Elevated δ15N Linked to Inhibited Nitrification Coupled to Ammonia Volatilization in Sediments of Shallow Alkaline-Hypersaline Lakes

Abstract

Alkaline lakes are among the most bioproductive aquatic ecosystems on Earth. The factors that ultimately limit productivity in these systems can vary, but nitrogen (N) cycling in particular has been shown to be adversely affected by high salinity, evidently due to the inhibition of nitrifying bacteria (i.e., those that convert ammonic species to nitrogen oxides). The coastal plain of Coorong National Park in South Australia, which hosts several alkaline lakes along 130 km of coastline, provides an ideal natural laboratory for examining how fine-scale differences in the geochemistry of such environments can lead to broad variations in nitrogen cycling through time, as manifest in sedimentary δ¹⁵N. Moreover, the lakes provide a gradient of aqueous conditions that allows us to assess the effects of pH, salinity, and carbonate chemistry on the sedimentary record. We report a wide range of δ¹⁵N values (3.8‰-18.6‰) measured in the sediments (0-35 cm depth) of five lakes of the Coorong region. Additional data include major element abundances, carbonate δ¹³C and δ¹⁸O values, and the results of principal component analyses. Stable nitrogen isotopes and wt% sodium (Na) display positive correlation (R² = 0.59, p < 0.001) across all lake systems. Principal component analyses further support the notion that salinity has historically impacted nitrogen cycling. We propose that the inhibition of nitrification at elevated salinity may lead to the accumulation of ammonic species, which, when exposed to the water column, are prone to ammonia volatilization facilitated by intervals of elevated pH. This process is accompanied by a significant isotope fractionation effect, isotopically enriching the nitrogen that remains in the lake water. This nitrogen is eventually buried in the sediments, preserving a record of these combined processes. Analogous enrichments in the rock record may provide important constraints on past chemical conditions and their associated microbial ecologies. Specifically, ancient terrestrial aquatic systems with high δ¹⁵N values attributed to denitrification and thus oxygen deficiency may warrant re-evaluation within the framework of this alternative. Constraints on pH as provided by elevated δ¹⁵N via ammonia volatilization may also inform critical aspects of closed-basin paleoenvironments and their suitability for a de novo origin of life.

FIGURE 1

Geographical context for the studied portion of the Coorong region, including the locations of all field sites in this study. Adapted from Wright (1999).

FIGURE 2

Correlation plots for the 13 different geochemical parameters measured in this study. Only sample depths > 4 cm are included in this plot; this filter tends to increase Pearson correlation coefficients (r) because it may serve to eliminate inconsistencies caused by high microbial respiration rates and/or rainwater dilution near the surface.

FIGURE 3

Geochemical trends (δ¹⁵N, TOC, and TIC) with depth at all study sites. Individual sites are distinguished by color‐coding. There is high intersystem variability in δ¹⁵N. The North Lagoon site (black) functions as an endmember because it is not a lake and is fed directly by the open ocean.

FIGURE 4

Scatterplots of δ¹⁵N data versus Na and Sr, from depths > 4 cm. There is a statistically moderate relationship between both δ¹⁵N versus Na (wt%) and δ¹⁵N versus Sr (wt%). There is also an apparent threshold in the δ¹⁵N versus Sr (wt%) data, where 96% of samples with δ¹⁵N > 11‰ also have Sr > 0.85 wt%.

FIGURE 5

(Top) Scatterplots of δ¹⁸O_carb versus δ¹³C_carb and δ¹⁸O_carb versus Na wt%, from depths > 4 cm. (Bottom) Coefficients of determination for the variables of interest at each sample site, as well as across all lakes. (a, b) Halite Lake and North Lagoon display a statistically strong relationship in δ¹⁸O_carb versus δ¹³C_carb space, indicating that Halite Lake may be evolved from a seawater source. This is in agreement with previous sedimentological and mineralogical evidence (Warren 1990).

FIGURE 6

A stepwise series (a–d) of principal component analyses (PCAs). Color‐coded ellipses that represent a 95% confidence interval. The values placed above and below the x‐ and y‐axes—each associated with a specific variable—are loading scores. Loading scores are equivalent to the r value of a given variable versus its corresponding principal component value; they serve as a general indicator of which variables have the strongest influence on the position of a given data point in principal component space. The rationale and significance of each PCA are described in Section 5.4.

FIGURE 7

A proposed schematic of hypersaline nitrogen cycling that results in elevated δ¹⁵N. The mechanism is described in detail in Section 5.5 and is broadly adapted from Isaji et al. (2019), although there are several key differences. These differences include a holistic consideration of near‐surface organic matter degradation, cation substitution in clays, and the recognition of a possible Sr toxicity threshold.