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Glycerol dibiphytanyl glycerol tetraethers (GDGTs) are distinctive archaeal membrane-spanning lipids with up to eight cyclopentane rings and/or one cyclohexane ring. The number of rings added to the GDGT core structure can vary as a function of environmental conditions, such as changes in growth temperature. This physiological response enables cyclic GDGTs preserved in sediments to be employed as proxies for reconstructing past global and regional temperatures and to provide fundamental insights into ancient climate variability. Yet, confidence in GDGT-based paleotemperature proxies is hindered by uncertainty concerning the archaeal communities contributing to GDGT pools in modern environments and ambiguity in the environmental and physiological factors that affect GDGT cyclization in extant archaea. To properly constrain these uncertainties, a comprehensive understanding of GDGT biosynthesis is required. Here in this paper, we identify 2 GDGT ring synthases, GrsA and GrsB, essential for GDGT ring formation in Sulfolobus acidocaldarius. Both proteins are radical S-adenosylmethionine proteins, indicating that GDGT cyclization occurs through a free radical mechanism. In addition, we demonstrate that GrsA introduces rings specifically at the C-7 position of the core GDGT lipid, while GrsB cyclizes at the C-3 position, suggesting that cyclization patterns are differentially controlled by 2 separate enzymes and potentially influenced by distinct environmental factors. Finally, phylogenetic analyses of the Grs proteins reveal that marine Thaumarchaeota, and not Euryarchaeota, are the dominant source of cyclized GDGTs in open ocean settings, addressing a major source of uncertainty in GDGT-based paleotemperature proxy applications.

The last glacial cycle provides the opportunity to investigate large changes in the Atlantic Meridional Overturning Circulation (AMOC) beyond the small fluctuations evidenced from direct measurements. Paleotemperature records from Greenland and the North Atlantic show an abrupt variability, called Dansgaard–Oeschger (DO) events, which is associated with abrupt changes of the AMOC. These DO events also have Southern Hemisphere counterparts via the thermal bipolar seesaw, a concept describing the meridional heat transport leading to asynchronous temperature changes between both hemispheres. However, temperature records from the North Atlantic show more pronounced DO cooling events during massive releases of icebergs known as Heinrich (H) events, contrary to ice-core–based temperature records from Greenland. Here, we present high-resolution temperature records from the Iberian Margin and a Bipolar Seesaw Index to discriminate DO cooling events with and without H events. We show that the thermal bipolar seesaw model generates synthetic Southern Hemisphere temperature records that best resemble Antarctic temperature records when using temperature records from the Iberian Margin as inputs. Our data-model comparison emphasizes the role of the thermal bipolar seesaw in the abrupt temperature variability of both hemispheres with a clear enhancement during DO cooling events with H events, implying a relationship that is more complex than a simple flip-flop between two climate states linked to a tipping point threshold.

• Seed dormancy is an important life history state that increases survival and fitness of seed plants, and thus it has attracted much attention. However, global biogeography, effects of paleoenvironment, evolutionary roles of dormancy transitions, and differences in adaptations of seed dormancy between life-forms are poorly understood. • We compiled global distribution records for seed dormancy of 12 743 species and their phylogeny to explore the biogeographic patterns, environmental drivers, and evolutionary transitions between seed dormancy and nondormancy. • Biogeographic patterns reveal a low proportion of dormancy in tropical rainforest regions and arctic regions and a high proportion of dormancy in remaining tropical, subtropical, and temperate regions for all species and woody species. Herbaceous plants show a greater proportion of dormancy in most global regions except arctic regions. Seasonal environments have a consistent positive influence on the dormancy pattern for both life-forms, but precipitation and temperature were important driving factors for woody and herbaceous plants, respectively. Seed dormancy was the dominating state during the evolutionary history of seed plants, and dormancy transitions had a significant relationship with paleotemperatures. Dormancy and nondormancy transitions in response to fluctuating environments during long-term evolutionary history may have played important roles in the diversification of seed plants. • Our results add to the current knowledge about seed dormancy from macro-adaptive perspectives and the potential adaptive mechanisms of seed plants.

