Origin, transport, and retention of fluvial sedimentary organic matter in South Africa's largest freshwater wetland, Mkhuze Wetland System

Sedimentary organic matter (OM) analyses along a 130 km-long transect of the Mkhuze River from the Lebombo Mountains to its outlet into Lake St. Lucia, Africa’s most extensive estuarine system, revealed the present active trapping function of a terminal freshwater wetland. A combination of organic bulk parameters, thermal analyses, and determination of plant waxes, and their corresponding stable carbon (δ13C) and hydrogen (δD) isotopic signatures in surface sediments and local plant species enabled characterization and comparison of sedimentary OM in terms of stability, degradation status, sources, and sinks within and among the respective sub-environments of the Mkhuze Wetland System. This approach showed that fluvial sedimentary OM originating from inland areas is mainly deposited on the floodplain and Mkhuze Swamps. In contrast to samples from upstream areas, a distinctly less degraded signature characterizes the sedimentary OM in the northern section of Lake St. Lucia. Although lake sedimentary plant waxes are similar in the observed wax distribution pattern and δ13C values, they exhibit considerably higher δD values. This offset in δD indicates that lakeshore vegetation dominates plant-derived sedimentary OM in the lake, elucidating the effective capturing of OM and its fate in a sub-tropical coastal freshwater wetland. These findings raise important constraints for environmental studies assuming watershed-integrated signals in sedimentary archives retrieved from downstream lakes or offshore.

A potential suite of climate markers of long-chain n-alkanes and alkenones preserved in the top sediments from the Pacific sector of the Southern Ocean

Investigating organic compounds in marine sediments can potentially unlock a wealth of new information in these climate archives. Here, we present pilot study results of organic geochemical features of long-chain n-alkanes and alkenones and individual carbon isotope ratios of long-chain n-alkanes from a newly collected, approximately 8 m long, located in the far reaches of the Pacific sector of the Southern Ocean. We analyzed a suite of organic compounds in the core. The results show abundant long-chain n-alkanes (C29–C35) with predominant odd-over-even carbon preference, suggesting an origin of terrestrial higher plant waxes via long-range transport of dust, possibly from Australia and New Zealand. The δ13C values of the C31n-alkane range from −29.4 to −24.8‰, in which the higher δ13C values suggest more contributions from C4 plant waxes. In the analysis, we found that the mid-chain n-alkanes (C23–C25) have a small odd-over-even carbon preference, indicating that they were derived from marine non-diatom pelagic phytoplankton and microalgae and terrestrial sources. Furthermore, the C26 and C28 with lower δ13C values (~−34‰) indicate an origin from marine chemoautotrophic bacteria. We found that the abundances of tetra-unsaturated alkenones (C37:4) in this Southern Ocean sediment core ranges from 11 to 37%, perhaps a marker of low sea surface temperature (SST). The results of this study strongly indicate that the δ13C values of long-chain n-alkanes and $$ {U}_{37}^{\mathrm{k}} $$ U 37 k index are potentially useful to reconstruct the detailed history of C3/C4 plants and SST change in the higher latitudes of the Southern Ocean.

A potential suite of climate markers of long-chain n-alkanes and alkenones preserved in the top sediments from the Pacific sector of the Southern Ocean

