The trace metal content of recent organic carbon-rich sediments; implications for Cretaceous black shale formation

[1] Many lakes show vertical stratification of their water masses, at least for some extended time periods. Density differences in water bodies facilitate an evolution of chemical differences with many consequences for living organisms in lakes. Temperature and dissolved substances contribute to density differences in water. The atmosphere imposes a temperature signal on the lake surface. As a result, thermal stratification can be established during the warm season if a lake is sufficiently deep. On the contrary, during the cold period, surface cooling forces vertical circulation of water masses and removal of gradients of water properties. However, gradients of dissolved substances may be sustained for periods much longer than one annual cycle. Such lakes do not experience full overturns. Gradients may be a consequence of external inflows or groundwater seepage. In addition, photosynthesis at the lake surface and subsequent decomposition of organic material in the deeper layers of a lake can sustain a gradient of dissolved substances. Three more geochemical cycles, namely, calcite precipitation, iron cycle, and manganese cycle, are known for sustaining meromixis. A limited number of lakes do not experience a complete overturn because of pressure dependence of temperature of maximum density. Such lakes must be sufficiently deep and lie in the appropriate climate zone. Although these lakes are permanently stratified, deep waters are well ventilated, and chemical differences are small. Turbulent mixing and convective deep water renewal must be very effective. As a consequence, these lakes usually are not termed meromictic. Permanent stratification may also be created by episodic partial recharging of the deep water layer. This mechanism resembles the cycling of the ocean: horizontal gradients result from gradients at the surface, such as differential cooling or enhanced evaporation in adjacent shallow side bays. Dense water parcels can be formed which intrude the deep water layer. In the final section, stratification relevant physical properties, such as sound speed, hydrostatic pressure, electrical conductivity, and density, are discussed. The assumptions behind salinity, electrical conductance, potential density, and potential temperature are introduced. Finally, empirical and theoretical approaches for quantitative evaluation from easy to measure properties conclude this contribution.

https://www.eoas.ubc.ca/~mjelline/453website/eosc453/E_prints/newfer09/boehrer_stratlakeRG08.pdf

Stratification of lakes

Examination of amino acids in particulate samples from a variety of marine environments (fresh phytoplankton to deep-sea sediments) revealed systematic compositional changes upon progressive degradation. These consistent trends have been used to derive a quantitative degradation index (DI) that is directly related to the reactivity of the organic material, as indicated by its lability to enzymatic decay and its first-order degradation rate constant. This direct link between molecular composition and degradation rate allows us to quantify the quality of organic matter based solely on its chemical composition.

https://aslopubs.onlinelibrary.wiley.com/doi/epdf/10.4319/lo.1999.44.7.1809

Linking diagenetic alteration of amino acids and bulk organic matter reactivity

In contrast to most other fields of noble gas geochemistry that mostly regard atmospheric noble gases as ‘contamination,’ air-derived noble gases make up the far largest and hence most important contribution to the noble gas abundance in meteoric waters, such as lakes and ground waters. Atmospheric noble gases enter the meteoric water cycle by gas partitioning during air/water exchange with the atmosphere.

In lakes and oceans noble gases are exchanged with the free atmosphere at the surface of the open water body. In ground waters gases partition between the water phase and the soil air of the quasi-saturated zone, the transition between the unsaturated and the saturated zone. Extensive measurements have shown that noble gas concentrations of open waters agree well with the noble gas solubility equilibrium according to (free) air/(free) water partitioning, whereby the aquatic concentration is directly proportional to the respective atmospheric noble gas abundance (Henry law, Aeschbach-Hertig et al. 1999b).

In applications in lakes and ground waters the gas specific Henry coefficient can simplifying be assumed to depend only on temperature and salinity of the water. Hence the equilibrium concentrations of noble gases implicitly convey information on the physical properties of the water during gas exchange at the air/water interface, i.e., air pressure, temperature and salinity of the exchanging water mass. The ubiquitous presence of atmospheric noble gases in the meteoric water cycle defines a natural baseline, which masks other noble gas components until their abundance is sufficiently large that these components can be separated against the natural atmospheric background. For most classical geochemical aspects this typical feature of natural waters may look at first sight as a disadvantage. In fact it turns out to be advantageous because in most cases the noble gas abundance in water can be understood as a binary mixture of two …

