Climate, Hydrology, and Lake Sediment
Abstract
Components of lake sediment such as endogenic minerals, organic compounds, and fossils as well as chemistry and isotopes of those components are sources of information about past continental climate. No matter the size of the lake, the components of their sedimentary records reflect some integration of catchment processes operating on the landscape, within the ground water, and in the lake. Climate change influences and often drives change among these processes, yet the integration of the processes commonly yields unique records in lakes residing under the same climate. An estimate of the actual climate history from lake records as opposed to the apparent climate history from a given lake depends on how well we understand the modern system, or in the absence of a modern system, making use of known modern systems. We single out the effect of hydrology on lake records to illustrate the importance of understanding the modern system. The Waubay Lakes Chain in NE South Dakota is a series of lakes at different elevations and spill points that may be hydrologically connected, even coalescing, or may be isolated, though some remain hydrologically open (Niehus et al. 1999). Pickerel is an "upland" open freshwater lake whose level remains nearly constant, while Waubay is a shallow lake at intermediate elevation having more changes in level and salinity. Bitter, at the lowest elevation, receives spillover from Waubay during high water periods, exhibits wide ranges of salinity and elevation including going dry. Pickerel has a short residence time with lower δ18O and TDS than Waubay and Bitter (08/1991: -4.1 permil, -1.5 permil, and -1.8 permil; 204 mg/L, 6,420 mg/L, and 13,128mg/L respectively). The three lakes see the same climate, but their hydrologies lead to these different characteristics that interpreted in climate terms would suggest Pickerel resides under a wet climate while Bitter resides under a dry climate. The sediment records from these lakes though incompletely known appear to contain proxies that reflect the modern characteristics. Fossil ostracode species in Pickerel sediment (Schwalb & Dean, 1998) indicate that euryhaline species Fabaeformiscandona rawsoni dominanted during the mid-Holocene (8.4 to 6ka), while Candona ohioensis, a freshwater species was common at other times. In contrast, in the last 1000 years, F. rawsoni is common at Waubay and C. ohioensis only appears during high lake periods (Shapley et al., 2005). The ostracode fossil record from Bitter isn't known, but the modern assemblage is made up of Limnocythere staplini, a saline water ostracode, along with F. rawsoni, and other taxa. The modern day ostracode fauna in Bitter then implies a more saline environment than the entire Holocene record from Pickerel. True climate of the region is wetter than indicated by modern day Bitter and perhaps Waubay and drier than indicated by the modern Pickerel limnology and ostracode assemblages. Hydrology thus modifies the actual climate signal creating an apparent climate signal. Recognition of an approximation of an actual climate signal requires an understanding of how a given hydrology modifies the actual climate signal and if possible examination of records from several lakes each having a different hydrology. In the absence of an extant lake, ostracode assemblages offer insight into the hydrologic behavior of the past lake and so how its hydrology may have modified the climate signal. Niehus et al. (1999) USGS Water-Resources Investigations Report 99-4122, 166p. Schwalb & Dean (1998) Quaternary Science Reviews 21:1541-1554. Shapley et al. (2005) The Holocene 15:29-41
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMPP51C1523I
- Keywords:
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- 4914 Continental climate records;
- 4942 Limnology (0458;
- 1845;
- 4239);
- 4944 Micropaleontology (0459;
- 3030)