Paleoflooding refers to any flooding event that took place in pre-history, physical evidence of which exists in Earth's geologic record. The phenomenon is apparent over various spatial and temporal scales. It often occurred on a large scale, and was the result of either glacial ice melt causing large outbursts of freshwater, or high sea levels breaching bodies of freshwater. If a freshwater outflow event was large enough that the water reached the ocean system, it caused changes in salinity that potentially affected ocean circulation and global climate. Freshwater flows could also accumulate to form continental glacial lakes, and this is another indicator of large-scale flooding. In contrast, periods of high global sea level (often during interglacials) could cause marine water to breach natural dams and flow into bodies of freshwater. Changes in salinity of freshwater and marine bodies can be detected from the analysis of organisms that inhabited those bodies at a given time, as certain organisms are more suited to live in either fresh or saline conditions.
Paleoflooding resulting from glacial melt
Champlain Sea The Champlain Sea was a body of salt water that underwent several episodes of freshening. Two main events were observed 11.4 and 13.0 ka BP (thousand years Before Present). Faunal and foraminifera indicators in core samples taken from the sea can be used to estimate its salinity throughout time. The ages of various portions and depths of core samples are determined through radiocarbon dating. The Champlain Sea was located to the north of present-day New York and Vermont, on the southern fringes of Quebec and was open to the Northern Atlantic Ocean on its northeast arm. During the last deglaciation as the Laurentide Ice Sheet retreated, two major glacial lakes formed to the west of the Champlain Sea – Lake Agassiz and Lake Algonquin (Fig. 1). As these lakes continued to expand, freshwater flooded eastward toward and into the Champlain Sea. However, uncertainty still exists regarding the location of the drainage and its exact effects on ocean salinity. Because of the Champlain Sea's openness to the Atlantic Ocean, changes in the salinity of the Champlain Sea could have translated into the Northern Atlantic, thus possibly causing changes in ocean circulation and climate. In fact, the melting of the Laurentide Ice Sheet was so extensive that its melt water entered the Gulf of Mexico, Arctic Ocean, and Hudson Bay (Fig. 2) in addition to the Champlain Sea and the Atlantic Ocean. Terrestrial plant material, seeds, and marine shells from Champlain Sea core samples have been used as proxies for paleosalinity. By studying δ13C (change in carbon-13) of marine mollusks, it can be inferred that when they existed in the Champlain Sea, conditions were brackish (mixture of fresh and salt water) about 10.8 ka BP. The δ13C value of a core sample Melo-1 (see Fig. 3 for location) indicates the amount of light carbon that is present. Biota preferentially intake light carbon, so the more that is present in a sample, the more biota that was present at that time. Additionally, core samples from the Champlain Sea indicate a change in assemblages from those that inhabit marine environments to those that live in much less saline conditions at about 11.4 to 11.2 ka BP (Fig. 4). In the specific core sample (core Melo-5, location noted in Fig. 3) analysis shown in Fig. 4, there is a change in assemblages from almost 100% E. clavata (which inhabits marine environments) to >50% E. albiumbilicatum (which prefers less saline conditions)—both species of Elphidium. This transition seems to be probable, as it has been corroborated by multiple studies. The overall decrease in salinity has been estimated to be from 25 psu to less than 15 psu (practical salinity units). The decrease in salinity starting at Melo-1 and moving to Melo-5 indicates a downstream translation of freshening.
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