Magnesium stable isotope ecology using mammal tooth enamel.

Geochemical inferences on ancient diet using bone and enamel apatite rely mainly on carbon isotope ratios (δ(13)C) and to a lesser extent on strontium/calcium (Sr/Ca) and barium/calcium (Ba/Ca) elemental ratios. Recent developments in nontraditional stable isotopes provide an unprecedented opportunity to use additional paleodietary proxies to disentangle complex diets such as omnivory. Of particular relevance for paleodietary reconstruction are metals present in large quantity in bone and enamel apatite, providing that biologically mediated fractionation processes are constrained. Calcium isotope ratios (δ(44)Ca) meet these criteria but exhibit complex ecological patterning. Stable magnesium isotope ratios (δ(26)Mg) also meet these criteria but a comprehensive understanding of its variability awaits new isotopic data. Here, 11 extant mammal species of known ecology from a single locality in equatorial Africa were sampled for tooth enamel and, together with vegetation and feces, analyzed for δ(26)Mg, δ(13)C, Sr/Ca, and Ba/Ca ratios. The results demonstrate that δ(26)Mg incorporated in tooth enamel becomes heavier from strict herbivores to omnivores/faunivores. Using data from experimentally raised sheep, we suggest that this (26)Mg enrichment up the trophic chain is due to a (26)Mg enrichment in muscle relative to bone. Notably, it is possible to distinguish omnivores from herbivores, using δ(26)Mg coupled to Ba/Ca ratios. The potential effects of metabolic and dietary changes on the enamel δ(26)Mg composition remain to be explored but, in the future, multiproxy approaches would permit a substantial refinement of dietary behaviors or enable accurate trophic reconstruction despite specimen-limited sampling, as is often the case for fossil assemblages.

Experimental Confirmation of Isotope Fractionation in Thiomolybdates Using Ion Chromatographic Separation and Detection by Multicollector ICPMS.

Molybdenum 98Mo/95Mo isotope ratios are a sediment paleo proxy for the redox state of the ancient ocean. Under sulfidic conditions, no fractionation between seawater and sediment should be observed if molybdate (MoO42-) is quantitatively transformed to tetrathiomolybdate (MoS42-) and precipitated. However, quantum mechanical calculations previously suggested that incomplete sulfidation could be associated with substantial fractionation. To experimentally confirm isotope fractionation in thiomolybdates, a new approach for determination of isotope ratios of individual thiomolybdate species was developed that uses chromatography (HPLC-UV) to separate individual thiomolybdates, collecting each peak and analyzing isotope ratios with multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Using commercially available MoO42- and MoS42- standards, the method was evaluated and excellent reproducibility and accuracy were obtained. For species with longer retention times, complete chromatographic peaks had to be collected to avoid isotope fractionation within peaks. Isotope fractionation during formation of thiomolybdates could be experimentally proven for the first time in the reaction of MoO42- with 20-fold or 50-fold excess of sulfide. The previously calculated isotope fractionation for MoS42- was confirmed, and the result for MoO2S22- was in the predicted range. Isotopic fractionation during MoS42- transformation with pressurized air was dominated by kinetic fractionation. Further optimization and online-coupling of the HPLC-MC-ICPMS approach for determination of low concentrations in natural samples will greatly help to obtain more accurate species-selective isotope information.

The oceanic budgets of nickel and zinc isotopes: the importance of sulfidic environments as illustrated by the Black Sea.

Isotopic data collected to date as part of the GEOTRACES and other programmes show that the oceanic dissolved pool is isotopically heavy relative to the inputs for zinc (Zn) and nickel (Ni). All Zn sinks measured until recently, and the only output yet measured for Ni, are isotopically heavier than the dissolved pool. This would require either a non-steady-state ocean or other unidentified sinks. Recently, isotopically light Zn has been measured in organic carbon-rich sediments from productive upwelling margins, providing a potential resolution of this issue, at least for Zn. However, the origin of the isotopically light sedimentary Zn signal is uncertain. Cellular uptake of isotopically light Zn followed by transfer to sediment does not appear to be a quantitatively important process. Here, we present Zn and Ni isotope data for the water column and sediments of the Black Sea. These data demonstrate that isotopically light Zn and Ni are extracted from the water column, probably through an equilibrium fractionation between different dissolved species followed by sequestration of light Zn and Ni in sulfide species to particulates and the sediment. We suggest that a similar, non-quantitative, process, operating in porewaters, explains the Zn data from organic carbon-rich sediments.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Variable Quaternary chemical weathering fluxes and imbalances in marine geochemical budgets.

