Diatom-based reconstruction of Middle to Late Holocene paleoenvironments in Lake Lonkoy, southern Pampas, Argentina

Abstract

Diatoms recovered from shallow Lake Lonkoy sediment core in southern Pampas, Argentina, were studied in order to assess past environmental conditions. A total of 47 samples were selected from a 1.42-m long core that spans the last 4840 cal. years. Diatoms were extracted by chemical cleaning and studied under light and scanning electron microscopy. A total of 89 diatom taxa were recorded, including 36 taxa that reached > 2% relative abundance and were used in statistical analyses. A cluster analysis allowed five significant diatom association zones (DAZ) to be defined. DAZ I (142-92 cm) was dominated by Cyclotella meneghiniana Kützing, Cocconeis placentula Ehrenberg and Nitzschia hustedtiana Salah, and a low abundance of aerophilic taxa, indicating brackish water conditions probably related to the Holocene sea-level high stand. DAZ II (90-48.5 cm) was dominated by C. meneghiniana and Aulacoseira granulata (Ehrenberg) Simonsen, representing a period of high nutrient loading and turbid conditions in the lake, associated with higher water levels and lower salinities than the environmental conditions in DAZ I. The upper 50 cm of the core (DAZ III to V) showed an increase in the dominance of brackish epiphytic and hypereutrophic benthic taxa, indicating an increase in nutrient concentrations associated with agricultural human impacts in the basin. Overall, diatoms allowed major paleoenvironmental changes to be inferred for the lake during the Holocene.

Mollusk Successions of Holocene Shallow-Lake Deposits From the Southeastern Pampa Plain, Argentina

“…This tidal inlet was characterized by deposits of estuarine mollusks such as Tagelus plebeius and H. australis, indicating a direct marine influence in that area. Our findings of H. australis in LK and SL suggest that this marine influence would have reached sites located even more distant, either directly or indirectly (as suggested by Hassan 2013).…”

“…The LK core was recovered with a 5-cm-diameter modified Livingston corer, in April 2009. The sedimentary section, which is 142 cm long, has been previously analyzed for pollen, NPPs, macrofossil remains (Stutz et al 2012), and diatoms (Hassan 2013), and it was divided into three lithological units (Table 1). Six AMS 14 C dates (from 4842 to 670 cal.…”

“…(Violante et al 2001, and references therein). Hassan (2013), based on the presence of saline-brackish diatoms for that moment in LK, pointed out that the rise in sea level would have elevated the water table and, therefore, induced a groundwater salinization in the southern Pampas region. This, in combination with a higher evaporation-precipitation ratio due to the eastward displacement of the arid-semiarid region (Stutz et al 2012), may have contributed to lake salinization.…”

Differential Preservation of Freshwater Diatoms and Mollusks in Late Holocene Sediments: Paleoenvironmental Implications

“…Root penetration was evident in many short cores taken from Lake Nahuel Rucá (Cristini and De Francesco 2012). These Pampean lakes increased their nutrient levels during the late Holocene (Stutz et al 2012;Hassan 2013) and, consequently, their biological activity. At ca.…”

“…Previous studies of these records indicated the occurrence of wide paleoenvironmental changes, particularly related to fluctuations in salinity, water depth, and nutrient status (Stutz et al 2010(Stutz et al , 2012De Francesco et al 2013;Hassan 2013). These environmental changes may have affected diatom and mollusk preservation in different ways.…”