Sedimentary and environmental evolution of an alpine lake in Sierra Nevada, western Mediterranean region, since the last deglaciation

Abstract

<p>The alpine wetlands from Sierra Nevada mountain range, located in the western Mediterranean region, are highly sensitive to climate and environmental changes and their sedimentary records preserve a high-quality signal of past natural environmental conditions. Previous studies from the Sierra Nevada show that alpine wetland formation occurred principally during the YD-Early Holocene transition in former glacier cirque areas and thus older sedimentary records are lacking from that area. In this study, we present the Laguna Seca (LS) record, which is the longest and oldest sedimentary record (14.1 meters and 18 cal kyr BP, respectively) ever retrieved in the alpine Sierra Nevada. In this record we have carried out detailed chronological and sedimentological analyses and we have obtained magnetic susceptibility, total organic carbon, and carbon/nitrogen data with the main goal of understanding how alpine environments of this region responded to climate variations since the last glacial-interglacial transition. Four climatically-controlled facies associations have been identified and interpreted in terms of transport mechanisms and paleoenvironments: (1) subaerial cohesionless debris flows during a paraglacial stage, (2) till or nival diamicton during a small glacier/nivation hollow stage, (3) massive mudstone by suspension settling of clays into standing water during a lacustrine stage from ~15.7 cal kyr BP to the present and; (4) frost-shattering breccia deposited inside the lacustrine stage, only in an area of the wetland, probably during the YD and related with a periglacial substage. The increase in summer insolation, temperatures and precipitation in the western Mediterranean area probably boosted a significant ice-melting and the glacier retreat in the Sierra Nevada, triggering the development of a deep lake in LS ~15.7 cal kyr BP with an important organic matter contribution until the end of the Early Holocene (except in the Younger Dryas that probably the lake level dropped). The general long-term aridification trend observed in the western Mediterranean region from the Middle Holocene to the present triggered the evolution from deep to ephemeral lacustrine conditions with an increase in aquatic productivity in the LS basin that ended up with the current summer desiccation of the lake.</p>