Swiss lakes as model oceans: A mixed carbonate-siliciclastic shelf drift hosting a 10-km-long chain of 170 submerged Neolithic mounds

Judith McKenzie contributed pivotal studies in using lacustrine sedimentary systems as natural analogues for the much larger marine environment. In this context, she introduced marine depositional processes and concepts to lakes, and in turn, used their smaller dimensions and more controllable boundary conditions to gain more insights into the controls of lithological and geochemical signals in any sedimentary system. One example of using this approach is the study of the prograding mixed carbonate-siliciclastic shallow-water shelf of Lake Constance, that is characterized by endogenic carbonate production ('lacustrine chalk'), water currents, progradation and related drift deposits, as they also have been investigated in the marine domain. A recent bathymetric survey of Lake Constance revealed ~170 mysterious mounds composed of loosely deposited rocks aligned in a ~10-km-long chain along the southern Swiss shoreline of Lake Constance in a water depth of 3–5 m. The mounds are 10–30 m in diameter and up to 1.5 m high. Over their entire length of occurrence, the mounds are estimated to be composed of ~60 million individual boulders, with a total weight of ~78,000 t. A ground penetrating radar (GPR) survey showed that the mounds are not linked to the glacial substrate but were rather deposited artificially on the edge of a prograding shelf composed of mixed carbonate-siliciclastic Late Glacial to Holocene lake sediments. Here, we present the results of a coring campaign with four piston cores along a GPR transect across one of the mounds. The cores recovered the full postglacial sedimentary succession all the way into the basal till that is overlain by lacustrine sediments dating back to ~14,400 cal. BP. The four cores are merged into a ~12.4-m-long composite section reflecting continuous sedimentation from the siliciclastic-dominated Late Glacial to the carbonate-rich Late Holocene. The stratigraphic horizon representing the mound's construction was radiocarbon-dated to ~5,600–5,300 cal. BP, placing them in the Neolithic period. This age was confirmed by radiocarbon dating of wood samples collected during underwater excavation of the mounds. Geochemical analysis of the Holocene sedimentary succession shows generally high carbonate contents (average of 69%). The interval from 5,750 to 4,950 cal. BP, a part of the mound period, is characterized by a Holocene minimum in carbonate content (average of 57%) and by larger mean grain sizes. Comparing these values to those from a recent surface-sediment depth transect indicates that this was a period of rather low lake levels, which might have favored mound construction. Correlations to nearby archaeological sites and to the general West-Central European lake-level record indicates that the mounds likely were built during a short phase of low lake levels during a general trend of climatic cooling followed by a lake-level transgression.