Geology topics

Sierra V. Petersen

Publications and source records attributed to Sierra V. Petersen.

3 recordsLinked to original sources

USGS2025

Molluscan isotope sclerochronology in marine palaeoclimatology: Taxa, technique and timespan issues

Study of the accretionary biomineralised hardparts of organisms (sclerochronology) can make a useful contribution to palaeoclimatology. Ontogenetic sequences of isotopic data (δ 18 O and Δ 47 values) from the shells of marine molluscs are a source of information on seasonal sea-surface temperatures that can be used for detailed and rigorous evaluation of the outputs of numerical climate models. In situations where there is significant seasonality, and where shell preservation is adequate, accurate information about winter and summer surface temperature can be obtained from shallow-water benthic forms (bivalves and gastropods), in particular the early ontogeny of fast-growing species. Accurate information about winter surface temperature can also be obtained from individuals that lived at mid-shelf depths (20–40 m), but summer seafloor values from these need upward adjustment to derive a plausible surface temperature if thermal stratification of the water column occurs in this season. Ontogenetic δ 18 O profiles from planktonic pteropod gastropods are a potential source of insight into seasonal surface temperatures in the ocean basins; these organisms merit investigation for provision of information to complement shelf data. Temperature profiles constructed from shell δ 18 O require an estimate of the δ 18 O value of ambient seawater, which can be derived by back-calculation from the Δ 47 -temperature supplied by the same shell material. Alternatively, through appropriate sampling and data processing, seasonal temperatures can be obtained directly from Δ 47 profiles. Climate parameters are defined in terms of the mean state over a period of 30 consecutive years, a statistic (e.g., for seasonal temperatures) which can be derived from the long isotopic temperature records obtainable from bivalve species that live for many tens or hundreds of years. Efforts should be made to acquire such records, especially averaged data from crossdated shells, to specify climate parameters for precise times in the past. Information for precise times would be of particular value for icehouse intervals like the late Cenozoic, characterised by high frequency (high amplitude) climate fluctuation. Short records from non-crossdated shells can nevertheless provide useful insights into climate, particularly if a large dataset is obtained, supplying a reliable picture of the mean state and range of variation in climate parameters over the interval represented by the shells.

Quaternary Science Reviews↗

USGS2023

Triple oxygen isotope compositions of globally distributed soil carbonates record widespread evaporation of soil waters

The stable isotopic composition of pedogenic carbonates is central to many studies of past climate and topography, providing a basis for our understanding of Earth's terrestrial history. A core assumption of many applications of oxygen isotope values (δ18O) of pedogenic carbonate is that they reflect the δ18O value of precipitation (rain/snow). This assumption is violated if soil carbonates form in evaporated soil waters. In this work, we develop a means to identify evaporation in ancient soils using the triple oxygen isotope composition (16O-17O-18O) of pedogenic carbonates. Both theoretical predictions of isotope kinetics during evaporation and studies of triple oxygen isotopes in other geological materials show that the deviation in the relationship between δ17O and δ18O from a reference line, evaluated using the parameter Δ'17O, is sensitive to evaporation. As a first step in developing the use of Δ'17O in ancient pedogenic carbonates, we report Δ'17O values from 47 near-modern pedogenic carbonate samples from globally distributed environments that vary in aridity (hyper-arid to humid). The Δ'17O values of pedogenic carbonate range from -154 to -60 per meg (as CaCO3, measured via O2, VSMOW-SLAP), corresponding to calculated soil water values of -66 to +27 per meg (VSMOW-SLAP) (using a carbonate-water triple oxygen isotope fractionation exponent of 0.5250 and clumped isotope-derived carbonate growth temperatures). The Δ'17O values indicate that evaporative modification of soil water from which pedogenic carbonate forms is common, especially in arid environments. Arid environments host pedogenic carbonates formed from soil waters ranging from highly to minimally evaporated, while humid environments host pedogenic carbonates formed from waters that are only minimally evaporated. The variability in Δ'17O within environments classified by the same aridity may relate to the fact that pedogenic carbonates record soil conditions only during times of carbonate mineralization, which may deviate from annual conditions. Thus, Δ'17O may be useful in understanding the specific circumstances of pedogenic carbonate formation but may not provide incontrovertible evidence of the magnitude of environmental aridity. Evaporative modification of δ18O values of pedogenic carbonates can be detected with Δ'17O, thereby improving estimates of δ18O of unevaporated waters. Our data show that evaporation must be (re)considered for all paleoclimate inferences based on the δ18O of pedogenic carbonate. The addition of Δ'17O will re-energize paleoclimate studies that use (or have avoided using) δ18O of pedogenic carbonate.

Geochimica et Cosmochimica Acta↗

USGS2017

Constraining the thermal history of the North American Midcontinent Rift System using carbonate clumped isotopes and organic thermal maturity indices

The Midcontinent Rift System (MRS) is a Late Mesoproterozoic (∼1.1 Ga) sequence of volcanic and sedimentary rocks exposed in the Lake Superior Region of North America. The MRS continues to be the focus of much research due to its economic mineral deposits as well as its archive of Precambrian life and tectonic processes. In order to constrain the post-depositional thermal history of the MRS, samples were analyzed for carbonate clumped isotope composition and organic thermal maturity. Clumped isotope values from sedimentary/early-diagenetic samples were partially reset during burial to temperatures between 68 and 75 °C. Solid-state reordering models indicate that maximum burial temperatures of 125–155 °C would reset the clumped isotope values to the observed temperature range prior to the onset of regional cooling and uplift. Clumped isotope results from late-stage veins in the White Pine Mine encompass a greater temperature range (49–116 °C), indicative of spatially variable hydrothermal activity and vein emplacement after burial temperatures fell below 100 °C during regional cooling and uplift. Clumped isotope and organic thermal maturity data do not indicate significant spatial differences in thermal history along the MRS. Observed variability in bulk organic matter composition and biomarker indices are therefore more likely a result of shifts in primary productivity or early-degradation processes. These results demonstrate that the MRS experienced a spatially consistent, relatively mild thermal history (125–155 °C) and is therefore a valuable archive for understanding the Late Mesoproterozoic environment.

Lake Superior↗