Böcker i Elements in Geochemical Tracers in Earth System Science-serien

In one form or another, iron speciation has had a long history as a paleoredox proxy. This Element covers the theory behind the proxy, methods involved in applying the technique, and potential complications in interpreting Fe speciation data.

Lithium isotopes are a relatively novel tracer of present and past silicate weathering processes. Given that silicate weathering is the primary long-term method by which CO2 is removed from the atmosphere, Li isotope research is going through an exciting phase. We show the weathering processes that fractionate dissolved and sedimentary Li isotope ratios, focusing on weathering intensity and clay formation. We then discuss the carbonate and silicate archive potential of past seawater I 7Li. These archives have been used to examine Li isotope changes across both short and long timescales. The former can demonstrate the rates at which the climate is stabilised from perturbations via weathering, a fundamental piece of the puzzle of the long-term carbon cycle.

Reconstruction of ocean paleoproductivity and paleochemistry is paramount to understanding global biogeochemical cycles such as the carbon, oxygen and sulfur cycles and the responses of these cycles to changes in climate and tectonics. Paleo-reconstruction involves the application of various tracers that record seawater compositions, which in turn may be used to infer oceanic processes. Several important tracers are incorporated into pelagic barite, an authigenic mineral that forms in the water column. Here we summarize the utility of pelagic barite for the reconstruction of export production and as a recorder of seawater S, O, Sr, Ca and Ba.

Diverse and abundant lipid biomarker assemblages have been reported from a variety of Proterozoic marine environments from the careful analysis of well-preserved rocks and oils. These molecular biosignatures have provided unique insights into the communities and the environmental conditions which characterized the Proterozoic marine biosphere. We summarize some of the major temporal patterns evident in Proterozoic lipid biomarkers found to date, whilst emphasizing the scale of local heterogeneity found within Neoproterozoic oceans from region to region, and their relationship with the evolving ecological, climatic and ocean/atmospheric redox conditions. Short commentaries on a selection of papers published from the last 15 years of biomarker literature are given. The focus here is on key studies, highlighted for further reading, which have helped to better constrain the timing of the ecological expansion of eukaryotes in Proterozoic oceans or which have impacted on our knowledge of the biological sources of Proterozoic biomarkers.

The attraction of selenium isotopes as a paleoenvironmental tracer lies in the high redox potential of selenium oxyanions (SeIV and SeVI), the dominant species in the modern ocean. The largest isotopic fractionations occur during oxyanion reduction, which makes selenium isotopes a sensitive proxy for the redox evolution of our planet. As a case study we review existing data from the Neoarchean and Paleoproterozoic, which show that significant isotopic fractionations are absent until 2.5 Ga, and prolonged isotopic deviations only appear around 2.3 Ga. Selenium isotopes have thus begun to reveal complex spatiotemporal redox patterns not reflected in other proxies.

How oxygen levels in Earth's atmosphere and oceans evolved has always been a central question in Earth System Science. Researchers have developed numerous tracers to tackle this question, utilizing geochemical characteristics of different elements. Iodine incorporated in calcium carbonate (including biogenic) minerals, reported as I/Ca, is a proxy for dissolved oxygen in seawater. Here we review the rationale behind this proxy, its recent applications and some potential future research directions.

Over million-year timescales, the geologic cycling of carbon controls long-term climate and the oxidation of Earth's surface. Inferences about the carbon cycle can be made from time series of carbon isotopic ratios measured from sedimentary rocks.

Magnesium is a major constituent in silicate and carbonate minerals, the hydrosphere and the biosphere. Magnesium is constantly cycled between these reservoirs. This review summarises some of the key motivations, successes and challenges facing the use of magnesium isotopes to construct a budget of seawater magnesium, present and past.

Ancient iron formations - iron and silica-rich chemical sedimentary rocks that formed throughout the Precambrian eons - provide a significant part of the evidence for the modern scientific understanding of palaeoenvironmental conditions in Archaean (4.0-2.5 billion years ago) and Proterozoic (2.5-0.539 billion years ago) times.

In this Element, we propose a refinement of molybdenum (Mo) geochemistry within aquatic systems, ancient rocks, and modern sediments through molecular geochemistry. Specifically, to intermediate sulfide concentrations governing Mo behavior below the "switch-point" and dominant sequestration pathways in low oxygen conditions.

In the marine environment barium isotope ( 138Ba) variations are driven by barite cycling. This Element examines these variations; evaluates their global, regional, local, and geological controls; and explores how 138Ba can be exploited to constrain the origin of enigmatic sedimentary sulfates and study marine biogeochemistry over Earth's history.

This Element focuses on the history of discovery of triple isotope effects, the conceptual framework behind these effects, and major lines of development in the past few years of triple oxygen isotope research.

The stable chromium (Cr) isotope system has emerged over the past decade as a new tool to track changes in the amount of oxygen in earth's ocean-atmosphere system. Cr isotopic signatures can provide novel insights into Cr redox cycling in both marine and terrestrial settings.

Paleosols formed in direct contact with the Earth's atmosphere, so they can record the composition of the atmosphere through weathering processes and products. Herein we critically review a variety of different approaches for reconstructing atmospheric O2 and CO2 over the past three billion years.

Stable isotope ratios of nitrogen compounds are preserved in the sedimentary record and track these changes over Earth's history, providing important insights into associated biogeochemical feedbacks. Here we review the use of nitrogen stable isotope geochemistry in unravelling the evolution of the global N cycle in deep time.

Vanadium isotope ratios (51V/50V) have potential to provide information about changes in past ocean oxygen contents. The oxygen abundance sensitivity range of V isotopes suggests that this paleoproxy could be particularly useful in tracing marine oxygenation changes throughout the Phanerozoic and potentially beyond.

Ce anomalies track changes in oxygen availability due to the anomalous redox-sensitivity of Ce compared with the other rare earth elements. This review focusses on the systematics of the Ce anomaly proxy, the preservation and extraction of the signal in sedimentary rocks, and the potential applications of the proxy.

The use of the trace element content of sedimentary pyrite as a proxy for the trace element composition of past oceans has recently emerged. The pyrite proxy has several potential advantages over bulk sample analysis, and these strengths make the pyrite trace element proxy is a valuable potential addition to the paleo-ocean chemistry tool kit.

Precise measurements of the calcium (Ca) isotopes have provided constraints on Ca cycling at global and local scales, and quantified rates of carbonate diagenesis in marine sedimentary systems. Key to applying Ca isotopes as a geochemical tracer is understanding the impact of multiple factors potentially impacting Ca isotopes in the rock record.

The TEX86 paleothermometer is based upon the distribution of archaeal membrane lipids ('GDGTs') in marine sediments. We review the principles of the TEX86 proxy and developments made over the last two decades. We also discuss its application as a paleotemperature proxy and explore existing challenges and limitations.

Tracking initial ocean (de)oxygenation is critical to better constrain the coevolution of life and environment. Development of thallium isotopes has provided evidence to track the global manganese oxide burial which responds to early (de)oxygenation for short-term climate events. This Element provides an introduction to the application of thallium isotopes, case studies, and future directions.