| 24 Jun 2026
Spatial variability of Fe and Mn in surface lake sediments and its implications for paleoredox studies – a case study of Lake Łazduny (Poland)
Abstract. Numerous lakes worldwide are deteriorating due to climate change and other human impacts. Specifically, low dissolved oxygen levels are threatening food webs and water security. Protection, mitigation, and future projections of these phenomena call for a better understanding of their past evolution. For decades, paleolimnology has provided information about past environments by studying sediment structure and geochemistry. Among the latter, iron (Fe) and manganese (Mn), and their ratios are well-established proxies of the past water oxygenation. However, the understanding of redox-sensitive elements' mobility calls for a still scarce use of spatial approaches, complementing typical investigations focused on temporal geochemical variability. To address that, we began with 33-month-long observations of limnological conditions (water temperature and dissolved oxygen concentration) in a small, deep lake experiencing seasonal anoxia, continued with characterization of major sediment structures, and concluded with geochemical and statistical analyses of collected material. We used 31 surface samples from different depths and investigated their sediment structures, bulk geochemistry (CNS and biogenic silica), elemental composition (micro-X-ray fluorescence), and Fe and Mn fractions. Our data indicate clear, testable links between oxygen availability and sediment structures, as well as their chemical composition. Anoxia promotes the deposition and preservation of laminations. Whereas seasonally migrating oxycline drives geochemical focusing, enriching the deepest sediments in Fe and Mn. This proves that both Fe and Mn are reliable indicators of deep-water redox conditions. Our study bridges modern limnology and paleolimnology and emphasizes the need to treat lakes and their sediments as a complete, complex system.
Given the very widespread use of Fe, Mn and Fe/Mn as proxies for oxygen status in lake water, this article is timely and important. There are, however, two aspects by which the paper could be strengthened.
First, the paper uses the term geochemical focusing without defining the phrase or explaining how it differs from physical focusing mechanisms. A clear definition for the phrase, and discussion of how geochemical focusing ties in with the traditional hydrodynamic sorting concept would be extremely valuable.
Second, much is made of the distinction between laminated and homogeneous parts of the sediment. It seems to me that a convincing case can be made that this distinction arises purely from depth. The extraordinary scatter plots shown in figure 7 show a continuity of composition across the boundary between laminated and nonlaminated sediments, such that regression residuals would, I expect, show no distinction between laminated and unlaminated sediments. Could it not simply be that both element composition and sediment structure of consequences of depth, rather than composition being a consequence of lamination? I think the paper would be strengthened if these issues are elaborated more fully.
Some specific points of greater or lesser significance
Line 12. A more specific word than Numerous would strengthen this statement
Line 24. By “deposition” do you mean “formation”?
Lines 68 and 69. This surprises me. In the few lakes where I have done a similar investigation I found maximum manganese (up to 25%) to be in shallow sediment under fully oxygenated water and presumably migrated laterally from deeper oxygen depleted water. Are you seeing hints of this at the SE end of the lake?
Line 76. I'm sure there is a simple explanation, but at the location you give I find your lake but with the name Jezioro Błękitne. I looked on Google Earth simply because I struggle to believe that any lake in this temperate landscape could be endorheic. There must be out flowing water because 1) the landscape has a positive water balance, and 2) the lake is not hypersaline. And, your map Figure 1 shows an outflow stream! I think the word endorheic is being misapplied here.
Line 264. How can you rule out enrichment of organically associated Fe in deep water resulting from abiotic hydrodynamic size sorting? This would then correlate with biogenic silica, also enriched in deep water due to hydrodynamic sorting.