In the Weeds: Aquatic Plant Biomarker Sources to Arctic Lake Sediments

In reconstructing past changes in precipitation or evaporation from the geologic record, paleoclimatologists sometimes employ the dual‐biomarker isotope method. This method requires that two co‐occurring sedimentary lipids are derived from different biological sources, and that their compound‐specific 2H/1H ratios record different aspects of the hydrologic cycle. Several studies have used this approach, typically by comparing the 2H/1H ratios of n‐alkyl lipids (δ2Hwax) thought to be sourced from aquatic versus terrestrial plants that gain their hydrogen atoms from lake water and soil water, respectively. Yet confidently fingerprinting n‐alkyl lipid sources continues to be a challenge because the wax inputs of different plant types vary across biomes, lake types, and time. New research in the Journal of Geophysical Research‐Biogeosciences by Hollister et al. (2022, https://doi.org/10.1029/2022jg006903) utilizes a combination of three independent metrics to demonstrate that for Arctic lakes, mid‐chain n‐alkanoic acids can be sourced principally from aquatic plants whereas long‐chain n‐alkanoic acids derive from a mixture of aquatic and terrestrial plants. By cataloging wax compound distributions and compound‐specific H and C isotopes of many new plant species, their efforts will strengthen future biomarker paleoclimatology and reinforce previous applications of the dual‐biomarker approach in high‐latitude lakes. The identification of a lake system with a strong aquatic plant wax signal in the sediments should motivate future targeting of similar lakes for reconstructing past moisture with the dual‐biomarker method.


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The dual-biomarker model usually assumes that the paired biomarkers are synthesized by terrestrial and aquatic plants. This is an important point of uncertainty, as sedimentary lipids represent a complex mixture of biologic sources (Gao et al., 2011;Peaple et al., 2021;Yang & Bowen, 2022). Ideally, the target biomarkers would be specific to a single plant or algal taxa, such as diols (Romero-Viana et al., 2013), alkenones (Nelson & Sachs, 2014a), dinosterol (Nelson & Sachs, 2014b), or highly branched isoprenoids (Corcoran et al., 2020). These biomarkers, however, are not always present in lake sediments, exist at too low a concentration for hydrogen isotope analysis, or simply require further calibration research in lakes (e.g., Longo et al., 2016). Most studies therefore represent δ 2 H AQ with mid-chain length n-alkane or n-alkanoic acid (n-acids) homologs and δ 2 H TERR with long-chain homologs, which are found universally in lake sediments. Some studies in the Arctic, however, challenge the outright use of mid-chain compounds (e.g., C 21 and C 23 n-alkanes; C 22 and C 24 n-acids) to represent the aquatic end member due to the potential for non-aquatic sourcing (Dion-Kirschner et al., 2020;McFarlin et al., 2019). Elucidating the sources of mid-chain compounds in Arctic lake systems is the focus of the recent study by Hollister et al. (2022).