Investigating organic compounds in marine sediments can potentially unlock a wealth of new information in these climate archives. Here we present pilot study results of organic geochemical features of long-chain n-alkanes and alkenones and individual carbon isotope ratios of long-chain n-alkanes from a newly collected, approximately 8-meter long, located in the far reaches of the Pacific sector of the Southern Ocean. We analyzed a suite of organic compounds in the core. The results show abundant long-chain n-alkanes (C29-C35) with predominant odd-over-even carbon preference, suggesting an origin of terrestrial higher plant waxes via long-range transport of dust, possibly from Australia and New Zealand. The δ13C values of the C31 n-alkane range from -29.4 to -24.8‰, in which the higher δ13C values suggest more contributions from C4 plant waxes. In the analysis, we found that the mid-chain n-alkanes (C23-C25) have a small odd-over-even carbon preference, indicating that they were derived from marine non-diatom pelagic phytoplankton and microalgae and terrestrial sources. Furthermore, the C26 and C28 with lower δ13C values (~ -34‰) indicate an origin from marine chemoautotrophic bacteria. We found that the abundances of tetra-unsaturated alkenones (C37:4) in this Southern Ocean sediment core ranges from 11-37%, perhaps a marker of low sea surface temperature (SST). The results of this study strongly indicate that the δ13C values of long-chain n-alkanes and index are potentially useful to reconstruct the detailed history of C3/C4 plants and SST change in the higher latitudes of the Southern Ocean.

Drought does not affect hydrogen isotope fractionation during lipid biosynthesis by the tropical plant Pachira aquatica

<p>Hydrogen isotope ratios (<sup>2</sup>H/<sup>1</sup>H) of plant waxes and other lipids preserved in sediments are increasingly used as a paleohydrologic proxy for past water isotopes. The relationship between precipitation <sup>2</sup>H/<sup>1</sup>H ratios and those of plant waxes in surface sediments is linearly correlated at a global scale. However, there are large residuals in this relationship, and the offsets in <sup>2</sup>H/<sup>1</sup>H ratios for the same compound produced by different species growing at the same site, as well as for different compounds produced within the same plant, can approach the magnitude of continental scale variability in precipitation isotopes. This indicates that lipid <sup>2</sup>H/<sup>1</sup>H ratios are influenced by significant factors besides the <sup>2</sup>H/<sup>1</sup>H ratios of local precipitation. One possibility is that plant metabolic responses to stresses such as drought cause changes in <sup>2</sup>H/<sup>1</sup>H fractionation during lipid synthesis.</p><p> </p><p>In order to assess the effects of drought on <sup>2</sup>H/<sup>1</sup>H fractionation during plant lipid synthesis, we grew Pachira aquatica seedlings in controlled growth chamber conditions, with half of the individual plants experiencing drought conditions (soil moisture content reduced to ~10%) and half serving as well-watered controls (soil moisture content ~25%). We used position-specific <sup>13</sup>C-pyruvate labeling to assess if there were changes in lipid production under drought, and focused on a diverse range of compounds including palmitic acid, n-C<sub>29</sub> and n-C<sub>31</sub>-alkanes, phytol, squalene, and sitosterol. We also measured natural abundance <sup>2</sup>H/<sup>1</sup>H ratios from the same compounds and from cryogenically extracted leaf water to quantify biosynthetic H isotope fractionation (ε<sub>Bio</sub>).</p><p> </p><p>Biosynthetic <sup>2</sup>H/<sup>1</sup>H fractionation spanned a 150‰ range among compounds, with palmitic acid being the least <sup>2</sup>H-depleted compound (ε<sub>Bio</sub> = -140 ± 10‰) and phytol being the most <sup>2</sup>H-depleted compound (ε<sub>Bio</sub> = -317 ± 7‰). These fractionation factors did not change under drought, although <sup>13</sup>C-pyruvate labeling indicated that the compounds were being actively produced. There was no change in the production rate of any compound under drought, however. Differential incorporation of <sup>13</sup>C depending on whether the 1<sup>st</sup> or 2<sup>nd</sup> carbon in pyruvate was labeled showed clear distinctions among compound classes, with the acetogenic compounds only becoming enriched from the C2 label, and isoprenoids using roughly equal proportions of carbon from each position. These results suggest that under this level of drought stress, Pachira aquatica did not make any changes to its lipid metabolism, and lipid <sup>2</sup>H/<sup>1</sup>H ratios were therefore unperturbed. If replicated in additional plants types and under more severe drought, this result is encouraging for the use of plant lipid <sup>2</sup>H/<sup>1</sup>H ratios as robust paleohydroclimate tracers.  </p>