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Phytoplankton, a diverse group of small photosynthetic algae inhabiting the sunlit region near the ocean surface, form the base of marine trophic webs ( 1 ). Whereas phytoplankton have evolved in tandem with the climate system for hundreds of millions of years ( 2 ), cumulative greenhouse gas emissions are causing rising ocean temperature, acidification, and oxygen loss at increasing rates ( 3 ). How phytoplankton will respond and adapt to these multistressors in the future ( 4 ), and how this will in turn threaten marine trophic webs and food supply for humans, remain important questions in oceanography. On page 1487 of this issue, Holm et al. ( 5 ) show that a physiological adaptation of phytoplankton to a warming world will lead to a reduction in their nutritional value, with negative consequences for marine ecosystems. 

The Cretaceous/Paleogene (K/Pg) boundary is marked by one of the largest mass extinctions in Earth’s history, with geological evidence for this event being expressed in hundreds of locations worldwide. An extensively studied section located near El Kef, northwestern Tunisia, is characterized by the classic iridium-rich K/Pg boundary layer, abundant and well-preserved microfossils, and apparently continuous sedimentation throughout the early Danian with no previously described structural complication. These features led to its designation in 1991 as the Global Stratigraphic Section and Point (GSSP) for the base of the Danian (i.e., the K/Pg boundary). However, the outcrop section has become weathered, and the “golden spike” marking the GSSP is difficult to locate. Therefore, the El Kef Coring Project aimed to provide a continuous record of unweathered sediments across the K/Pg transition in cores recovered from five rotary-drilled holes located close to the El Kef GSSP. Here, we present new, high-resolution lithologic, biostratigraphic, and geochemical data from these cores. The recovered stratigraphic successions of each hole (all drilled within ∼75 m of one another) are unexpectedly different, and we identified a formerly unknown unconformity within planktic foraminiferal biozone P1b. Our results provide evidence that sedimentation at El Kef was not as continuous or free from structural complication as previously thought. Despite these challenges, we present a new composite section from the five El Kef holes and an age model correlated to the orbitally tuned record at Walvis Ridge, South Atlantic Ocean, which is critical in placing the paleoenvironmental and paleoecological records from El Kef in a global context. 

BrGDGT lipids from the deepest oceans to the high Arctic share fundamental relationships with temperature, pH, and one another. 

Abstract. Elevated organic matter (OM) concentrations are found in hadalsurface sediments relative to the surrounding abyssal seabed. However, theorigin of this biological material remains elusive. Here, we report on thecomposition and distribution of cellular membrane intact polar lipids (IPLs)extracted from surface sediments around the deepest points of the AtacamaTrench and adjacent bathyal margin to assess and constrain the sources oflabile OM in the hadal seabed. Multiscale bootstrap resampling of IPLs'structural diversity and abundance indicates distinct lipid signatures inthe sediments of the Atacama Trench that are more closely related to thosefound in bathyal sediments than to those previously reported for the upperocean water column in the region. Whereas the overall number of unique IPLstructures in hadal sediments contributes a small fraction of the total IPLpool, we also report a high contribution of phospholipids with mono- anddi-unsaturated fatty acids that are not associated with photoautotrophicsources and that resemble traits of physiological adaptation to highpressure and low temperature. Our results indicate that IPLs in hadalsediments of the Atacama Trench predominantly derive from in situ microbialproduction and biomass, whereas the export of the most labile lipidcomponent of the OM pool from the euphotic zone and the overlying oxygenminimum zone is neglectable. While other OM sources such as the downslopeand/or lateral transport of labile OM cannot be ruled out and remain to bestudied, they are likely less important in view of the lability ofester-bond IPLs. Our results contribute to the understanding of themechanisms that control the delivery of labile OM to this extreme deep-seaecosystem. Furthermore, they provide insights into some potentialphysiological adaptation of the in situ microbial community to high pressure andlow temperature through lipid remodeling. 

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.

 

Glycerol dialkyl glycerol tetraethers (GDGTs), both archaeal isoprenoid GDGTs (isoGDGTs) and bacterial branched GDGTs (brGDGTs), have been used in paleoclimate studies to reconstruct environmental conditions. Since GDGTs are produced in many types of environments, their relative abundances also depend on the depositional setting. This suggests that the distribution of GDGTs also preserves useful information that can be used more broadly to infer these depositional environments in the geological past. Here, we combined existing iso‐ and brGDGT relative abundance data with newly analyzed samples to generate a database of 1,153 samples from several modern sedimentary settings. We observed a robust relationship between the depositional environment and the relative abundances of GDGTs in our samples. This data set was used to train and test theBranched andisoGDGT Machine learningClassification (BIGMaC) algorithm, which identifies the environment a sample comes from based on the distribution of GDGTs with high precision and recall (F1 = 0.95). We tested the model on the sedimentary record from the Giraffe kimberlite pipe, an Eocene maar in subantarctic Canada, and found that the BIGMaC reconstruction agrees with independent stratigraphic and palynological information, provides new information about the paleoenvironment of this site, and helps improve its paleotemperature reconstruction. In contrast, we also include an example from the PETM‐aged Cobham lignite as a cautionary example that illustrates the limitations of the algorithm. We propose that in cases where paleoenvironments are unknown or are changing, BIGMaC can be applied in concert with other proxies to generate more refined paleoclimate records.