Fingerprinting n-Alkyl Sources to Arctic Lakes
The production of mid-chain n-alkanes and n-acids by aquatic plants and moss is well-established (Bush & McInerney, 2013;Ficken et al., 2000;Gao et al., 2011;Nichols et al., 2006), including for Arctic lakes (Gorbey et al., 2021;Kjellman et al., 2020;Thomas et al., 2016). Yet, as the database of Arctic plant wax distributions grows, it is becoming apparent that many land plants also make mid-chain compounds (Berke et al., 2019;Daniels et al., 2018;Dion-Kirschner et al., 2020;Gorbey et al., 2021;Kjellman et al., 2020;Wilkie et al., 2013). At Little Sugarloaf Lake in SW Greenland, Dion-Kirschner et al. (2020) found that the concentration of n-alkyl lipids was up to 30 times higher in terrestrial plants than in aquatic plants, and as a result, the terrestrial inputs of mid-chain n-alkanes and n-acids swamp out aquatic inputs to lake sediments. Moreover, McFarlin et al. (2019) report that δ 2 H of mid-chain compounds correlates poorly with δ 2 H of lake water across a suite of lakes in Greenland.
To further fingerprint the sources of n-alkyl lipids in Arctic lakes, Hollister et al. (2022) traveled to Qaupat Lake on Baffin Island, Canada. The setting is in the low Artic-a landscape characterized by low shrub tundra and spotted with small lakes. There, the authors cataloged n-alkane and n-acid distributions, compound-specific δ 2 H and compound-specific δ 13 C for many terrestrial and aquatic plants, soils, suspended particulate matter, and lake surface sediments. In the article, they determined that in Qaupat Lake, the mid-chain n-alkanes and n-acids are largely derived from submerged aquatic vegetation (SAV), particularly the submerged mosses. Interestingly, the sedimentary n-alkyl distributions alone are insufficient to partition plant sources because the submerged aquatic vegetation and the abundant willow shrubs have similar compound distributions. By incorporating δ 2 H wax and δ 13 C wax into the analysis, they were able to distinguish the plant sources more clearly. Hollister et al. (2022) combine their geochemical results with vegetation mapping in and around the lake to gain a mechanistic understanding of the sources, transport, and deposition of the plant waxes. The authors propose that a combination of factors explain the aquatic origin of mid-chain n-alkyl compounds at Qaupat Lake ( Figure 1). These include (a) the presence of abundant SAV growing in the littoral-benthic habitat, (b) a short transport distance of aquatic plant material to the deposition site, (c) low terrestrial primary productivity in the catchment, and (d) low-relief topography and limited rainfall which together limit erosion of waxes from the soil. These mechanisms may explain the contrasting findings from Qaupat Lake (Hollister et al., 2022) and Little Sugarloaf Lake in Western Greenland (Dion-Kirschner et al., 2020), which are in similar bioclimate zones but have some differences in catchment relief, precipitation, and terrestrial productivity.

Future Opportunities
This new article supports existing paleoclimate δ 2 H wax records from Qaupat Lake (Gorbey et al., 2021), while providing guidance on what watershed traits should be considered in future site selection for using the dual-biomarker method. It would be useful to build on the work of Hollister et al. (2022) to evaluate other aspects of the proxy system in the Arctic as well. For example, while sedimentary plant waxes in temperate forest biomes are dominated by local vegetation (Freimuth et al., 2019), it is unknown to what extent aeolian transport (Conte & Weber, 2002;Gao et al., 2014) in the more open landscape of the Arctic contributes to a regional mixing of terrestrial plant wax signatures. Observations of aerosols or dry deposition would contribute to an improved leaf wax budget for the study lake. Similarly, the spring snowmelt freshet is an important hydrological and geochemical event in the high latitudes (Holmes et al., 2012) and corresponds with the timing of maximum n-alkane and n-acid deposition into a lake in Alaska (Daniels et al., 2017). Observing plant waxes during the spring snowmelt freshet, and across the entire seasonal cycle, will be important for further evaluating lipid sources to Arctic lake sediments.
In addition to the ongoing lipid fingerprinting addressed by Hollister et al. (2022), the dual-biomarker proxy system will profit from research on how leaf water and lake water δ 2 H signatures relate to climatic parameters in the Arctic (Anderson et al., 2013;Cluett et al., 2021;Tondu et al., 2013). Previous high-latitude paleoclimate studies diverge on whether lake water 2 H-enrichment outpaces leaf water 2 H-enrichment under arid conditions (Balascio et al., 2013(Balascio et al., , 2016, or vice versa (Kjellman et al., 2020;Thomas et al., 2018;Zhao et al., 2022). Direct comparisons of leaf water and lake water 2 H/ 1 H ratios in the Arctic are needed to further understand how climatic, biotic, and physiographic traits are expressed in the proxy system and to further guide site selection for paleoclimate studies.
The contrasting results from Hollister et al. (2022) and Dion-Kirschner et al. (2020) highlight the variability of plant wax production, transport, and deposition in high latitude lakes. Continued efforts to fingerprint the sources n-alkyl compounds, and to build a mechanistic understanding of biomarker and isotope systematics, will support applications of the dual-biomarker isotope method for reconstructing past changes in Arctic humidity and precipitation seasonality.  Hollister et al. (2022) that control the sources of mid-chain n-alkyl lipids to Arctic lakes. The mid-chain compounds at Qaupat Lake, Baffin Island appear to be derived from aquatic plants. The lake and catchment are characterized by abundant submerged aquatic vegetation (SAV), low terrestrial productivity, and low soil erosion related to the low-relief topography and dry conditions (e.g., left lake). In contrast, mid-chain compounds at Little Sugarloaf Lake in Greenland are derived from terrestrial plants (e.g., right lake).