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

Vertical mixing in Uberlingersee is studied by releasing sulfur hexafluoride (SF6) as a tracer at a central hypolimnic depth of 60 m and measuring its subsequent vertical dispersion over a period of three months. The experiment started with a streaky tracer injection of 1 liter gaseous SF6 (STP) in August 1990. At that time the lake showed a typical strong summer stratification which in a weakened form lasts until November. From the SF6 profiles of fifteen surveys at three sampling sites vertical diffusivitiesKz are calculated compensating internal seiche displacement and horizontal tracer loss. Except of the bottom region no sampling site or time period is marked by significant differences in the hypolimnicKz profile. So vertical mixing in the whole Uberlingersee is described by mean diffusivities decreasing from 1.7 cm2/s at 120 m depth to 0.4 cm2/s in 30 m. The minimal value of 0.3 cm2/s in the thermocline region at 20 m depth is only based on observations in autumn. For a strong summer stratification it is certainly lower. The gradient-flux-method for heat was applied to compute a meanKz (T) profile from continuously measured temperature profiles. Significant differences resulting from the two tracers showed, that theKz (T) values are underestimated by up to a factor of 5 if cooling by lateral exchange is neglected. Particularly, internal seiche pumping of colder water from the adjacent Lake Obersee over the separating sill of Mainau into the deep Uberlingersee basin is observed in 1990 from August onward, obviously controlling the heat budget below the sill level.

Vertical mixing in Überlingersee (Lake Constance) traced by SF6 and heat

Coupling between groundwater and surface water at Lake Willersinnweiher, a gravel pit lake in the Upper Rhine Graben without any surface in- or outflow, was investigated using both a groundwater model and the tracers 18O and SF6. Based on groundwater modeling, recharge and discharge areas around the lake as well as the residence time of the lake water were determined in a regional context. The uncertainty of the simulated flow field was assessed by sensitivity analysis. The tracers 18O and SF6 were measured in several observation wells and piezometers around the lake as well as in the lake’s water column. They were used to verify groundwater flow directions found in the modeling. We found that the groundwater-lake interaction model had a large uncertainty even though relatively detailed information on input data was available. Independent information obtained from the environmental tracers allowed us to improve and verify the model.

Coupling of groundwater and surface water at Lake Willersinnweiher : Groundwater modeling and tracer studies

Horizontal mixing processes in the hypolimnion of the western part of Lake Constance are studied by measuring the dispersion of a sulfur hexafluoride (SF6) tracer plume. Only 1 liter gaseous SF6 (STP) was released at a central hypolimnic depth of 60 m in August 1990. Over a period of 3 months the horizontal dispersion of the tracer plume was measured by 19 surveys using a new, vertically integrating sampling device. The observed horizontal dispersion is marked by strong storm-induced stirring events. Nevertheless mean turbulent diffusion coefficients for the whole period can be computed. They rise about linear from 0.7 ⋅ 105 cm2/s to 3.0 ⋅ 105 cm2/s with the distance from the western end of the lake. For the hypolimnion of Uberlingersee, a sill-separated basin in the western part of Lake Constance, a simple budget model gives an exchange time of 67 ± 6 days with the main basin (Obersee).

A SF6 tracer study of horizontal mixing in Lake Constance

Regular measurements—extending over about one year—of water temperature, of electric conductivity, and of the concentration of the stable isotope helium-3 are analyzed with regard to the lateral exchange of water between the main part of Lake Constance—the Obersee—and its smaller neighboring basin, the Uberlingersee. In the absence of a significant density stratification (as occurs in late winter and early spring), different wind speeds over the two basins cause different vertical mixing leading to horizontal concentration gradients between the basins. Wind-induced or density-driven horizontal currents are not strong enough to completely reduce the gradients. In contrast, during periods of strong vertical stratification (summer and autumn), internal waves extending along the whole lake cause periodical horizontal currents and mixing and thus lead to homogeneous concentration distributions along the horizontal axis. The seasonal variation of horizontal gradients couples the nutrient balance of Uberlingersee to the chemistry of the main lake.

Behavior of a Medium-Sized Basin Connected to a Large Lake

Vertical transport in a small eutrophic lake was investigated. The study site is a gravel pit lake in the Upper Rhine Graben near Ludwigshafen, Germany (800 × 250 m, mean depth 9 m, maximum depth 20 m). The governing mixing processes show patterns influenced by a strong density stratification. From weekly measured temperature profiles, we quantitatively estimated profiles of effective vertical transport coefficients K
 z
 . In the stratified regions, K
 z
 is at the level of heat conduction (~1.4·10-7 m2/s). Towards the lake bottom, we found a characteristic increase of K
 z
 to values up to 10−5 m2/s. Using a bottom-mounted 1,200 kHz-ADCP, we performed velocity measurements with a high vertical and temporal resolution (0.2 to 0.4 m and 5 min, respectively) of the horizontal currents. Using these horizontal flow data and stability profiles, Richardson numbers were analysed for a period of 5 weeks during summer stratification. The simultaneous measurements of currents and vertical diffusion coefficients allowed us to check a given parameterisation of K
 z
 (Ri) and to discuss its applicability to the investigated system.

Johann Ilmberger

Papers

Vertical mixing in Überlingersee (Lake Constance) traced by SF6 and heat

Coupling of groundwater and surface water at Lake Willersinnweiher : Groundwater modeling and tracer studies

A SF6 tracer study of horizontal mixing in Lake Constance

Behavior of a Medium-Sized Basin Connected to a Large Lake

Parameterisation of the vertical transport in a small thermally stratified lake