Sediment diagenesis, geochemical cycles, and budgets in Lake Superior

This work characterizes the sediment geochemistry of Lake Superior, the world's largest freshwater lake by area and one of the most oligotrophic freshwater systems. The work covered 13 locations in the lake with multi-year, multi-season field surveys, quantified diagenetic rates and fluxes (of C, N, P, Fe, and S), investigate their temporal and spatial heterogeneity, and develop quantitative mechanistic understandings that can be applied across systems and scales.

This work has generated several exciting findings including 1) Deep oxygen penetration and strong oxygen dynamics in carbon-poor sediments responding to bottom oxygen levels and carbon fluxes, 2) nitrogen effluxes and low denitrification rates, explaining the accumulation of nitrate in the water column of Lake Superior, 3) Anammox (anaerobic ammonium oxidation) contributing significantly to nitrogen loss in the lake, 4) organic sulfur supporting sulfate reduction in low-sulfate environments, such as those in ancient Oceans, and 5) phosphorus flux in deeply oxygenated sediments controlled by organic matter mineralization.



Publications: - Li, J., Y. Zhang, and S. Katsev. 2018. Phosphorous recycling in deeply oxygenated sediments in Lake Superior controlled by organic matter mineralization. Limnology and Oceanography. 63: 1372- 1385 (link)- Fakhraee, M., J. Li and S. Katsev. 2017. Significant role of organic sulfur in supporting sedimentary sulfate reduction in low-sulfate environments. Geochimica et Cosmochimica Acta. 213: 502-516 (link)- Crowe, S. A., A. H. Treusch, M. Forth, J. Li, C. Magen, D. E. Canfield, B. Thamdrup, S. Katsev. 2017. Novel anammox bacteria and nitrogen loss from Lake Superior. Scientific Reports. 7: 13757 (PDF)- Li, J., and S. Katsev. 2014. Nitrogen cycling in deeply oxygenated sediments: Results in Lake Superior and implication to marine sediments. Limnol. Oceanogr. 59 (2): 465–481 (PDF) - Li, J. S. A. Crowe, D. Miklesh, M. Kistner, D. E. Canfield, and S. Katsev. 2012. Carbon mineralization and oxygen dynamics in sediments with deep oxygen penetration, Lake Superior. Limnol. Oceanogr. 57:1634-1650 (PDF)

Strong geographic variability in this "freshwater sea"

Sediments in Lake Superior exhibit strong lateral variability in their visual appearance, as well as vertical positions of characteristic diagenetic layers (Fig. 1)

Fig. 1 Optical images of Lake Superior sediment cores

Deep oxygen penetration and strong oxygen dynamics

Oxygen penetration in the sediments of Lake Superior are extremely deep. Whereas in most lakes and coastal ocean oxygen penetrates into sediments only by several mm, in Lake Superior we typically measure oxygen penetration of 3-12 cm in the offshore locations, and at one location even 27 cm! This is unusual for lakes, but typical for the deep ocean.

These sediments with deep penetration of oxygen are particularly sensitive to changes in external conditions such as bottom oxygen levels and organic carbon fluxes. The observed changes in oxygen penetration in Lake Superior include seasonal variability of up to 2 cm (Fig. 2), as well as decadal changes of a up to 6 cm, indicated by the multiple iron layers mismatching the present depth of oxygen penetrations (Fig 3).

Fluctuations in the depth of the oxygen penetration shift the sediment oxic/anoxic boundary, and affect the dynamics of biogeochemical reactions in the sediments and the nutrient fluxes across the sediment-water interface.

Fig. 2 Vertical distributions of dissolved oxygen in sediments of Lake Superior.
Fig. 3 The highly dynamic lake-bottom environment leads to frequent migrations in the sediment oxic-anoxic boundary and is recorded in priminent Fe-rich layers that are ubiquitous in Lake Superior (also see Fig. 1).