 

Abstract. Marine fronts delineate the boundary between distinct water masses and,through the advection of nutrients, are important facilitators of regionalproductivity and biodiversity. As the modern climate continues to change, themigration of frontal zones is evident, but a lack of information about theirstatus prior to instrumental records hinders future projections. Here, wecombine data from lipid biomarkers (archaeal isoprenoid glycerol dibiphytanylglycerol tetraethers and algal highly branched isoprenoids) with planktic andbenthic foraminifera assemblages to detail the biological response of themarine Arctic and polar front migrations on the North Iceland Shelf (NIS) overthe last 8 kyr. This multi-proxy approach enables us to quantify thethermal structure relating to Arctic and polar front migration and test howthis influences the corresponding changes in local pelagic productivity. Ourdata show that following an interval of Atlantic water influence, the Arcticfront and its associated high pelagic productivity migrated southeastward tothe NIS by ∼6.1 ka. Following a subsequent trend in regionalcooling, Polar Water from the East Greenland Current and the associated polarfront spread onto the NIS by ∼3.8 ka, greatly diminishinglocal algal productivity through the Little Ice Age. Within the last century,the Arctic and polar fronts have moved northward back to their currentpositions relative to the NIS and helped stimulate the productivity thatpartially supports Iceland's economy. Our Holocene records from the NISprovide analogues for how the current frontal configuration and theproductivity that it supports may change as global temperatures continue torise. 

Abstract. Distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs) are frequently employed for reconstructing terrestrial paleotemperaturesfrom lake sediment archives. Although brGDGTs are globally ubiquitous, the microbial producers of these membrane lipids remain unknown, precluding afull understanding of the ways in which environmental parameters control their production and distribution. Here, we advance this understanding inthree ways. First, we present 43 new high-latitude lake sites characterized by low mean annual air temperatures (MATs) and high seasonality, fillingan important gap in the global dataset. Second, we introduce a new approach for analyzing brGDGT data in which compound fractional abundances (FAs)are calculated within structural groups based on methylation number, methylation position, and cyclization number. Finally, we perform linear andnonlinear regressions of the resulting FAs against a suite of environmental parameters in a compiled global lake sediment dataset(n = 182). We find that our approach deconvolves temperature, conductivity, and pH trends in brGDGTs without increasing calibration errorsfrom the standard approach. We also find that it reveals novel patterns in brGDGT distributions and provides a methodology for investigating thebiological underpinnings of their structural diversity. Warm-season temperature indices outperformed MAT in our regressions, with the mean temperature of months abovefreezing yielding the highest-performing model (adjusted R2 = 0.91, RMSE = 1.97 ∘C, n = 182). The naturallogarithm of conductivity had the second-strongest relationship to brGDGT distributions (adjusted R2 = 0.83, RMSE = 0.66,n = 143), notably outperforming pH in our dataset (adjusted R2 = 0.73, RMSE = 0.57, n = 154) and providing a potential newproxy for paleohydrology applications. We recommend these calibrations for use in lake sediments globally, including at high latitudes, and detailthe advantages and disadvantages of each. 

Summer warming is driving a greening trend across the Arctic, with the potential for large-scale amplification of climate change due to vegetation-related feedbacks [Pearson et al.,Nat. Clim. Chang.(3), 673–677 (2013)]. Because observational records are sparse and temporally limited, past episodes of Arctic warming can help elucidate the magnitude of vegetation response to temperature change. The Last Interglacial ([LIG], 129,000 to 116,000 y ago) was the most recent episode of Arctic warming on par with predicted 21st century temperature change [Otto-Bliesner et al.,Philos. Trans. A Math. Phys. Eng. Sci.(371), 20130097 (2013) and Post et al.,Sci.Adv. (5), eaaw9883 (2019)]. However, high-latitude terrestrial records from this period are rare, so LIG vegetation distributions are incompletely known. Pollen-based vegetation reconstructions can be biased by long-distance pollen transport, further obscuring the paleoenvironmental record. Here, we present a LIG vegetation record based on ancient DNA in lake sediment and compare it with fossil pollen. Comprehensive plant community reconstructions through the last and current interglacial (the Holocene) on Baffin Island, Arctic Canada, reveal coherent climate-driven community shifts across both interglacials. Peak LIG warmth featured a ∼400-km northward range shift of dwarf birch, a key woody shrub that is again expanding northward. Greening of the High Arctic—documented here by multiple proxies—likely represented a strong positive feedback on high-latitude LIG warming. Authenticated ancient DNA from this lake sediment also extends the useful preservation window for the technique and highlights the utility of combining traditional and molecular approaches for gleaning paleoenvironmental insights to better anticipate a warmer future.