A 740,000-yr-long Mohawk Lake record, Mohawk Valley, northeastern California, USA

ABSTRACT Mohawk Valley lies in northeastern California on the margin of the northernmost Sierra Nevada and was occupied by Mohawk Lake during much of the middle and late Pleistocene. Throughout that time, the Sierra Nevada ice cap repeatedly extended northward into Mohawk Lake, and ice-contact deltaic sediments were deposited along the valley margins and in the valley bottom. Nearly 200 m of lacustrine and deltaic sediments are now well exposed along streams draining the Sierra Nevada. Tephra beds deposited within the deltaic sediments allow correlation of stratigraphic sections around the valley margin and, together with geomorphic evidence of former lake levels, permit interpretation of a Mohawk Lake history as far back as 740 ka. Mohawk Valley changed from a through-flowing fluvial setting to an intermittent closed basin sometime before 740 ka. After this change occurred, relatively small lakes intermittently formed in Mohawk Valley until ca. 600 ka, when the lake dramatically deepened. Mohawk Lake fluctuated in size over the next ~400,000 yr and increased in size to its highest levels after ca. 200 ka, possibly due to drainage integration with the upstream Lake Beckwourth. After this time, Mohawk Lake spilled over its westward sill, incrementally eroding and lowering lake levels until Mohawk Lake was emptied by ca. 7 ka.

A summary of 19th and early 20th Century researchers of glacial Lake Agassiz, North America: from Noah's Flood to Upham's bathtub and beyond

During the last North American deglaciation, meltwater collected along the margins of the Laurentide Ice Sheet in proglacial lakes, the largest of these being glacial Lake Agassiz, which existed for over five thousand years starting ~13,950 cal. years B.P. Lake Agassiz was first described in 1823 by mineralogist William H. Keating of the Long Expedition at a time when diluvianism was often used to explain ancient lakes. Subsequent researchers also recognized the existence of an ancient lake, but the first connections of this lake to a possible glacial source came in 1873. Starting in 1879, Warren Upham spent the next fifteen years researching and publishing on Lake Agassiz, eventually publishing his seminal work, the U.S. Geological Survey's Monograph 25 The Glacial Lake Agassiz. Some of Upham's interpretations were later challenged by William A. Johnston, who favored a more complex lake history.

Reconstructing Lake History Changes from Zooplankton Subfossil in the Sediment of Wuliangsuhai Lake, China

The sediment samples of zooplankton subfossil at different depths were collected from a large shallow lake Wuliangsuhai Lake, China, in order to reconstruct lake past changes. The lake began to switch to eutrophication gradually in the middle 1980s, creating a sediment discontinuity layer (about 32.5 cm depth) that can be visually used to separate sediments derived from plankton and macrophyte. Inferences about the timing and trajectory of eutrophication were made using sediment zooplankton subfossil reconstruction. The changes in composition of the zooplankton fragments were rather gradual up the core, whereas the total numbers of fragments show the most marked changes from 32.5 cm to present. Changes in the composition of zooplankton fragments (in 32.5 cm depth) show a gradual shift from a “pelagic community” dominated by B. longirostris, reflecting clearwater conditions with high predation pressure, to a more “benthic” community, dominated by plant associated Chydorids. The results suggest that it is possible to reconstruct past eutrophication trends of the lake by using zooplankton subfossil, and that anthropogenic pollutant loading is the key factor in the eutrophication of Lake Wuliangsuhai.

Long-Term Records of Continental Diatoms Paleolimnology and Biochronology

The great abundance and diversity of diatoms in lacustrine sediments and their ability to adjust to rapid changes in physical, chemical, and biological conditions makes them ideal for the study of lake history. Continuous diatom records from long-lived lakes have the potential to answer questions of basin history, climate variability, ecological change, and evolution. Isolated Tertiary outcrops provide a more limited record of environmental conditions, but as the ability to correlate individual exposures improves through the use of techniques such as tephrochronology, it is becoming possible to evaluate the timing of environmental or evolutionary changes on a regional basis.