Nitrogen cycling in sediments explains the enigmatic nitrate rise in the lake

As organic materials are mineralized in the sediments, ammonium is released and increase with depth. In the oxic zone, NH4 can be oxidized by oxygen to produce NO3, a process call nitrification. In the anoxic zone where there is no oxygen, denitrification or anammox start to occur, producing dinitrogen (N2). The deep oxygen penetration in the offshore sediments in Lake Superior, which covers ~ 80% of the lake floor, leads to low N2 production rates and net nitrate fluxes from the sediments into the water column (Fig. 4).

Accounting for both the nearshore and offshore sediments, we construct nitrogen budget for the lake (Fig. 5), which suggest important role of sediments: sediments remove > 75% of nitrogen (20% by burial and 55% by N2 production); sediment contribute 77% of nitrate input into the water column.

The large nitrate fluxes from sediments explain the enigmatic rise of nitrate in the water column for the last century (Fig. 6).

Fig. 4 Typical profiles of O2, NO3 and NH4 in Lake Superior sediments, offshore (left) vs nearshore (right). Offshore sediments have deep oxygen penetration while nearshore sediments have shallow oxygen penetration. This leads to major differences in the sediments: deep vs shallow nitration penetration, low and high denitrification rates, NO3 fluxes out vs in.
Fig. 5 Nitrogen budget in Lake Superior suggests important role of sediments.
Fig. 6 Nitrate in the water column of Lake Superior has increased for nearly 5 fold for the last century.

Relationships developed in Lake Superior sediment predict nitrogen removal rates in abyssal ocean

The total oxygen uptake (TOU) was previously found to correlate linearly with the denitrification rates in the continental slope and shelf sediments (Fig. 7). The relationship is reasonable when denitrification is limited by the supply of nitrate, a product of nitrification that consumes oxygen. However, in carbon-poor sediments as those in offshore Lake Superior, we found the relationship being stronger than linear (Fig. 7). This is because denitrification is regulated by both nitrate and the amount and reactivity of organic carbon, affected by the deep oxygen penetration non-linearly. Whereas the nearshore sediments in Lake Superior conform to the same relationship as marine coastal sediments, the deeply oxygenated sediments are characterized by significantly lower denitrification rates.

Denitrification rates strongly depend on the depth of oxygen penetration (Fig. 8A). The deeply oxygenated abyssal sediments in the Ocean may not conform to the same relationships as coastal and continental shelf sediments (Fig.7). Using the relationship we developed based on Lake Superior sediments, we estimate the rates of denitrification in deep Ocean sediments (Fig. 8B).

Fig. 7 Denitrification rates vs. sediment oxygen uptake. Whereas the nearshore sediments in Lake Superior conform to the same relationship as marine coastal sediments, the deeply oxygenated sediments are characterized by significantly lower denitrification rates.
Fig. 8 (A) The rates of reactive nitrogen removal vs. oxygen penetration; (B) Denitrification rates vs. water depth. The relationship developed in Lake Superior well predicts the rates of denitrification in ocean sediments.

Oxygen controls the fluxes of phosphorus

In contrast to organic rich systems where P effluxes are sensitive to redox conditions, phosphate effluxes in organic-poor well-oxygenated Lake Superior are controlled by the rates of organic phosphorus mineralization, similar to marine sediments (Fig. 9). Similar behavior should be expected in other well-oxygenated freshwater systems, such as other large oligotrophic lakes. The efficiency of P recycling in Lake Superior sediment (12% of deposited P is returned to the water column), however, is substantially lower than in marine sediments.

We construct a P budget for the Lake. While burial into sediments is the dominant sink for P in the lake, sediments still contribute up to 40% of total water column P inputs. The current P budget for the lake appears imbalanced, with P sinks significantly exceeding sources, indicating the need to reconcile P loading estimates from different sources. Phosphorus inputs from shoreline erosion is largely unquantified. There may have been a significant and underappreciated role of sediment resuspension that enhances P remobilization from sediments beyond diffusive and bioirrigation fluxes.

Fig. 9 Flux of PO4 out of sediment vs. total oxygen uptake
Fig. 10 Sources and sinks of phosphorus in Lake Superior (umol m-2 d-1). Sediment recycles 12% of P deposited, while 88% of P sedimentation is permanently buried.