Calcium isotopes in deep time: Potential and limitations

Calcium is an essential element in the biogeochemical cycles that regulate the long-term climate state of Earth. The removal of CO2 from the ocean-atmosphere system is controlled by the burial of carbonate sediments (CaCO3), ultimately linking the global calcium and carbon cycles. This fundamental link has driven the development of the stable calcium isotope proxy with applica-tion to both ancient skeletal and non-skeletal bulk carbonate sediments. Calcium isotope ratios (44/40Ca) have been used to track long-term changes in seawater chemistry (e.g., aragonite vs. calcite seas) and to elucidate short-term climatic perturbations associated with mass extinction events. However, developments in the calcium isotope proxy have shown that 44/40Ca values in carbonate minerals also are sensitive to changes in precipitation rates, mineralogy and diagene-sis, thereby complicating the application of the proxy to the reconstruction of global cycles. First, inorganic carbonate precipitation experiments have demonstrated that carbonate 44/40Ca values are sensitive to precipitation rates with higher rates generally leading to larger fractiona-tion. Second, 44/40Ca values are sensitive to carbonate mineralogy with inorganic aragonite and calcite being on average ~1.5‰ and ~0.9‰ depleted relative to contemporaneous seawater, re-spectively. The effects of both changes in carbonate mineralogy and precipitation rates affect primary and secondary minerals, but are particularly pronounced during carbonate diagenesis where relatively slow rates of recrystallization and neomorphism can lead to significant changes in bulk sediment 44/40Ca values. Third, changes in faunal composition expressed in skeletal fossil archives can lead to large changes in carbonate 44/40Ca values that are decoupled from changes in global cycles. Nevertheless, when these factors are appropriately considered the ap-plication of calcium isotopes in ancient carbonate sediments becomes a powerful tool for under-standing biogeochemical processes that operate over many scales; from diagenetic changes with-in the sediment pore-space, to regional changes across ancient carbonate platforms, and to global changes in seawater chemistry through time. Importantly, the processes that contribute to varia-bility in carbonate 44/40Ca values are likely to impact other carbonate-bound proxies, highlight-ing the potential for calcium isotopes as a hint to better understand the variability of other iso-tope systems

Controls on the Precipitation of Carbonate Minerals Within Marine Sediments

The vast majority of carbonate minerals in modern marine sediments are biogenic, derived from the skeletal remains of organisms living in the ocean. However, carbonate minerals can also precipitate abiotically within marine sediments, and this carbonate mineral precipitation within sediments has been suggested as a third major, and isotopically distinct, sink in the global carbon cycle, particularly important earlier in Earth history. Here we present a global compilation of pore fluid data and compare the sulfate, calcium, phosphate and magnesium concentrations with pore fluid alkalinity to explore the emerging relationships and explore what drives carbonate mineral precipitation in sediments. Our data compilation shows that the gradient of pore fluid sulfate concentrations correlates strongly with the gradient of alkalinity as well as with the gradient of calcium, and that these correlations improve dramatically in sediments where methane is present. We also note that sedimentary pore fluids that are high in phosphate concentration are also high in alkalinity, which may indicate suppression of carbonate mineral precipitation in the presence of sedimentary phosphate. Our data can be used to highlight sediments where both dolomite formation and dolomitization of previously deposited calcium carbonate minerals is occurring. We explore how carbonate mineral saturation state changes as a function of calcium concentrations, alkalinity, and pH, and suggest a reason why calcium concentrations are never fully depleted in sedimentary pore fluids. We conclude that carbonate minerals precipitate in sediments with methane, where the anaerobic oxidation of this methane helps promote particularly high saturation states for carbonate minerals

Local and Regional Indian Summer Monsoon Precipitation Dynamics During Termination II and the Last Interglacial

To date Indian summer monsoon (ISM) dynamics have been assessed by changes in stalagmite δ18O. However, stalagmite δ18O is influenced by multiple environmental factors (e.g., atmospheric moisture transport, rainfall amount at the study site, and ISM seasonality), precluding simple and clear reconstructions of rainfall amount or variability. This study aims to disentangle these environmental factors by combining δ18O, δ44Ca, and elemental data from a stalagmite covering Termination II and the last interglacial from Mawmluh Cave, NE India, to produce a semiquantitative reconstruction of past ISM rainfall. We interpret δ18O as a mixed signal of rainfall source dynamics and rainfall amount and coupled δ44Ca and X/Ca ratios as indicators of local infiltration rate and prior calcite precipitation in the karst zone. The wettest conditions in our studied interval (135 and 100 kyrs BP; BP = before present, with the present being 1950 CE) occurred during Marine Isotope Stage 5e. Our multiproxy data set suggests a likely change in seasonal distribution of Marine Isotope Stage 5e rainfall compared to the Holocene; the wet season was longer with higher‐than‐modern dry season rainfall. Using the last interglacial as an analogue for future anthropogenic warming, our data suggest a more erratic ISM behavior in a warmer world

