfur cycles in marine and marginal marine environments. Despite the important role VOSCs play in global biogeochemical sulfur cycling, less is known about how the local geochemical conditions influence production and consumption of VOSCs. We present a study of dimethyl sulfide (DMS), methanethiol (MeSH), and dimethylsulfoniopropionate (DMSP) in sulfide-rich (sulfidic) and iron-rich (ferruginous) salt marsh sediment from north Norfolk, UK. Initial results illustrate the importance of minimizing time between sampling in remote field locations and laboratory analysis, due to rapid degradation of VOSCs. With rapid analysis of sediment from different depths, we observe high concentrations of DMS, MeSH, and DMSP, with concentrations in surface sediment an order of magnitude higher than those in previous studies of surface water. We measure systematic differences in the concentration and depth distribution of MeSH and DMS between sediment environments; DMS concentrations are higher in ferruginous sediment, and MeSH concentrations are higher in sulfidic sediment. With repeated measurements over a short time period, we show that the degradation patterns for DMS and MeSH are different in the ferruginous versus sulfidic sediment. We discuss potential biogeochemical interactions that could be driving the observed differences in VOSC dynamics in ferruginous and sulfidic sediment.This work was supported by a Churchill Scholarship to J. V. W., NERC Grant NE/S001352/1 to A.V.T. and J. D. T., NERC Grant NE/K01546X/1 to K. R. R., and NERC Grants NE/P012671/1, NE/N002385/1, and NE/M004449/1 to J. D. T. Initial analyses were supported by ERCStG307582 (CARBONSINK) to A. V. T

Semiquantitative Estimates of Rainfall Variability During the 8.2 kyr Event in California Using Speleothem Calcium Isotope Ratios

A multiproxy record from a fast-growing stalagmite reveals variable hydroclimate on the California coast across the 8.2 kyr event and a precursor event likely caused by initial drainage of proglacial Lake Agassiz. Using speleothem δ Ca, we develop the first semiquantitative estimates of paleorainfall variability for California through calibration with measurements of the modern climate and cave environment. We find that the magnitude of rainfall variability during the 8.2 kyr event approached the multiyear variability observable in the recent past (1950–2019) and the magnitude of variability during the precursor event likely exceeded this range. Additionally, we observe other instances of multidecadal variability comparable in magnitude to the precursor event during the record. Our work suggests that speleothem calcium isotope ratios are a powerful semiquantitative means to reconstruct paleorainfall, although numerous factors must be assessed in each cave system before applying this approach. 4

Testing for ocean acidification during the Early Toarcian using δ44/40Ca and δ88/86Sr

During the Early Toarcian, volcanic gases released by the Karoo-Ferrar large igneous province are widely believed to have caused severe environmental disturbances, including ocean acidification. Here we show records of δ Ca and δ Sr through the Early Toarcian, as recorded in three groups of biogenic calcite: Megateuthididae belemnites, Passaloteuthididae belemnites, and brachiopods of the species Soaresirhynchia bouchardi. We evaluate the data to eliminate the influence on isotopic composition of varying temperature, calcification rate, and salinity, through the section that may mask the environmental signals. Neither δ Ca nor δ Sr show negative isotope excursions across the suggested acidification interval as would be expected had acidification occurred. A profile of δ B, re-interpreted from a published study, shows no variation through the interval. Taken together, these data provide little support for ocean acidification at this time. In our belemnites, values of δ Sr are independent of temperature or Sr/Ca. For brachiopods, too few data are available to determine whether such dependences exist. Values of δ Ca show a weak temperature control of magnitude +0.020 ± 0.004 ‰/°C (2 s.d.). In belemnites, δ Ca also correlates positively with Mg/Ca and Sr/Ca. 44/40 88/86 44/40 88/86 11 88/86 44/40 44/4

Physical weathering of carbonate host-rock by precipitation of soluble salts in caves: A case study in El Orón-Arco Cave (Region of Murcia, SE Spain)

The dissolution of carbonate host-rock by freshwater in phreatic or vadose conditions is the most common mechanism for the formation of caves; however, circulation of saline solutions through carbonate materials and precipitation of soluble salts may also play an important role. We studied the stable isotope composition (δ18O and δ34S of sulfate, δ18O and δD of structurally-bound gypsum hydration water and 87Sr/86Sr) and salinity of fluid inclusions in gypsum speleothems found in El Orón-Arco Cave (Cartagena, SE Spain). We suggest that physical weathering of carbonate host-rock was driven by precipitation of soluble sea-salts (mostly gypsum and halite), and this process controlled the recent geomorphological evolution of the cave. The Triassic carbonate host-rock shows clear evidence for salt weathering, including gypsum/halite infillings in cracks of the bedrock, mechanical spalling of the carbonate, and detachment of rock fragments that lead to the formation cave voids and in-situ accumulations of piles of unsorted rubble. Sulfur and oxygen isotopes of gypsum sulfate (3.0‰ < δ18O < 11.6‰ and 16.7‰ < δ34S < 20.7‰) are generally lower than modern seawater sulfate and suggest contributions from a 34S-depleted source (i.e. oxidation of pyrite). The δ18O and δD of gypsum hydration water are relatively low compared to expected values for the evaporation of pure seawater to gypsum saturation, suggesting that gypsum precipitation involved a secondary calcium-sulfate source or recycling of gypsum from previous stages, along with mixing of seawater and meteoric water seepage to the cave. The 87Sr/86Sr in gypsum shows intermediate values between modern seawater and Triassic carbonate values because of interaction between the solution and the bedrock. The salinities of the speleothem-forming solutions are relatively high (13.2 ± 3.2 wt% eq. NaCl) compared to gypsum formed from evaporated brackish solutions (i.e. ~4–8 wt% eq. NaCl) and indicate dissolution of earlier evaporites before secondary gypsum precipitation. This cave-forming mechanism, which is related to saline water circulation and precipitation of evaporitic minerals, may be common in other coastal caves

The role of microbial sulfate reduction in calcium carbonate polymorph selection

Microbial sulfate reduction is a dominant metabolism in many marine sedimentary environments. The influence of this metabolism on the kinetics of CaCO3 growth, as well as the dominant polymorphs precipitated, is poorly understood. To investigate the role of microbial metabolism on CaCO3 precipitation and polymorph selection, we conducted growth experiments with the sulfate reducing bacteria (D. bizertensis) in media with varying Mg/Ca and different seeding materials (calcite and kaolinite). Our results suggest that sulfate reducing bacteria both induce carbonate mineral precipitation through an increase in alkalinity and serve as a nucleation sites for the growing carbonate mineral; the majority of the carbonate minerals produced were on cell material rather than mineral seeds. We also find the Mg/Ca and presence of phosphate in the media play a key role in controlling the rates of carbonate mineral precipitation and calcium carbonate polymorph selection. In media where the Mg/Ca is greater than 2, crystalline monohydrocalcite (MHC) is the primary carbonate mineral produced. Although phosphate concentrations have a lesser effect on which polymorph initially precipitates, a series of transformation experiments suggests that the presence of phosphate stabilizes MHC crystals and prevents its transformation to more stable calcium carbonate polymorphs. Collectively, these results suggest that the polymorph of microbially-mediated calcium carbonate cements is determined by the solution chemistry upon nucleation.Ministry of Education Malaysia, SLAI (Skim Latihan Akademik IPTA) scholarship to CY