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The hydrogen isotopic composition of lake water (δ2Hlw) contains hydrologic information and can be used as a recorder of lake water hydrology, including the extent of evaporation of the lake system. Initial studies indicate that the hydrogen isotopes of highly branched isoprenoids (δ2HHBI), synthesized by lake diatoms and preserved in lake sediments are a promising proxy for constraining past δ2Hlw values that are free from terrestrial in- fluences. However, there are many aspects of this proxy, including the seasonality of HBI production, that are unknown and need to be addressed more fully before the proxy can by widely applied. To determine when HBIs are produced throughout the year, and whether there are seasonal biases in δ2Hlw reconstructions, we deployed two sediment traps at Brown’s Lake, in northeastern Ohio. We present HBI concentrations, δ2HHBI values, HBI carbon isotopes and bulk sediment carbon isotopes from sediment traps collected monthly for 26 months to investigate seasonality of HBIs. We observed HBIs in each of the monthly sediment traps throughout the study interval with an increase in HBI concentration during September and October, suggesting that HBIs are made throughout the year with greater production during fall. We calculated the difference between δ2HHBI and δ2Hlw values (ε2HHBI/lw) and observe a range in ε2HHBI/lw values of up to 64‰, which we speculate is related to changes in the diatom communities that synthesize HBIs throughout the year and between different years. Different diatom communities may have different biosynthetic pathways or metabolisms that result in isotope effects. This study is the first that examines the seasonality of HBIs in lake sediments and provides framework for interpreting the seasonality of hydroclimate records generated from δ2HHBI values in temperate eutrophic lakes. 

Diatom-derived highly branched isoprenoid lipids (HBIs) are found extensively in marine sediments, but to date are only reported in a few lacustrine sediments. To expand on prior lake studies, we collected lake surface sediment samples, water samples, and filtered photic zone water from 50 lakes from the Great Plains to the northeastern United States. Samples were collected in May and June and a few sites were revisited in September and October. Studied lakes vary in climate, water chemistry (e.g., pH, salinity, alkalinity), size, and trophic states. They also vary in their diatom species compositions with 344 diatom taxa reported. We characterized HBI assemblages in each lake and found 11 different HBI compounds including one C20:0 HBI, five C20:1 HBI isomers, C21:0 HBI, C25:2 HBI, two C25:3 HBIs, and C25:4 HBI. C20:0 HBI was present in all but two lakes and was often the most abundant HBI present. HBIs were also detected in nearly all the water filter samples indicating they are produced in the photic zone. C20:0 HBI was present in all freshwater lakes, but not present or at very low con- centration in the highest salinity lakes, which were dominated by C21:0 HBI and C25 HBIs. Many of the lakes were dominated by diatom genera and species that are not known to be HBI-producing genera, suggesting there are unrecognized HBI-producing diatom taxa. This inventory, illustrating the widespread presence and diversity of HBIs from lakes across large differences in water chemistries and climate, further suggests that HBIs may be useful diatom biomarkers for paleoclimate applications. 

Abstract Arctic precipitation is predicted to increase this century. Records of past precipitation seasonality provide baselines for a mechanistic understanding of the dynamics controlling Arctic precipitation. We present an approach to reconstruct Arctic precipitation seasonality using stable hydrogen isotopes (δ²H) of aquatic plant waxes in neighboring lakes with contrasting water residence times and present a case study of this approach in two lakes on western Greenland. Residence time calculations suggest that growing season lake water δ²H in one lake reflects summer precipitation δ²H, while the other reflects amount‐weighted annual precipitation δ²H and evaporative enrichment. Aquatic plant wax δ²H in the “summer lake” is relatively constant throughout the Holocene, perhaps reflecting competing effects of local summer warmth and increased distal moisture transport due to a strengthened latitudinal temperature gradient. In contrast, aquatic plant wax δ²H in the “mean annual lake” is 100‰²H depleted from 6 to 4 ka relative to the beginning and end of the record. Because there are relatively minor changes in summer precipitation δ²H, we interpret the 100‰²H depletion in mean annual precipitation to reflect an increase in winter precipitation amount, likely accompanied by changes in winter precipitation δ²H and decreased evaporative enrichment. Thus, unlike the “summer lake,” the “mean annual lake” records changes in winter precipitation. This dual‐lake approach may be applied to reconstruct past changes in precipitation seasonality at sites with strong precipitation isotope seasonality and minimal lake water evaporative enrichment. 

Abstract Lacustrine δ²H and δ¹⁸O isotope proxies are powerful tools for reconstructing past climate and precipitation changes in the Arctic. However, robust paleoclimate record interpretations depend on site‐specific lake water isotope systematics, which are poorly described in the eastern Canadian Arctic due to insufficient modern lake water isotope data. We use modern lake water isotopes (δ¹⁸O and δ²H) collected between 1994–1997 and 2017–2021 from a transect of sites spanning a Québec‐to‐Ellesmere Island gradient to evaluate the effects of inflow seasonality and evaporative enrichment on the δ²H and δ¹⁸O composition of lake water. Four lakes near Iqaluit, Nunavut sampled biweekly through three ice‐free seasons reflect mean annual precipitation isotopes with slight evaporative enrichment. In a 23° latitudinal transect of 181 lakes, through‐flowing lake water δ²H and δ¹⁸O fall along local meteoric water lines. Despite variability within each region, we observe a latitudinal pattern: southern lakes reflect mean annual precipitation isotopes, whereas northern lakes reflect summer‐biased precipitation isotopes. This pattern suggests that northern lakes are more fully flushed with summer precipitation, and we hypothesize that this occurs because the ratio of runoff to precipitation increases with latitude as vegetation cover decreases. Therefore, proxy records from through‐flowing lakes in this region should reflect precipitation isotopes with minimal influence of evaporation, but vegetation changes in lake catchments across a latitudinal transect and through geologic time may influence the seasonality of lake water isotopic compositions. Thus, we recommend that future lake water isotope proxy records are considered in context with temperature and ecological proxy records. 

We report an improved measurement of the valence $u$and $d$quark distributions from the forward-backward asymmetry in the Drell-Yan process using $8.6{\mathrm{fb}}^{-1}$of data collected with the D0 detector in $p\overline{p}$collisions at $\sqrt{s}=1.96$. This analysis provides the values of new structure parameters that are directly related to the valence up and down quark distributions in the proton. In other experimental results measuring the quark content of the proton, $d$quark contributions are mixed with those from other quark flavors. In this measurement, the $u$and $d$quark contributions are separately extracted by applying a factorization of the QCD and electroweak portions of the forward-backward asymmetry. Published by the American Physical Society2024