The geochemical and ultrastructural properties of thermally altered skeletal carbonate are expected to be compromised to varying degrees by disequilibrium processes between solids and the ambient aqueous fluids. When assessing the alteration history of carbonates, it is important to apply models that quantitatively describe these diagenetic processes on multiple geochemical systems, such that they can be identified in natural samples. Carbonate clumped isotope analysis provides a unique tool for validating such models and can be combined with other geochemical tools/proxies to more comprehensively describe the processes and products. Here, we have analyzed bivalve shells that have undergone hydrothermal alteration (experimental diagenesis) in high water/rock ratio experiments at 130 and 160°C, demonstrating that non‐linear changes in ∆47 and ∆48 values can be attributed to heterogeneous replacement of precursor carbonates. Importantly, this model predicts decoupled ∆47 and ∆48 values, despite all reactions occurring at clumped isotope equilibrium with respect to the experimental temperature. We demonstrate that the rapid, thermally induced re‐equilibration occurs in a “closed system” with minimal exchange with the ambient fluid, similar to the results of heating experiments conducted without an extraneous fluid. Later, stages of alteration occur in an “open system” wherein internal fluid is exchanged with the external fluid at a similar rate to recrystallization and neomorphism. In these experiments, some oxygen from the original inorganic‐organic composite‐biomineral is inherited, indicating restrictions on the availability of fluid oxygen. Our experiments and models validate a novel application for dual‐clumped isotopes for reconstructing hydrothermal temperatures and fluid δ18O compositions. Plain Language Summary Hydrothermal alteration has the potential to alter biominerals that are often used as a record of ancient climates. Here, we present a method to identify these reactions in natural samples. Key Points Hydrothermal alteration of carbonate often results in precipitation of minerals that are isotopically distinct from the original material Mixtures of isotopically distinct carbonates exhibit non‐linear clumped isotope mixing effects Analyses of dual‐clumped isotopes can identify the alteration of carbonate and be used to study the process that caused the alteration

Glycerol dibiphytanyl glycerol tetraethers (GDGTs) are distinctive archaeal membrane-spanning lipids with up to eight cyclopentane rings and/or one cyclohexane ring. The number of rings added to the GDGT core structure can vary as a function of environmental conditions, such as changes in growth temperature. This physiological response enables cyclic GDGTs preserved in sediments to be employed as proxies for reconstructing past global and regional temperatures and to provide fundamental insights into ancient climate variability. Yet, confidence in GDGT-based paleotemperature proxies is hindered by uncertainty concerning the archaeal communities contributing to GDGT pools in modern environments and ambiguity in the environmental and physiological factors that affect GDGT cyclization in extant archaea. To properly constrain these uncertainties, a comprehensive understanding of GDGT biosynthesis is required. Here, we identify 2 GDGT ring synthases, GrsA and GrsB, essential for GDGT ring formation in Sulfolobus acidocaldarius. Both proteins are radical S-adenosylmethionine proteins, indicating that GDGT cyclization occurs through a free radical mechanism. In addition, we demonstrate that GrsA introduces rings specifically at the C-7 position of the core GDGT lipid, while GrsB cyclizes at the C-3 position, suggesting that cyclization patterns are differentially controlled by 2 separate enzymes and potentially influenced by distinct environmental factors. Finally, phylogenetic analyses of the Grs proteins reveal that marine Thaumarchaeota, and not Euryarchaeota, are the dominant source of cyclized GDGTs in open ocean settings, addressing a major source of uncertainty in GDGT-based paleotemperature proxy applications.

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. Plain Language Summary Branched glycerol dialkyl glycerol tetraethers are fats used by bacteria to build their cell walls. Bacteria build their cell walls with different kinds of brGDGTs in response to ambient temperature. BrGDGTs are often preserved in lake sediments, making them a useful tool for reconstructing past climate. While working on samples from Lake Baikal, Russia, we noticed unexpectedly warm temperatures during the last ice age estimated from brGDGTs. These warm temperatures coincided with unusually high relative amounts of 6‐methyl brGDGTs. This observation spurred the analysis of a large data set of published globally distributed lake brGDGT data. We found that samples from middle and high latitude lakes with relatively more 6‐methyl brGDGTs tended to have higher than expected brGDGT‐estimated temperatures. We use our findings to refine the equations that relate brGDGT distributions to surface air temperature in middle and high latitude lake sediments. Key Points A non‐thermal effect on brGDGT paleotemperature estimates in lakes is identified by the isomer ratio IR6Me The calibration of MBT′5Me to temperature in middle and high latitude lakes is strengthened if samples with IR6Me > 0.4 are excluded IR6Me does not appear to identify non‐thermal effects on tropical lake samples, but this could change as the data set is expanded

