Description: Palmitic acid (PA) is ubiquitous in the biosphere and its hydrogen isotopic composition (δ2HPA) was proposed as a potential paleoenvironmental proxy for salinity, with δ2HPA values increasing with salinity. In this study, we analyzed 40 surface sediment samples from Baffin Bay and the Labrador Sea to examine the isotopic composition of PA in relation to local environmental variables, including salinity. In contrast to expectations, our results show a negative relationship between the δ2HPA and sea-surface salinity, raising questions about its pertinence/usefulness as a salinity proxy. Instead, our results suggest that the relative abundance of distinct organisms that employ different metabolisms is key in determining the hydrogen isotopic fractionations in PA. Whereas we show that PA is mostly produced through photoautotrophic metabolisms by diatoms and dinoflagellates, varying contributions from heterotrophic metabolisms may obscure the stable isotope composition of PA. Surprisingly, we found no correlation between the stable carbon isotopic composition of the sedimentary organic matter (δ13Corg) and palmitic acid (δ13CPA), implying major differences in either the dominant organisms producing sedimentary PA or in carbon isotope fractionation during lipid biosynthesis. We also found that the presence of extended sea-ice cover leads to enriched carbon and hydrogen isotopic compositions in PA. These enriched values suggest heterotrophic biodegradation in the water column and/or in the sediment as well as an increase in grazing activities. We propose that sea-ice cover and surface water oxygenation modulate the relative impact of phototrophic and heterotrophic metabolisms, and therefore the isotopic composition of marine sedimentary PA.

Description: As Earth's atmospheric temperatures and human populations increase, more people are becoming vulnerable to natural and human-induced disasters. This is particularly true in Central America, where the growing human population is experiencing climate extremes (droughts and floods), and the region is susceptible to geological hazards, such as earthquakes and volcanic eruptions, and environmental deterioration in many forms (soil erosion, lake eutrophication, heavy metal contamination, etc.). Instrumental and historical data from the region are insufficient to understand and document past hazards, a necessary first step for mitigating future risks. Long, continuous, well-resolved geological records can, however, provide a window into past climate and environmental changes that can be used to better predict future conditions in the region. The Lake Izabal Basin (LIB), in eastern Guatemala, contains the longest known continental records of tectonics, climate, and environmental change in the northern Neotropics. The basin is a pull-apart depression that developed along the North American and Caribbean plate boundary ∼ 12 Myr ago and contains 〉 4 km of sediment. The sedimentological archive in the LIB records the interplay among several Earth System processes. Consequently, exploration of sediments in the basin can provide key information concerning: (1) tectonic deformation and earthquake history along the plate boundary; (2) the timing and causes of volcanism from the Central American Volcanic Arc; and (3) hydroclimatic, ecologic, and geomicrobiological responses to different climate and environmental states. To evaluate the LIB as a potential site for scientific drilling, 65 scientists from 13 countries and 33 institutions met in Antigua, Guatemala, in August 2022 under the auspices of the International Continental Scientific Drilling Program (ICDP) and the US National Science Foundation (NSF). Several working groups developed scientific questions and overarching hypotheses that could be addressed by drilling the LIB and identified optimal coring sites and instrumentation needed to achieve the project goals. The group also discussed logistical challenges and outreach opportunities. The project is not only an outstanding opportunity to improve our scientific understanding of seismotectonic, volcanic, paleoclimatic, paleoecologic, and paleobiologic processes that operate in the tropics of Central America, but it is also an opportunity to improve understanding of multiple geological hazards and communicate that knowledge to help increase the resilience of at-risk Central American communities.