Leaf Wax Hydrogen Isotopes as a Hydroclimate Proxy in the Tropical Pacific

Abstract

Hydrogen isotope ratios of sedimentary leaf waxes (δ 2 H Wax values) are increasingly used to reconstruct past hydroclimate. Here, we add δ 2 H Wax values from 19 lakes and four swamps on 15 tropical Pacific islands to an updated global compilation of published data from surface sediments and soils. Globally, there is a strong positive linear correlation between δ 2 H values of mean annual precipitation (δ 2 H P values) and the leaf waxes n-C 29-alkane (R 2 = 0.74, n = 665) and n-C 28-acid (R 2 = 0.74, n = 242). Tropical Pacific δ 2 H Wax values fall within the predicted range of values based on the global calibration, and the largest residuals from the global regression line are no greater than those observed elsewhere, despite large uncertainties in δ 2 H P values at some Pacific sites. However, tropical Pacific δ 2 H Wax values in isolation are not correlated with estimated δ 2 H P values from isoscapes or from isotope-enabled general circulation models. Palynological analyses from these same Pacific sediment samples suggest no systematic relationship between any particular type of pollen distribution and deviations from the global calibration line. Rather, the poor correlations observed in the tropical Pacific are likely a function of the small range of δ 2 H P values relative to the typical residuals around the global calibration line. Our results suggest that δ 2 H Wax values are currently most suitable for use in detecting large changes in precipitation in the tropical Pacific and elsewhere, but that ample room for improving this threshold exits in both improved understanding of δ 2 H variability in plants, as well as in precipitation. Plain Language Summary Past precipitation patterns are difficult to reconstruct, limiting our ability to understand Earth's climate system. Geochemists reconstruct past precipitation by measuring the amount of heavy hydrogen naturally incorporated into the waxy coating of leaves, which is preserved in mud that accumulates in lakes, soils, and oceans. Heavy hydrogen in leaf waxes is strongly correlated with local precipitation, allowing us to learn about rainfall intensity, temperature, and cloud movement. However, no existing calibration studies include sites from the tropical Pacific, home to the most intense rainfall on the planet and populations that rely on rain for drinking water and farming. We measured heavy hydrogen in leaf waxes from tropical Pacific islands and show that although values are within the global calibration error, no precipitation relationship exists within the region. Plant type distributions do not explain the lack of correlation, which is best attributed to poorly constrained estimates of heavy hydrogen in local rain and the relatively small range of variability within the region. At present, heavy hydrogen from ancient leaf waxes can show large changes in past precipitation, but improved processlevel understanding is needed to use this tool to understand smaller changes in the tropical Pacific and elsewhere. LADD ET AL.

Metabolic exchange between pathways for isoprenoid synthesis and implications for biosynthetic hydrogen isotope fractionation

“…Paleoclimate proxies are used to understand the Earth's climate system and inform predictions of future change. One such proxy is based on the hydrogen isotope composition (δ 2 H = ( 2 H/ 1 H) Sample /( 2 H/ 1 H) VSMOW − 1) of leaf wax n-alkanes and nalkanoic acids, which are strongly correlated with δ 2 H values of mean annual precipitation on a global scale (Sachse et al, 2012;McFarlin et al, 2019;Ladd et al, 2021). Reconstructing δ 2 H values of precipitation is useful for learning about past changes in temperature, moisture transit pathways, and precipitation intensity, depending on location (Bowen et al, 2019), and sedimentary leaf wax δ 2 H values are increasingly used for this purpose (e.g.…”

Mixed Signals From the Stable Isotope Composition of Precipitation and Plant Waxes in the Northern Tropical Andes

“…It is worth noticing that besides their high p ‐value, the relationship between δ 2 H p and δ 2 H wax is negative (Figure 7), which is opposite to the most common direction (positive slope) observed in this correlation elsewhere (Garcin et al., 2012; Polissar & Freeman, 2010; Sachse et al., 2004, 2012). Whereas such negative correlation has been reported in other tropical places like the islands from the South Pacific Convergence Zone (Ladd et al., 2021), an explanation remains elusive. Thus, further studies investigating the modern relationship between δ 2 H p and δ 2 H wax in terrestrial tropical regions are needed.…”

“…The hydrogen (δ 2 H wax ) and carbon (δ 13 C wax ) isotope compositions of plant waxes can reflect the physical, chemical, biological, and ecological conditions of the environment in which they grow, such as the precipitation/evaporation balance (hydrological cycle), temperature, and the composition of the plant community (i.e., C 3 and C 4 plants; Diefendorf & Freimuth, 2017). Terrestrial plants, which produce predominantly long‐chain waxes, incorporate and fractionate the hydrogen isotope composition of the source water (Goldsmith et al., 2019; Ladd et al., 2021; McFarlin et al., 2019; Sachse et al., 2012). The main source of water for terrestrial plants is soil moisture, which in turn derives its original isotopic composition from the local precipitation.…”

Contrasting Common Era climate and hydrology sensitivities from paired lake sediment dinosterol hydrogen isotope records in the South Pacific Convergence Zone

“…Strontium isotope ratios of otoliths provide clear support that young eels enter Lac Lalolalo from the ocean presumably through small, hidden underground connections (Bareille et al, 2015) and become trapped in the lake as they mature. Details about surface sediments from 'Uvea, 'Upolu, Efate lakes, including compound-specific isotope analyses and pollen assemblages, have been recently described (Ladd et al, 2021;Maloney et al, 2019) and a preliminary assessment of microfossils in 'Uvea sediment is described in Appendix A. Cores/sections: G5, G9, L1, L1A, L2, L2A, L3, L3A. Core top sample includes G7 (0-2 cm), G5 (0-2 cm), and a 0-1cm section collected in 1995 (Maloney et al 2019) 3…”

“…Comparing the precipitation rate reconstructions from all three locations indicates that the precipitation estimates inferred from d 2 Hdinosterol values may be most useful in a relative sense (comparing variability at a single site) as opposed to an absolute sense (comparing values between sites hundreds of kilometers apart). Tracking additional lake-specific physical and ecological factors that influence d 2 Hlakewater, such as the extent of vegetation cover over shallow lakes that can act as a barrier to evaporation (Ladd et al, 2021;Maloney et al, 2019), may be necessary to confidently compare reconstructed precipitation rates across space. Another consideration is that evaporation rates and/or the observed relationship between precipitation amount and the isotopic composition of precipitation may vary through time.…”