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Plant leaf waxes and their isotopic composition are important tracers of ecological, environmental, and climate variability, with strong preservation potential in sedimentary archives. However, they represent an integrated, and often complicated, signal of vegetation and hydrology within a watershed. Here, we report a new approach for examining complex mixtures of n-alkanes in sediments and their isotope values: non-negative matrix factorization (NMF). NMF identifies the endmembers in a mixture from the integrated n-alkane data and provides quantitative information on the relative importance of those endmembers across samples. We apply this approach to a synthetic dataset and two previously published datasets to illustrate its uses. Our application of NMF to re-analyse previously published data reveals new insights into past climate and ecological change. We demonstrate that NMF allows a user to 1) identify potential mixing problems, 2) evaluate which specific compounds in a mixture carry the isotope signal that can best address a given scientific objective, 3) determine compound concentrations after excluding contributions from particular endmember sources, and 4) calculate isotope values of different sources. NMF provides a quantitative approach for evaluating the influence of endmember mixing on molecular concentrations and isotope values within a dataset. The re-analysis of two published datasets reveals new quantitative insight into Holocene Arctic climate and Neogene vegetation change. 

Understanding the transport mechanisms of terrestrial biomarkers to marine sediments is critical for interpreting past environmental and climate changes from these valuable archives. Here, we produce new estimates of two classes of terrestrial plant biomarkers, n-alkane waxes and pentacyclic triterpene methyl ethers (PTMEs), from a transect of marine core top sediments that span the full length of the West African margin. We determine the chain length distributions, mass accumulation rates, carbon isotope signatures (δ13C) of n-alkanes and the mass accumulation rates of PTMEs and assess the extent to which these proxy characteristics reflect vegetation and climate patterns within source areas on adjacent land. We achieve this via comparisons with a variety of satellite-based vegetation and climate data sets and with atmospheric back trajectory and river basin estimates. The mass accumulation rate of grass-produced PTMEs to core top marine sediments shows good spatial agreement with the presence of C4 grasses on land and appears to have shorter transport distances than n-alkanes. The mass accumulation rate of n-alkanes roughly corresponds to changes in the landscape net primary productivity. The δ13C signature of n-alkanes records changes in landscape C3 versus C4 vegetation balance while longer chain length n-alkane distributions indicate drier conditions and grassier vegetation. Apparent discrepancies between the zonal distribution of biomarkers in the marine sediments versus the observed vegetation patterns can mostly be explained by the influence of long-range atmospheric transport, with modest contributions from river inputs. 

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane-spanning lipids synthesized by bacteria in numerous substrates. The degree of methylation of the five methyl brGDGTs in both soils and lake sediments, described by the MBT′5Me index, is empirically related to surface atmospheric temperature. This relationship in lakes is generally assumed to reflect lake surface temperatures captured by brGDGT production in the water column and exported to lake sediments, and the MBT′5Me index has been applied to brGDGTs in lake sediment successions to reconstruct changes in temperature through time. We analyzed the relationship between MBT′5Me and the isomerization of brGDGTs (IR6Me) in globally distributed surficial lake sediments and demonstrated that the relationship, and calibrations, of MBT′5Me and temperature in middle and high latitude lakes are sensitive to incompletely understood factors related to IR6Me. IR6Me does not appear to track a non-thermal influence of brGDGT methylation in tropical lakes, but this could change as the data set is expanded. We address ongoing challenges in the application of the MBT′5Me paleothermometer in middle and high latitude lakes with new MBT′5Me-temperature calibrations based on grouping lakes by IR6Me. We demonstrate how IR6Me can distinguish samples with a significant non-thermal influence on MBT′5Me by targeting anomalously warm temperatures during the Last Glacial Maximum from newly analyzed piston and gravity core samples from Lake Baikal, Russia. 

Savanna ecosystems were the landscapes for human evolution and are vital to modern Sub-Saharan African food security, yet the fundamental drivers of climate and ecology in these ecosystems remain unclear. Here we generate plant-wax isotope and dust flux records to explore the mechanistic drivers of the Northwest African monsoon, and to assess ecosystem responses to changes in monsoon rainfall and atmospheric pCO2. We show that monsoon rainfall is controlled by low-latitude insolation gradients and that while increases in precipitation are associated with expansion of grasslands into desert landscapes, changes in pCO2 predominantly drive the C3/C4 composition of savanna ecosystems. 

Abstract Creeping faults are difficult to assess for seismic hazard because they may participate in rupture even though they likely cannot nucleate large earthquakes. The creeping central section of the San Andreas fault in California (USA) has not participated in a historical large earthquake; however, earthquake ruptures nucleating in the locked northern and southern sections may propagate through the creeping section. We used biomarker thermal maturity and K/Ar dating on samples from the San Andreas Fault Observatory at Depth to look for evidence of earthquakes. Biomarkers show evidence of many earthquakes with displacements >1.5 m in and near a 3.5-m-wide patch of the fault. We show that K/Ar ages decrease with thermal maturity, and partial resetting occurs during coseismic heating. Therefore, measured ages provide a maximum constraint on earthquake age, and the youngest earthquakes here are younger than 3 Ma. Our results demonstrate that creeping faults may host large earthquakes over longer time scales. 

Abstract Precisely targeted measurements of trace elements using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) reveal inter-chamber heterogeneities in specimens of the planktic foraminiferTrilobatus (Globigerinoides) sacculifer. We find that Mg/Ca ratios in the final growth chamber are generally lower compared to previous growth chambers, but final chamber Mg/Ca is elevated in one of thirteen sample intervals. Differences in distributions of Mg/Ca values from separate growth chambers are observed, occurring most often at lower Mg/Ca values, suggesting that single-chamber measurements may not be reflective of the specimen’s integrated Mg/Ca. We compared LA-ICPMS Mg/Ca values to paired, same-individual Mg/Ca measured via inductively coupled plasma optical emission spectrometry (ICP-OES) to assess their correspondence. Paired LA-ICPMS and ICP-OES Mg/Ca show a maximum correlation coefficient of R = 0.92 (p < 0.05) achieved by applying a weighted average of the last and penultimate growth chambers. Population distributions of paired Mg/Ca values are identical under this weighting. These findings demonstrate that multi-chamber LA-ICPMS measurements can approximate entire specimen Mg/Ca, and is thus representative of the integrated conditions experienced during the specimen’s lifespan. This correspondence between LA-ICPMS and ICP-OES data links these methods and demonstrates that both generate Mg/Ca values suitable for individual foraminifera palaeoceanographic reconstructions. 

Abstract The El Niño Southern Oscillation (ENSO) is highly dependent on coupled atmosphere-ocean interactions and feedbacks, suggesting a tight relationship between ENSO strength and background climate conditions. However, the extent to which background climate state determines ENSO behavior remains in question. Here we present reconstructions of total variability and El Niño amplitude from individual foraminifera distributions at discrete time intervals over the past ~285,000 years across varying atmospheric CO₂levels, global ice volume and sea level, and orbital insolation forcing. Our results show a strong correlation between eastern tropical Pacific Ocean mixed-layer thickness and both El Niño amplitude and central Pacific variability. This ENSO-thermocline relationship implicates upwelling feedbacks as the major factor controlling ENSO strength on millennial time scales. The primacy of the upwelling feedback in shaping ENSO behavior across many different background states suggests accurate quantification and modeling of this feedback is essential for predicting ENSO’s behavior under future climate conditions. 

The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO₂) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO₂beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO₂record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO₂thresholds in biological and cryosphere evolution.