The Iso2k database: a global compilation of paleo-δ18O and δ2H records to aid understanding of common era climate

Reconstructions of global hydroclimate during the Common Era (CE; the past ~2,000 years) are important for providing context for current and future global environmental change. Stable isotope ratios in water are quantitative indicators of hydroclimate on regional to global scales, and these signals are encoded in a wide range of natural geologic archives. Here we present the Iso2k database, a global compilation of previously published datasets from a variety of natural archives that record the stable oxygen (δ18O) or hydrogen (δ²H) isotopic composition of environmental waters, which reflect hydroclimate changes over the CE. The Iso2k database contains 756 isotope records from the terrestrial and marine realms, including: glacier and ground ice (205); speleothems (68); corals, sclerosponges, and mollusks (145); wood (81); lake sediments and other terrestrial sediments (e.g., loess) (158); and marine sediments (99). Individual datasets have temporal resolutions ranging from sub-annual to centennial, and include chronological data where available. A fundamental feature of the database is its comprehensive metadata, which will assist both experts and non-experts in the interpretation of each record and in data synthesis. Key metadata fields have standardized vocabularies to facilitate comparisons across diverse archives and with climate model simulated fields. This is the first global-scale collection of water isotope proxy records from multiple types of geological and biological archives. It is suitable for evaluating hydroclimate processes through time and space using large-scale synthesis, model-data intercomparison and (paleo)data assimilation. The Iso2k database is available for download at: https://doi.org/10.25921/57j8-vs18 (Konecky and McKay, 2020) and is also accessible via through the NOAA/WDS Paleo Data landing page: https://www.ncdc.noaa.gov/paleo/study/29593

Chromium Isotope Systematics in Modern and Ancient Microbialites

Changes in stable chromium isotopes (denoted as δ53Cr) in ancient carbonate sediments are increasingly used to reconstruct the oxygenation history in Earth’s atmosphere and oceans through time. As a significant proportion of marine carbonate older than the Cambrian is microbially-mediated, the utility of δ53Cr values in ancient carbonates hinges on whether these sediments accurately capture the isotope composition of their environment. We report Cr concentrations (Cr) and δ53Cr values of modern marginal marine and non-marine microbial carbonates. These data are supported by stable C and O isotope compositions, as well as rare earth elements and yttrium (REY) concentrations. In addition, we present data on ancient analogs from Precambrian strata. Microbial carbonates from Marion Lake (Australia, δ53Cr ≈ 0.99‰) and Mono Lake (USA, ≈0.78‰) display significantly higher δ53Cr values compared with ancient microbialites from the Andrée Land Group in Greenland (720 Ma, ≈0.36‰) and the Bitter Springs Formation in Australia (800 Ma, ≈−0.12‰). The δ53Cr values are homogenous within microbialite specimens and within individual study sites. This indicates that biological parameters, such as vital effects, causing highly variable δ53Cr values in skeletal carbonates, do not induce variability in δ53Cr values in microbialites. Together with stable C and O isotope compositions and REY patterns, δ53Cr values in microbialites seem to be driven by environmental parameters such as background lithology and salinity. In support, our Cr and δ53Cr results of ancient microbial carbonates agree well with data of abiotically precipitated carbonates of the Proterozoic. If detrital contamination is carefully assessed, microbialites have the potential to record the δ53Cr values of the waters from which they precipitated. However, it remains unclear if these δ53Cr values record (paleo-) redox conditions or rather result from other physico-chemical parameters

Globally coherent water cycle response to temperature change during the past two millennia

The response of the global water cycle to changes in global surface temperature remains an outstanding question in future climate projections and in past climate reconstructions. The stable hydrogen and oxygen isotope compositions of precipitation (δprecip), meteoric water (δMW) and seawater (δSW) integrate processes from microphysical to global scales and thus are uniquely positioned to track global hydroclimate variations. Here we evaluate global hydroclimate during the past 2,000 years using a globally distributed compilation of proxies for δprecip, δMW and δSW. We show that global mean surface temperature exerted a coherent influence on global δprecip and δMW throughout the past two millennia, driven by global ocean evaporation and condensation processes, with lower values during the Little Ice Age (1450–1850) and higher values after the onset of anthropogenic warming (~1850). The Pacific Walker Circulation is a predominant source of regional variability, particularly since 1850. Our results demonstrate rapid adjustments in global precipitation and atmospheric circulation patterns—within decades—as the planet warms and cools