Rivers are the dominant source of many elements and isotopes to the ocean. But this input from the continents is not balanced by the loss of the elements and isotopes through hydrothermal and sedimentary exchange with the oceanic crust, or by temporal changes in the marine inventory for elements that are demonstrably not in steady state. To resolve the problem of the observed imbalance in marine geochemical budgets, attention has been focused on uncertainties in the hydrothermal and sedimentary fluxes. In recent Earth history, temporally dynamic chemical weathering fluxes from the continents are an inevitable consequence of periodic glaciations. Chemical weathering rates on modern Earth are likely to remain far from equilibrium owing to the physical production of finely ground material at glacial terminations that acts as a fertile substrate for chemical weathering. Here we explore the implications of temporal changes in the riverine chemical weathering flux for oceanic geochemical budgets. We contend that the riverine flux obtained from observations of modern rivers is broadly accurate, but not representative of timescales appropriate for elements with oceanic residence longer than Quaternary glacial-interglacial cycles. We suggest that the pulse of rapid chemical weathering initiated at the last deglaciation has not yet decayed away and that weathering rates remain about two to three times the average for an entire late Quaternary glacial cycle. Taking into account the effect of the suggested non-steady-state process on the silicate weathering flux helps to reconcile the modelled marine strontium isotope budget with available data. Overall, we conclude that consideration of the temporal variability in riverine fluxes largely ameliorates long-standing problems with chemical and isotopic mass balances in the ocean.

Dawn of diverse shelled and carbonaceous animal microfossils at ~ 571 Ma.

The Ediacaran-Cambrian transition documents a critical stage in the diversification of animals. The global fossil record documents the appearance of cloudinomorphs and other shelled tubular organisms followed by non-biomineralized small carbonaceous fossils and by the highly diversified small shelly fossils between ~ 550 and 530 Ma. Here, we report diverse microfossils in thin sections and hand samples from the Ediacaran Bocaina Formation, Brazil, separated into five descriptive categories: elongate solid structures (ES); elongate filled structures (EF); two types of equidimensional structures (EQ 1 and 2) and elongate hollow structures with coiled ends (CE). These specimens, interpreted as diversified candidate metazoans, predate the latest Ediacaran biomineralized index macrofossils of the Cloudina-Corumbella-Namacalathus biozone in the overlying Tamengo Formation. Our new carbonate U-Pb ages for the Bocaina Formation, position this novel fossil record at 571 ± 9 Ma (weighted mean age). Thus, our data point to diversification of metazoans, including biomineralized specimens reminiscent of sections of cloudinids, protoconodonts, anabaritids, and hyolithids, in addition to organo-phosphatic surficial coverings of animals, demonstrably earlier than the record of the earliest known skeletonized metazoan fossils.

Nickel and zinc micronutrient availability in Phanerozoic oceans.

Nickel and zinc are both bio-essential micronutrients with a nutrient-like distribution in the modern ocean, but show key differences in their biological functions and geochemical behavior. Eukaryotic phytoplankton, and especially diatoms, have high Zn quotas, whereas cyanobacteria generally require relatively more Ni. Secular changes in the relative availability of these micronutrients may, therefore, have affected the evolution and diversification of phytoplankton. In this study, we use a large compilation of Ni and Zn concentration data for Phanerozoic sediments to evaluate long-term changes in Ni and Zn availability and possible links to phytoplankton evolution. Modern data suggest that organic-rich sediments capture the dissolved deep ocean Ni/Zn ratio, regardless of local depositional conditions. We use this observation to constrain Ni/Zn ratios for past oceans, based on data from the sedimentary record. This record highlights long-term changes in the relative availability of these micronutrients that can be linked to the (bio)geochemical conditions on the Earth's surface. Early Palaeozoic oceans were likely relatively Ni rich, with sedimentary Ni/Zn ratios for this interval mostly being around ~1 or higher. A comparison with Phanerozoic strontium-, carbon-, and sulfur-isotopic records suggests that the late Palaeozoic decrease in sulfidic conditions and increase in hydrothermal inputs and organic-carbon burial rates caused a shift towards more Zn-rich conditions. Mesozoic and Cenozoic sediments show relatively Zn-rich oceans for these time intervals, with sedimentary Ni/Zn ratios mostly being around ~1 or lower. These observations imply that the diversification of the dominant groups of modern eukaryotic phytoplankton occurred in relatively Zn-rich oceans and that these organisms still carry this signature in their stoichiometries. However, the Phanerozoic transition to a more Zn-rich ocean pre-dates the origin and diversification of modern eukaryotes and, therefore, this transition was likely not the main direct cause for eukaryotic diversification in the Mesozoic and Cenozoic Eras.

Dominance of benthic fluxes in the oceanic beryllium budget and implications for paleo-denudation records.

