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Organic matter accumulation and preservation in aquatic systems can be linked to variations in organic matter sources as well as primary productivity. These changes can be used to determine contemporaneous environmental variations in the catchment area. The source, quality and distribution of lacustrine organic matter (LOM) have been determined in a sedimentary core covering the last ~1,500 years from Laguna Mar Chiquita, a saline lake in central Argentina. Petrophysical, sedimentological and geochemical data along with results of high-resolution pigment analyses provide a unique dataset that allow characterization of both source and type of LOM. Climatically triggered changes in the Laguna Mar Chiquita catchment led to changes in the water salinity and lake trophic state that in turn influenced primary productivity and thus organic matter accumulation. Distinctive high lake water levels and associated low salinity characterize the Medieval Climatic Anomaly (MCA) as well as the last quarter of the twentieth century and beginning of the twentyfirst century. Conversely, extremely low lake levels with resulting high salinity correspond with the Little Ice Age (LIA). High-resolution sedimentary pigment analyses in these two end–members (i.e., LIA and MCA) do not indicate major changes in the structure of phototrophic communities signifying that the biota survives a large range of salinity and temperature. High lake stands (low salinity) correspond with increased primary productivity as shown by high pigment concentrations along with a decrease in authigenic carbonates and evaporites. These high lake level intervals have been previously linked to a warmer and more humid climate. Conversely, low lake levels (hyper saline waters) correlate with decreasing lake productivity and precipitation of both authigenic carbonates and evaporites. These sediments correspond to cooler intervals with a negative moisture balance. The relative similarity between LOM indicated by the pigment stratigraphy for both intervals show no clear evidence of a linkage between human impact and lake productivity. Hence, Laguna Mar Chiquita has been naturally productive even prior to agriculture expansion by the end of the twentieth century.

Authigenic minerals can form in the water column and sediments of lakes, either abiotically or mediated by biological activity. Such minerals have been used as paleosalinity and paleoproductivity indicators and reflect trophic state and early diagenetic conditions. They are also considered potential indicators of past and perhaps ongoing microbial activity within sediments. Authigenic concretions, including vivianite, were described in late glacial sediments of Laguna Potrok Aike, a maar lake in southernmost Argentina. Occurrence of iron phosphate implies specific phosphorus sorption behavior and a reducing environment, with methane present. Because organic matter content in these sediments was generally low during glacial times, there must have been alternative sources of phosphorus and biogenic methane. Identifying these sources can help define past trophic state of the lake and diagenetic processes in the sediments. We used scanning electron microscopy, phosphorus speciation in bulk sediment, pore water analyses, in situ ATP measurements, microbial cell counts, and measurements of methane content and its carbon isotope composition (δ13CCH4) to identify components of and processes in the sediment. The multiple approaches indicated that volcanic materials in the catchment are important suppliers of iron, sulfur and phosphorus. These elements influence primary productivity and play a role in microbial metabolism during early diagenesis. Authigenic processes led to the formation of pyrite framboids and revealed sulfate reduction. Anaerobic oxidation of methane and shifts in pore water ion concentration indicated microbial influence with depth. This study documents the presence of active microbes within the sediments and their relationship to changing environmental conditions. It also illustrates the substantial role played by microbes in the formation of Laguna Potrok Aike concretions. Thus, authigenic minerals can be used as biosignatures in these late Pleistocene maar sediments.

Stratigraphic shifts in the oxygen isotopic (δ18O) and trace element (Mg and Sr) composition of biogenic carbonate from tropical lake sediment cores are often interpreted as a proxy record of the changing relation between evaporation and precipitation (E/P). Holocene δ18O and Mg and Sr records from Lakes Salpeten and Peten Itza, Guatemala were apparently affected by drainage basin vegetation changes that influenced watershed hydrology, thereby confounding paleoclimatic interpretations. Oxygen isotope values and trace element concentrations in the two lowland lakes were greatest between ~ 9000 and 6800 14C-yr BP, suggesting relatively high E/P, but pollen data indicate moist conditions and extensive forest cover in the early Holocene. The discrepancy between pollen- and geochemically-inferred climate conditions may be reconciled if the high early Holocene δ18O and trace element values were controlled principally by low surface runoff and groundwater flow to the lake, rather than high E/P. Dense forest cover in the early Holocene would have increased evapotranspiration and soil moisture storage, thereby reducing delivery of meteoric water to the lakes. Carbonate δ18O and Mg and Sr decreased between 7200 and 3500 14C-yr BP in Lake Salpeten and between 6800 and 5000 14C-yr BP in Lake Peten Itza. This decline coincided with palynologically documented forest loss that may have led to increased surface and groundwater flow to the lakes. In Lake Salpeten, minimum δ18O values (i.e., high lake levels) occurred between 3500 and 1800 14C-yr BP. Relatively high lake levels were confirmed by 14C-dated aquatic gastropods from subaerial soil profiles ~ 1.0–7.5 m above present lake stage. High lake levels were a consequence of lower E/P and/or greater surface runoff and groundwater inflow caused by human-induced deforestation.

A combined bulk and detailed geochemical study of the sedimentary organic matter in Lake Albano, central Italy, provides critical data to track the response of this aquatic system to the environmental changes of variable amplitude that occurred during the Holocene. Rock‐Eval pyrolysis of this predominantly laminated, organic carbon‐rich sedimentary sequence shows changes in hydrogen and oxygen indices that are related to variations in the dominance of the primary producers. These variations are further confirmed by the pigments and the carbon isotopic composition of bulk organic matter showing that cyanobacteria dominated the lake waters during the early and late Holocene whereas diatoms have been the main producers during the middle Holocene. Sharp decreases in productivity, 2–3 centuries long, are identified at ca. 8.2, 6.4 and 3.8 ka. B.P. Changes in temperature and/or effective moisture are suggested as the most probable causes, although human impact cannot be ruled out for the latest part of the Holocene.