Redox controls on solute transfer between shallow sediments and bottom-waters: Chemical and isotopic evidence from two Antarctic Dry Valley Lakes
Abstract
The water chemistry studies of the McMurdo Dry Valley Lakes of Antarctica indicate a complex history of recent and past climate fluctuations. During warmer periods, the reduction in ice-cover results in augmented productivity and deposition of organic matter, coupled with a greater influx of oxic waters due to ice-melt. During cooler periods, the ice cover increases, and the oxidation of organic matter on the lake bottoms results in euxinic conditions. In this investigation, we present chemical and isotopic evidence that interstitial waters in sediments and bottom-water chemistry of Lake Fryxell and Lake Joyce are influenced by these changes. At Lake Fryxell, the deep-waters are characterized by 3He/4He and 36Cl/Cl ratios that indicate mixing between radiogenically derived 4He and 36Cl from basement rocks and He and Cl from meteoric sources. In contrast, Lake Joyce presents 3He/4He ratios which are and order of magnitude lower, while the 36Cl/Cl ratios are a order of magnitude higher. Both values indicate some limitations in the exchange between deep waters and oxic surface waters. The waters also become more depleted in deuterium and O-18 with increasing depth. Iodine bottom-water concentrations in both Lake Fryxell and Lake Joyce are roughly an order or magnitude less than the sediment pore waters directly below them. Bottom sediments in both lakes have most of the solid- phase iodine bound to it in the upper 5cm. Upon deeper burial, the sediment-bound iodine appears to be released into deeper pore waters and then rescavenged by the uppermost sediments. In both lakes, oxidation of organic matter, coupled with sulfate reduction, is consistent with the observed iodine concentrations. In the Lake Fryxell sediments, however, the iodine is less effectively retained in the upper sediments than in Lake Joyce, and seeps back into the water column. Interestingly, both the deepest waters of Lakes Joyce and Lake Fryxell have high 129I/I ratios suggesting that anthropogenic 129I may have been scavenged by organic matter at the lake surface, then settled to the lake bottom and was released into the deep waters. Apart from this anomaly, 129I/I ratios decrease with depth. Sediments host 129I/I ratios which are significantly lower than those of the lake water, indicate the either the scavenging of iodine has persisted in these lakes for extended periods of time or that there is a deep subsurface brine source of iodine. The presence and isotopic composition of iodine in the lake waters and shallow sediments is consistent with diagenesis and oxidation of organic matter in sediments. It also appears to be indirectly coupled to non-reactive lake water constituents such as helium and chloride. The melt-water input and ice cover changes which influence 36Cl/Cl and 3He/4He ratios also ultimately determine the oxidation state of the lake waters and the immediate subsurface.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.H14D..07S
- Keywords:
-
- 0746 Lakes (9345);
- 0793 Biogeochemistry (0412;
- 0414;
- 1615;
- 4805;
- 4912);
- 1806 Chemistry of fresh water;
- 1830 Groundwater/surface water interaction