Mineralogy, early marine diagenesis, and the chemistry of shallow-water carbonate sediments

Shallow-water carbonate sediments constitute the bulk of sedimentary carbonates in the geologic record and are widely used archives of Earth’s chemical and climatic history. One of the main limitations in interpreting the geochemistry of ancient carbonate sediments is the potential for post-depositional diagenetic alteration. In this study, we use paired measurements of calcium (44Ca/40Ca or δ44Ca) and magnesium (26Mg/24Mg or δ26Mg) isotope ratios in sedimentary carbonates and associated pore-fluids as a tool to understand the mineralogical and diagenetic history of Neogene shallow-water carbonate sediments from the Bahamas and southwest Australia. We find that the Ca and Mg isotopic composition of bulk carbonate sediments at these sites exhibits systematic stratigraphic variability that is related to both mineralogy and early marine diagenesis. The observed variability in bulk sediment Ca isotopes is best explained by changes in the extent and style of early marine diagenesis from one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the fluid (fluid-buffered) to one where the composition of the diagenetic carbonate mineral is determined by the chemistry of the precursor sediment (sediment-buffered). Our results indicate that this process, together with variations in carbonate mineralogy (aragonite, calcite, and dolomite), plays a fundamental and underappreciated role in determining the regional and global stratigraphic expressions of geochemical tracers (δ13C, δ18O, major, minor, and trace elements) in shallow-water carbonate sediments in the geologic record. Our results also provide evidence that a large shallow-water carbonate sink that is enriched in 44Ca can explain the mismatch between the δ44/40Ca value of rivers and deep-sea carbonate sediments and call into question the hypothesis that the δ44/40Ca value of seawater depends on the mineralogy of primary carbonate precipitations (e.g. ‘aragonite seas’ and ‘calcite seas’). Finally, our results for sedimentary dolomites suggest that paired measurements of Ca and Mg isotopes may provide a unique geochemical fingerprint of mass transfer during dolomitization to better understand the paleo-environmental information preserved in these enigmatic but widespread carbonate minerals

Worldwide, mortality risk is high soon after initiation of hemodialysis.

Mortality rates for maintenance hemodialysis patients are much higher than the general population and are even greater soon after starting dialysis. Here we analyzed mortality patterns in 86,886 patients in 11 countries focusing on the early dialysis period using data from the Dialysis Outcomes and Practice Patterns Study, a prospective cohort study of in-center hemodialysis. The primary outcome was all-cause mortality, using time-dependent Cox regression, stratified by study phase adjusted for age, sex, race, and diabetes. The main predictor was time since dialysis start as divided into early (up to 120 days), intermediate (121-365 days), and late (over 365 days) periods. Mortality rates (deaths/100 patient-years) were 26.7 (95% confidence intervals 25.6-27.9), 16.9 (16.2-17.6), and 13.7 (13.5-14.0) in the early, intermediate, and late periods, respectively. In each country, mortality was higher in the early compared to the intermediate period, with a range of adjusted mortality ratios from 3.10 (2.22-4.32) in Japan to 1.15 (0.87-1.53) in the United Kingdom. Adjusted mortality rates were similar for intermediate and late periods. The ratio of elevated mortality rates in the early to the intermediate period increased with age. Within each period, mortality was higher in the United States than in most other countries. Thus, internationally, the early hemodialysis period is a high-risk time for all countries studied, with substantial differences in mortality between countries. Efforts to improve outcomes should focus on the transition period and the first few months of dialysis

The fraternity of female friendly societies

In this chapter, the structured reciprocity of female friendly societies, even those with overt patrons, is presented as a categorization which is also applicable to men’s societies. The first part addresses the notion of independence, then the focus is on the financial aspect of the Southill Female Friendly Society, SFFS, which existed between 1844 and 1948 for women of that Bedfordshire village in England who were of ‘a good and honest character’, in good health and aged between 14 and 45 when they joined. Members had few other opportunities to reduce the risks associated with illness other than accept the uneven reciprocity of the SFFS. The patrons may also have seen the SFFS as an investment opportunity. Then the attractions of Southill, with its healthy housing and relatively liberal interpretation of relief legislation, are presented as evidence of another important attribute of successful friendly societies, their centrality to social networking. Next is considered how far mutuality and philanthropy were interwoven within the SFFS and elsewhere. An assessment of the roles of civil engagement and moral regulation within friendly societies follows and the final section suggests that a notion of fraternity which emphasizes flexible reciprocity can net together both vast international brotherhoods and tiny village societies in a way which illuminate understandings of nineteenth-century society