/pdf/noble-gases-in-lakes-and-ground-waters-2o4xahnmm4.pdf

Noble Gases in Lakes and Ground Waters

. The present paper is the result of a workshop sponsored by the DFG Research Center/Cluster of Excellence MARUM "The Ocean in the Earth System", the International Graduate College EUROPROX, and the Alfred Wegener Institute for Polar and Marine Research. The workshop brought together specialists on organic matter degradation and on proxy-based environmental reconstruction. The paper deals with the main theme of the workshop, understanding the impact of selective degradation/preservation of organic matter (OM) in marine sediments on the interpretation of the fossil record. Special attention is paid to (A) the influence of the molecular composition of OM in relation to the biological and physical depositional environment, including new methods for determining complex organic biomolecules, (B) the impact of selective OM preservation on the interpretation of proxies for marine palaeoceanographic and palaeoclimatic reconstruction, and (C) past marine productivity and selective preservation in sediments. It appears that most of the factors influencing OM preservation have been identified, but many of the mechanisms by which they operate are partly, or even fragmentarily, understood. Some factors have not even been taken carefully into consideration. This incomplete understanding of OM breakdown hampers proper assessment of the present and past carbon cycle as well as the interpretation of OM based proxies and proxies affected by OM breakdown. To arrive at better proxy-based reconstructions "deformation functions" are needed, taking into account the transport and diagenesis-related molecular and atomic modifications following proxy formation. Some emerging proxies for OM degradation may shed light on such deformation functions. The use of palynomorph concentrations and selective changes in assemblage composition as models for production and preservation of OM may correct for bias due to selective degradation. Such quantitative assessment of OM degradation may lead to more accurate reconstruction of past productivity and bottom water oxygenation. Given the cost and effort associated with programs to recover sediment cores for paleoclimatological studies, as well as with generating proxy records, it would seem wise to develop a detailed sedimentological and diagenetic context for interpretation of these records. With respect to the latter, parallel acquisition of data that inform on the fidelity of the proxy signatures and reveal potential diagenetic biases would be of clear value.

/pdf/selective-preservation-of-organic-matter-in-marine-3u1ry40ph3.pdf

Selective preservation of organic matter in marine environments; processes and impact on the sedimentary record

Why some mediterranean sapropels survived burn-down (and others did not)

. Due to its high activities in groundwater, the radionuclide 222Rn is a sensitive natural tracer to detect and quantify groundwater inflow into lakes, provided the comparatively low activities in the lakes can be measured accurately. Here we present a simple method for radon measurements in the low-level range down to 3 Bq m−3, appropriate for groundwater-influenced lakes, together with a concept to derive inflow rates from the radon budget in lakes. The analytical method is based on a commercially available radon detector and combines the advantages of established procedures with regard to efficient sampling and sensitive analysis. Large volume (12 l) water samples are taken in the field and analyzed in the laboratory by equilibration with a closed air loop and alpha spectrometry of radon in the gas phase. After successful laboratory tests, the method has been applied to a small dredging lake without surface in- or outflow in order to estimate the groundwater contribution to the hydrological budget. The inflow rate calculated from a 222Rn balance for the lake is around 530 m³ per day, which is comparable to the results of previous studies. In addition to the inflow rate, the vertical and horizontal radon distribution in the lake provides information on the spatial distribution of groundwater inflow to the lake. The simple measurement and sampling technique encourages further use of radon to examine groundwater-lake water interaction.

/pdf/tracing-and-quantifying-groundwater-inflow-into-lakes-using-3cyghbyi8k.pdf

Tracing and quantifying groundwater inflow into lakes using a simple method for radon-222 analysis

This paper describes a tracer experiment with sulfurhexafluoride (SF6) in the monimolimnion of the meromictic mining lake Merseburg-Ost 1b. In October 1998, 1.1 mmol ( 160 mg) of the conservative gas SF6 was released at the site of greatest depth 3.5 m above the sediment to observe its vertical spreading. An easy-to-use system to collect ∼0.5 L water samples using evacuated flasks was developed. The headspace technique, gas chromatographic separation and ECD-detection were used to determine SF6. The main objective is the evaluation of mean vertical diffusion coefficients for SF6 and heat from periodically measured SF6 and CTD profiles. During the study period of 11/2 years, the heat transport was estimated to be molecular in the stratified portion. A thermal flux from the sediments of 0.23 W/m2 was found necessary to balance the heat. In the region of high stability (N2∼10-2 s-2) diffusivities for SF6 were ∼ 10-8 m2/s, whereas in the lower part of the monimolimnion both tracers resulted in K∼ 7 · 10-6 m2/s. We found K to be approximately proportional to N-2.4 ± 0.2.

Tracer experiment with sulfur hexafluoride to quantify the vertical transport in a meromictic pit lake

Localising and quantifying groundwater inflow into lakes using high-precision 222Rn profiles

[1] From a small meromictic lake, we present observations of a circulation pattern that has not been documented in the limnological literature before. While surface cooling drives a vertical circulation of the upper water layer (mixolimnion), the deeper water layer (monimolimnion) is not included because of its higher salt concentration. However, double diffusion (higher diffusivity of heat than of dissolved substances) facilitates the faster escape of heat from the monimolimnion compared to dissolved substances during cold periods. As a consequence, interfacial cooling drives a vertical circulation within the monimolimnion without breaking the stratification toward the mixolimnion. In the presented case, the geochemical setting does not permit dissolved substances to accumulate in the mixolimnion. As a consequence, the system approaches the case of two immiscible layers in thermal contact. Below the interface, a convection layer is formed that exceeds the staircase layer thickness of double diffusion when conservative salts are involved. Finally, the entire lake circulates as two separate convection layers. This has a decisive impact on the formation of gradients and the redistribution of dissolved substances in lakes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009GC002389

Johann Ilmberger

Papers

Why some mediterranean sapropels survived burn-down (and others did not)

Tracing and quantifying groundwater inflow into lakes using a simple method for radon-222 analysis

Tracer experiment with sulfur hexafluoride to quantify the vertical transport in a meromictic pit lake

Localising and quantifying groundwater inflow into lakes using high-precision 222Rn profiles

Double-diffusive deep water circulation in an iron-meromictic lake