The ratio of atmosphere-derived 10Be to continent-derived 9Be in marine sediments has been used to probe the long-term relationship between continental denudation and climate. However, its application is complicated by uncertainty in 9Be transfer through the land-ocean interface. The riverine dissolved load alone is insufficient to close the marine 9Be budget, largely due to substantial removal of riverine 9Be to continental margin sediments. We focus on the ultimate fate of this latter Be. We present sediment pore-water Be profiles from diverse continental margin environments to quantify the diagenetic Be release to the ocean. Our results suggest that pore-water Be cycling is mainly controlled by particulate supply and Mn-Fe cycling, leading to higher benthic fluxes on shelves. Benthic fluxes may help close the 9Be budget and are at least comparable to, or higher (~2-fold) than, the riverine dissolved input. These observations demand a revised model framework, which considers the potentially dominant benthic source, to robustly interpret marine Be isotopic records.

Negligible glacial-interglacial variation in continental chemical weathering rates.

Chemical weathering of the continents is central to the regulation of atmospheric carbon dioxide concentrations, and hence global climate. On million-year timescales silicate weathering leads to the draw-down of carbon dioxide, and on millennial timescales chemical weathering affects the calcium carbonate saturation state of the oceans and hence their uptake of carbon dioxide. However, variations in chemical weathering rates over glacial-interglacial cycles remain uncertain. During glacial periods, cold and dry conditions reduce the rate of chemical weathering, but intense physical weathering and the exposure of carbonates on continental shelves due to low sea levels may increase this rate. Here we present high-resolution records of the lead isotope composition of ferromanganese crusts from the North Atlantic Ocean that cover the past 550,000 years. Combining these records with a simple quantitative model of changes in the lead isotope composition of the deep North Atlantic Ocean in response to chemical weathering, we find that chemical weathering rates were two to three times lower in the glaciated interior of the North Atlantic Region during glacial periods than during the intervening interglacial periods. This decrease roughly balances the increase in chemical weathering caused by the exposure of continental shelves, indicating that chemical weathering rates remained relatively constant on glacial-interglacial timescales. On timescales of more than a million years, however, we suggest that enhanced weathering of silicate glacial sediments during interglacial periods results in a net draw-down of atmospheric carbon dioxide, creating a positive feedback on global climate that, once initiated, promotes cooling and further glaciation.

A biomarker based on the stable isotopes of nickel.

The new stable isotope systems of transition metals are increasingly used to understand and quantify the impact of primitive microbial metabolisms on the modern and ancient Earth. To date, little effort has been expended on nickel (Ni) isotopes but there are good reasons to believe that this system may be more straightforward, and useful in this respect, than some others. Here, we present Ni stable isotope data for abiotic terrestrial samples and pure cultures of methanogens. The dataset for rocks reveals little isotopic variability and provides a lithologic baseline for terrestrial Ni isotope studies. In contrast, methanogens assimilate the light isotopes, yielding residual media with a complementary heavy isotopic enrichment. Methanogenesis may have evolved during or before the Archean, when methane could have been key to Earth's early systems. Our data suggest significant potential in Ni stable isotopes for identifying and quantifying methanogenesis on the early planet. Additionally, Ni stable isotope fractionation may well prove to be the fundamental unambiguous trace metal biomarker for methanogens.

A humid corridor across the Sahara for the migration of early modern humans out of Africa 120,000 years ago.

It is widely accepted that modern humans originated in sub-Saharan Africa approximately 150-200 thousand years ago (ka), but their route of dispersal across the currently hyperarid Sahara remains controversial. Given that the first modern humans north of the Sahara are found in the Levant approximately 120-90 ka, northward dispersal likely occurred during a humid episode in the Sahara within Marine Isotope Stage (MIS) 5e (130-117 ka). The obvious dispersal route, the Nile, may be ruled out by notable differences between archaeological finds in the Nile Valley and the Levant at the critical time. Further west, space-born radar images reveal networks of-now buried-fossil river channels that extend across the desert to the Mediterranean coast, which represent alternative dispersal corridors. These corridors would explain scattered findings at desert oases of Middle Stone Age Aterian lithic industries with bifacial and tanged points that can be linked with industries further to the east and as far north as the Mediterranean coast. Here we present geochemical data that demonstrate that water in these fossil systems derived from the south during wet episodes in general, and penetrated all of the way to the Mediterranean during MIS 5e in particular. This proves the existence of an uninterrupted freshwater corridor across a currently hyperarid region of the Sahara at a key time for early modern human migrations to the north and out of Africa.

Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes.

