Organic matter preservation in the carbonate matrix of a recent microbial mat – Is there a ‘mat seal effect’?

Phototrophic mats (microbial mats with a phototrophic top layer) are complex systems in terms of microbial diversity, biogeochemical cycles and organic matter (OM) turnover. It has been proposed that these mats were a predominant life form in Proterozoic shallow water settings, prior to the emergence of bioturbating organisms in the Ediacaran–Cambrian transition. For most of the Precambrian, microbial mats were not only quantitative important carbon fixing systems, but also influenced the transfer and transformation of OM before it entered the geosphere. The profound alteration of compound inventories during transit through microbial mats, implying substantial consequences for OM preservation in the Proterozoic, was recently proposed as a “mat-seal effect” [Pawlowska et al. (2012) Geology 41, 103–106]. To obtain a better understanding of the early diagenetic fate of primary produced OM in microbial mats, we studied a recent calcifying mat from a hypersaline lake in Kiritimati, which showed in the deeper mat layers a maximum ¹⁴C_carbonate age of ∼1500 years. We particularly focused on OM entrapped in the carbonate matrix, because of the better potential of such biomineral-encapsulated OM to reach the geosphere before degradation (and remineralization). Our data indicate that selective preservation is important in phototrophic mats. While a diagenetic transformation of lipid fatty acids (FAs) was evident, their fatty acyl-derived hydrocarbon moieties were not introduced into protokerogen, which was instead mainly comprised of cyanobacterial and/or algal biomacromolecules. Our data support the proposed major impact of the “mat-seal effect” on OM turnover and preservation; i.e. the suppression of biosignatures derived from the upper mat layers, while signals of heterotrophic microbes thriving in deeper mat layers become preferentially preserved (e.g. high hopane/sterane ratios). This mechanism may have broad consequences for the interpretation of biomarkers from Proterozoic shelf environments, because biosignatures of phototrophic mat dwellers as well as planktonic signals may have become heavily biased by the production and turnover of OM in microbial mat systems.     

The hydrological response of surface water to recent climate variability: A remote sensing case study from the central tropical Pacific

For small tropical islands with limited freshwater resources, understanding how island hydrology is influenced by regional climate is important, considering projected hydroclimate and sea level changes as well as growing populations dependent on limited groundwater resources. However, the relationship between climate variability and hydrologic variability for many tropical islands remains uncertain due to local hydroclimatic data scarcity. Here, we present a case study from Kiritimati, Republic of Kiribati (2°N, 157°W), utilizing the normalized difference vegetation index to investigate variability in island surface water area, an important link between climate variability and groundwater storage. Kiritimati surface water area varies seasonally, following wet and dry seasons, and interannually, due to hydroclimate variability associated with the El Niño/Southern Oscillation. The NIÑO3.4 sea surface temperature index, satellite‐derived precipitation, precipitation minus evaporation, and local sea level all had significant positive correlations with surface water area. Lagged correlations show sea level changes and precipitation influence surface water area up to 6 months later. Differences in the timing of surface water area changes and variable climate‐surface water area correlations in island subregions indicate that surface hydrology on Kiritimati is not uniform in response to climate variations. Rather, the magnitude of the ocean–atmosphere anomalies and island–ocean connectivity determine the extent to which sea level and precipitation control surface water area. The very strong 2015–2016 El Niño event led to the largest surface water area measured in the 18‐year data set. Surface water area decreased to pre‐event values in a similarly rapid manner (<6 months) after both the very strong 2015–2016 event and the 2009–2010 moderate El Niño event. Future changes in the frequency and amplitude of interannual hydroclimate variability as well as seasonal duration will thus alter surface water coverage on Kiritimati, with implications for freshwater resources, flooding, and drought.