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Coastal freshwater ecosystems are economically and ecologically important and provide multiple environmental services worldwide. They sequester carbon at rates ten times faster, and store five times more carbon per unit area than mature tropical forests. Vulnerability of these carbon sinks to marine inundation, however, is expected to increase in response to global sea-level rise (GSLR). To better understand the implications of future GSLR, we investigated the geochemical and biological consequences of episodic Holocene marine incursions into Lake Izabal, a large coastal freshwater ecosystem on the Caribbean coast of Central America. About 8,300 cal yr BP, marine incursion transformed Lake Izabal into a sulfur-rich anoxic waterbody, altered its biogeochemical cycles, eliminated several aquatic species, and reduced sediment organic carbon (OC) concentration by as much as to 90%. After that Early Holocene seawater incursion, it took almost 5,000 years for the lacustrine ecosystem to return to low-salinity status. And even when it did, the system did not fully recover to pre-inundation conditions. Some freshwater taxa failed to return, and sediment carbon content remained lower than pre-inundation values. A subsequent, but less intense marine incursion ca. 1,900 cal yr BP led to the formation of a sulfur-rich, hypoxic, brackish-water ecosystem that triggered a similar biodiversity loss and further sediment OC decline. These findings suggest that future marine incursions into coastal freshwater ecosystems, driven by ongoing GSLR, could have dramatic consequences, leading to losses of environmental services, including the ability of these systems to maintain high rates of blue carbon storage. 

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) have shown great promise in lacustrine temperature reconstructions across different continents. While brGDGTs have been reported from many different regions and global brGDGT-temperature calibrations have been developed with various methods, southern North America remains an understudied area with little available data. In this study, we analyzed 101 lake surface sediment samples across Mexico and Central America and compared their distributions with those in other lacustrine systems. Nine major brGDGTs were found in all samples. We investigated the relationships between the distribution of the fractional abundances of the nine major brGDGTs and temperature and developed regional calibrations for Mean Annual Temperature using three different approaches, including a novel machine learning method – Ridge Regression. All the regional calibrations provide similar results with very close error ranges (RMSE = 3.1 ◦C). The majority of global brGDGT-temperature calibrations tend to reconstruct lower temperatures when it is below 15 ◦C. Interestingly, regional brGDGT calibrations appear to reduce the “cold bias”, but the various global and regional calibrations tested here are not significantly different in their predictive capability.