The Paleocene Eocene Thermal Maximum (PETM) represents a major carbon cycle and climate perturbation that was associated with ocean de-oxygenation, in a qualitatively similar manner to the more extensive Mesozoic Oceanic Anoxic Events. Although indicators of ocean de-oxygenation are common for the PETM, and linked to biotic turnover, the global extent and temporal progression of de-oxygenation is poorly constrained. Here we present carbonate associated uranium isotope data for the PETM. A lack of resolvable perturbation to the U-cycle during the event suggests a limited expansion of seafloor anoxia on a global scale. We use this result, in conjunction with a biogeochemical model, to set an upper limit on the extent of global seafloor de-oxygenation. The model suggests that the new U isotope data, whilst also being consistent with plausible carbon emission scenarios and observations of carbon cycle recovery, permit a maximum ~10-fold expansion of anoxia, covering <2% of seafloor area.

New isotope constraints on the Mg oceanic budget point to cryptic modern dolomite formation.

The oceanic magnesium budget is important to our understanding of Earth's carbon cycle, because similar processes control both (e.g., weathering, volcanism, and carbonate precipitation). However, dolomite sedimentation and low-temperature hydrothermal circulation remain enigmatic oceanic Mg sinks. In recent years, magnesium isotopes (δ26Mg) have provided new constraints on the Mg cycle, but the lack of data for the low-temperature hydrothermal isotope fractionation has hindered this approach. Here we present new δ26Mg data for low-temperature hydrothermal fluids, demonstrating preferential 26Mg incorporation into the oceanic crust, on average by εsolid-fluid ≈ 1.6‰. These new data, along with the constant seawater δ26Mg over the past ~20 Myr, require a significant dolomitic sink (estimated to be 1.5-2.9 Tmol yr-1; 40-60% of the oceanic Mg outputs). This estimate argues strongly against the conventional view that dolomite formation has been negligible in the Neogene and points to the existence of significant hidden dolomite formation.

The residence time of Southern Ocean surface waters and the 100,000-year ice age cycle.

From 1.25 million to 700,000 years ago, the ice age cycle deepened and lengthened from 41,000- to 100,000-year periodicity, a transition that remains unexplained. Using surface- and bottom-dwelling foraminifera from the Antarctic Zone of the Southern Ocean to reconstruct the deep-to-surface supply of water during the ice ages of the past 1.5 million years, we found that a reduction in deep water supply and a concomitant freshening of the surface ocean coincided with the emergence of the high-amplitude 100,000-year glacial cycle. We propose that this slowing of deep-to-surface circulation (i.e., a longer residence time for Antarctic surface waters) prolonged ice ages by allowing the Antarctic halocline to strengthen, which increased the resistance of the Antarctic upper water column to orbitally paced drivers of carbon dioxide release.

Timing and causes of North African wet phases during the last glacial period and implications for modern human migration.

We present the first speleothem-derived central North Africa rainfall record for the last glacial period. The record reveals three main wet periods at 65-61 ka, 52.5-50.5 ka and 37.5-33 ka that lead obliquity maxima and precession minima. We find additional minor wet episodes that are synchronous with Greenland interstadials. Our results demonstrate that sub-tropical hydrology is forced by both orbital cyclicity and North Atlantic moisture sources. The record shows that after the end of a Saharan wet phase around 70 ka ago, North Africa continued to intermittently receive substantially more rainfall than today, resulting in favourable environmental conditions for modern human expansion. The encounter and subsequent mixture of Neanderthals and modern humans - which, on genetic evidence, is considered to have occurred between 60 and 50 ka - occurred synchronously with the wet phase between 52.5 and 50.5 ka. Based on genetic evidence the dispersal of modern humans into Eurasia started less than 55 ka ago. This may have been initiated by dry conditions that prevailed in North Africa after 50.5 ka. The timing of a migration reversal of modern humans from Eurasia into North Africa is suggested to be coincident with the wet period between 37.5 and 33 ka.

Rise to modern levels of ocean oxygenation coincided with the Cambrian radiation of animals.

The early diversification of animals (∼ 630 Ma), and their development into both motile and macroscopic forms (∼ 575-565 Ma), has been linked to stepwise increases in the oxygenation of Earth's surface environment. However, establishing such a linkage between oxygen and evolution for the later Cambrian 'explosion' (540-520 Ma) of new, energy-sapping body plans and behaviours has proved more elusive. Here we present new molybdenum isotope data, which demonstrate that the areal extent of oxygenated bottom waters increased in step with the early Cambrian bioradiation of animals and eukaryotic phytoplankton. Modern-like oxygen levels characterized the ocean at ∼ 521 Ma for the first time in Earth history. This marks the first establishment of a key environmental factor in modern-like ecosystems, where animals benefit from, and also contribute to, the 'homeostasis' of marine redox conditions.