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Project : Models for carbon and oxygen isotope trends of Holocene carbonate lacustrine sequences form the northern Great Plains, EE UU Period: 1995- 1999. Funding: Petroleum Research Fund (EE UU) Investigators: Kerry Kelts (LRC, U of MN) and Blas Valero Garcés (IPE) Sites: Moon Lake and Medicine Lake
Medicine Lake (South Dakota, USA) Bibliography Valero-Garcés, B.L., Laird, K., Fritz, S., Kelts, K., Ito, E. & Grimm, E. 1997. Holocene climate in the northern Great Plains inferred from sediment stratigraphy, stable isotopes, carbonate geochemistry, diatoms, and pollen at Moon Lake, North Dakota. Quaternary Research, 48: 359-369. ABSTRACT. Seismic stratigraphy, sedimentary facies pollen stratigraphy, diatom-inferred salinity, stable isotope , stable isotope ( d18O and d13C), and chemical composition (Sr/Ca and Mg/Ca) of authigenic carbonates from Moon Lake cores (North Dakota, USA) provide a congruent Holocene record of effective moisture for the eastern Northern Great Plains. Between 11,700 14C yr B.P. and 9500 14C yr B.P., the climate was cool and moist. A gradual decrease in effective moisture occurred between 9500 and 7100 yr B.P. A change at about 7400 14C yr B.P. inaugurated the most arid period during the Holocene. Between 7100 and 4000 14C yr B.P., three arid phases occurred at 6600 - 6200 14C yr B.P., 5400 - 5200 14C yr B.P., and 4800 - 4600 14C yr B.P.). Effective moisture generally increased after 4000 14C yr B.P., but periods of low effective moisture occurred at 2900- 2800 14C yr B.P., and 1200 - 800 14C yr B.P. The data also suggest high climatic variability during the last few centuries. Despite the overall congruence, the biological (diatom), sedimentological, isotopic and chemical proxies were occasionally out of phase. At these times the evaporative process was not the only control of lake-water chemical and isotopic composition.Figure 1: The Moon Lake record. Valero Garcés, B.L., Kelts K. & Ito E.,1995. Oxygen and Carbon Isotope Trends and Sedimentological Evolution of a Meromictic and Saline Lacustrine System: The Holocene Medicine Lake Basin, North American Great Plains, USA. Palaeogeography, Palaeoclimatology, Palaeoecology. 117: 253 - 278. ABSTRACT. The succession of sedimentary subfacies in the Medicine Lake Basin (South Dakota, USA) documents numerous changes in total dissolved solids (TDS), lake level, and the nature and stability of meromixis during the Holocene. The lake formed about 10.6 ka BP when coarse clastics and massive fine-grained muds were deposited in a well-mixed freshwater system. Subsequently the lake became saline and meromictic, depositing a finely laminated unit composed of variegated and gray muds, organic facies, gypsum and aragonite laminae. Phototrophic bacterial plate communities (BPC) at the chemocline were significant producers of pelagic organic matter. Relatively high lake levels (at least 10 m higher than today) but saline and meromictic conditions predominated through the early to mid Holocene. At the end of this phase, lake level dropped and stable meromixis was disrupted. Sands and massive or banded muds were deposited in sublittoral areas and benthic microbial communities (BMC) spread over the lake, replacing BPCs as the predominant microbial producers. When lake level rose again, massive and laminated units were deposited under less saline conditions. Another very pronounced episode of low lake level and high salinity occurred during the late Holocene. The most recent sediments are sapropels, massive or banded muds, and littoral sands. The d18O record from authigenic aragonite shows more positive values during the arid mid-Holocene interval, with an abrupt increase at the end of this period, followed by generally more negative values during the cooler and wetter late Holocene. The small range of d 18O values (3 ‰) suggests that the lake was well buffered against high-frequency climatic cycles, probably as a result of groundwater modulation. We postulate that meromixis and the type of photosynthetic microbial communities dominating the basinal region of the lake system - either BPCs or BMCs- exerted major controls on the carbon cycle, and consequently, on the development of isotopic trends linking d 13C and d 18O values. More negative d 13C values during the early and mid Holocene are interpreted as a consequence of the predominance of 12C-enriched microbial carbon from BPCs.Examples of both covariant and inverse covariant isotope trends between d 13C and d 18O values occur in the Medicine Lake record. Inverse covariant trends characterize long periods of stable meromixis when BPCs predominate over BMCs. We hypothesize that well-developed water stratification caused by greater influx of relatively 18O-depleted freshwater induces higher productivity at the chemocline and, consequently, more positive d 13C values in the precipitated carbonates. Unstable phases of meromixis or holomictic conditions are characterized by a predominance of BMCs or other biogenic producers over BPCs, and a number of other factors combine to promote covariant trends.The Medicine Lake record is consistent with the paleoclimatic interpretation of a more arid early and middle Holocene followed by an abrupt transition to a relatively cooler and wetter late Holocene. The occurrence of a pronounced dry phase within the generally wet late Holocene was previously documented in other Northern Great Plains sites, and supports regional scale correlations. The general trend toward wetter and cooler conditions in the late Holocene was punctuated by more arid episodes. The sedimentary succession in Medicine Lake documents a much more fluctuating and abrupt climate behavior than suggested by the vegetation paleorecord. Valero Garcés, B.L.& Kelts, K.,1995. A sedimentary facies model for perennial and meromictic saline lakes: Holocene Medicine Lake Basin, South Dakota, USA.Journal of Paleolimnology., 14: 123 - 149. ABSTRACT. Medicine lake is a small (about 1 km2), shallow (up to 10 m deep), saline (50 - 170 g l-1) and meromictic lake formed after the retreat of the Wisconsin ice in the North American Great Plains. Based on a detailed sedimentological analysis of cores, we describe and interpret 13 sedimentary subfacies grouped in 9 associations which characterize the following lacustrine subenvironments: clastic littoral (freshwater and saline), springs, microbial mats, bench slope, and pelagial (oxic, alternating oxic-anoxic, anoxic and hypersaline, and organic-dominated). Lateral distribution and vertical evolution of subfacies in our model are controlled by climate fluctuations, climate-related limnological parameters (lake level, TDS and brine composition, and redox conditions), and autocyclic processes (progressive infilling of the basin and higher sedimentation rate in the pelagial realm). Microbial and chemical processes govern deposition in this system, and meromixis plays a decisive role in lake dynamics. Phototropic bacterial plate communites at the chemocline dominated as pelagial organic producers during stable meromictic periods, whereas benthic microbial communities developed during mixed water periods. Water stratification during the Holocene was mainly controlled by three parameters:1) basin morphometry, 2) lake level, and 3) differences in TDS values between mixolimnion and monimolimnion waters. Sedimentary facies analysis is a powerful descriptive and interpretative tool that greatly contributes to deciphering the high resolution paleoenvironmental information archived in lake sequences. Depositional and paleoenvironmental models provide a dynamic framework for integrating paleolimnological data and other proxy paleorecords. Medicine lake serves as a facies model for shallow, perennial hypersaline, meromictic lakes in modern and ancient lacustrine basins. The sediment sequence from Medicine lake cores is consistent with the general paleoclimatic evolution of the northern Great Plains since the retreat of ice sheets. Our study reveals a plethora of rapid fluctuations in the water cycle both during the middle and the late Holocene. These augment prior paleoclimate reconstructions based on diatom studies of the lower Holocene freshwater to saline transition and on pollen profiles which show little variability during the subsequent long prairie grass episode.
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