Benthic carbonates in perennially ice-covered Lake Fryxell (Mc-Murdo Dry Valleys, Antarctica) precipitated from pore waters in microbial mats as calcite rhombs, acicular botryoids and interfering bundles. Carbonates span the pronounced Lake Fryxell oxycline; variations in carbonate-associated manganese and iron concentrations are consistent with local oxycline conditions and seasonal fluctuations in pore water oxygenation. Precipitation is most abundant in shallow oxic waters, but extended through the oxycline during a discrete episode lasting multiple years, as evidenced by patterns of cathodoluminescence consistent with predicted seasonal changes in redox modulating dissolved manganese and iron concentrations. Carbonates did not precipitate in isotopic equilibrium with the water column, and are enriched in 18 O relative to predicted equilibrium values. Carbonate layer 18 O values vary by >20‰ at the mm-scale, suggesting precipitation was driven by mixing of isotopically heterogeneous fluids in the mat pore waters. Correlation of carbonate geochemistry and mat morphology with historical observations indicates that precipitation postdates recent lake level rise. Further investigation of the physical and geochemical carbonate proxies from Lake Fryxell and other ice-covered lakes in the Dry Valleys promises to provide a valuable framework for interpreting Antarctic carbonates as records of modern and ancient climate, Antarctic biogeochemical and hydrological systems, and the drivers of carbonate precipitation at polar climate extremes.

The habitability and ecology of Earth is fundamentally shaped by surface temperature, but the temperature history of our planet is not easily reconstructed, especially before the evolution of early biomineralizing animals. This work presents a billion-year-long, high-resolution, mineral-specific record of oxygen isotope measurements in shallow marine rocks. Clumped isotope paleothermometry results from four minerals resolves previous ambiguity in seawater oxygen isotope composition and confirms that long-term cooling punctuated by short-lived temperature extremes are dominant components of this record. We consider post-depositional effects by comparing Phanerozoic rock and fossil records, and identify temporal and spatial controls on alteration. Furthermore, this record is suggestive of key differences in dolomite (CaMg(CO3)2) formation processes between the Neoproterozoic (1000–538.8 Ma) and Phanerozoic (538.8–0 Ma), consistent with previous suggestions based on petrographic and sedimentological observations. This record, when viewed alongside the fossil record, suggests temperature change is tightly coupled to extinction and origination in the history of life and carbon cycle perturbations over the last billion years.

Quantitative scanning electron microscopy (SEM) quartz microtextural analysis can reveal the transport histories of modern and ancient sediments. However, because workers identify and count microtextures differently, it is difficult to directly compare quantitative microtextural data analyzed by different workers. As a result, the defining microtextures of certain transport modes and their probabilities of occurrence are not well constrained. We used principal component analysis (PCA) to directly compare modern and ancient aeolian, fluvial, and glacial samples from the literature with 9 new samples from active aeolian and glacial environments. Our results demonstrate that PCA can group microtextural samples by transport mode and differentiate between aeolian and fluvial/glacial transport modes across studies. The PCA ordination indicates that aeolian samples are distinct from fluvial and glacial samples, which are in turn difficult to disambiguate from each other. Ancient and modern sediments are also shown to have quantitatively similar microtextural relationships. Therefore, PCA may be a useful tool to constrain the ambiguous transport histories of some ancient sediment grains. As a case study, we analyzed two samples with ambiguous transport histories from the Cryogenian Bråvika Member (Svalbard). Integrating PCA with field observations, we find evidence that the Bråvika Member facies investigated here includes aeolian deposition and may be analogous to syn-glacial Marinoan aeolian units including the Bakoye Formation in Mali and the Whyalla Sandstone in South Australia.

The potential for carbonate clumped isotope thermometry to independently constrain both the formation temperature (T∆47 ) of carbonate minerals and fluid oxygen isotope composition allows insight into long-standing questions in the Earth sciences, but remaining discrepancies between calibration schemes hamper interpretation of T∆47 measurements. To address discrepancies between calibrations, we designed and analyzed a sample suite (41 total samples) with broad applicability across the geosciences, with an exceptionally wide range of formation temperatures, precipitation methods, and mineralogies. We see no statistically significant offset between sample types, although comparison of calcite and dolomite remains inconclusive. When data are reduced identically, the regression defined by this study is nearly identical to that defined by four previous calibration studies that used carbonate-based standardization; we combine these data to present a composite carbonate-standardized regression equation. Agreement across a wide range of temperature and sample types demonstrates a unified, broadly applicable clumped isotope thermometer calibration.