The period of relatively warm climate from 11,000 to 5,000 years ago was marked by considerable temporal and spatial variability. Model simulations relate this complexity to the influence of the waning Laurentide ice sheet. The Holocene thermal maximum, a period of relatively warm climate between 11,000 and 5,000 years ago 1 , 2 , is most clearly recorded in the middle and high latitudes 2 , 3 of the Northern Hemisphere, where it is generally associated with the local orbitally forced summer insolation maximum. However, proxy-based reconstructions have shown that both the timing and magnitude of the warming vary substantially between different regions 2 , 3 , 4 , suggesting the involvement of extra feedbacks and forcings. Here, we simulate the Holocene thermal maximum in a coupled global ocean–atmosphere–vegetation model. We find that before 7,000 years ago, summers were substantially cooler over regions directly influenced by the presence of the Laurentide ice sheet, whereas other regions of the Northern Hemisphere were dominated by orbital forcing. Our simulations suggest that the cool conditions arose from a combination of the inhibition of Labrador Sea deep convection by the flux of meltwater from the ice sheet, which weakened northward heat transport by the ocean, and the high surface albedo of the ice sheet. We thus conclude that interglacial climate is highly sensitive to relatively small changes in ice-sheet configuration.

The Middle–Late Permian Emeishan Large Igneous Province (ELIP) in southwestern China represents a classic example of a mantle plume origin. To constrain the thermal regime of the ELIP and contemporaneous magmatic activity in the northeastern Sichuan Basin, maximum paleotemperature profiles of deep boreholes were reconstructed using vitrinite reflectance (Ro) and apatite fission track data. Two heating patterns were identified: (1) heating of the overlying lithosphere by magma storage regions and/or magmatic activity related to the mantle plume, which resulted in a relatively strong geothermal field and (2) direct heating of country rock by stock or basalt. Borehole Ro data and reconstructed maximum paleotemperature profiles near the ELIP exhibit abrupt tectonothermal unconformities between the Middle and Late Permian. The profiles in the lower subsections (i.e., pre-Middle Permian) exhibited significantly higher gradients than those in the upper subsections. Distal to the basalt province, high paleo-geotemperatures (hereafter, paleotemperatures) were inferred, despite deformation of the paleogeothermal curve due to deep faults and igneous rocks within the boreholes. In contrast, Ro profiles from boreholes without igneous rocks (i.e., Late Permian) contained no break at the unconformity. Paleotemperature gradients of the upper and the lower subsections and erosion at the Middle/Late Permian unconformity revealed variations in the thermal regime. The inferred spatial distribution of the paleothermal regime and the erosion magnitudes record the magmatic and tectonic-thermal response to the Emeishan mantle plume.

Distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs) are frequently employed for reconstructing terrestrial paleotemperatures from lake sediment archives. Although brGDGTs are globally ubiquitous, the microbial producers of these membrane lipids remain unknown, precluding a full understanding of the ways in which environmental parameters control their production and distribution. Here, we advance this understanding in three ways. First, we present 43 new high-latitude lake sites characterized by low mean annual air temperatures (MATs) and high seasonality, filling an 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 and nonlinear 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 errors from the standard approach. We also find that it reveals novel patterns in brGDGT distributions and provides a methodology for investigating the biological underpinnings of their structural diversity. Warm-season temperature indices outperformed MAT in our regressions, with the mean temperature of months above freezing yielding the highest-performing model (adjusted R2 = 0.91, RMSE = 1.97 ∘C, n = 182). The natural logarithm 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 new proxy for paleohydrology applications. We recommend these calibrations for use in lake sediments globally, including at high latitudes, and detail the advantages and disadvantages of each.

Qarhan Salt Lake, located in the Qaidam Basin on the Qinghai-Tibet Plateau in northwestern China, is the largest potash production base in China. The main potash deposit in Qarhan Salt Lake is a comprehensive deposit in which the liquid potassium resources dominated by potassium-rich and lithium-rich brine coexist with the low-grade solid potassium salt deposit composed of solid potash minerals (polyhalite, carnallite, sylvite). Polyhalite is one of the representative potassium minerals, widely distributed in the Bieletan area of Qarhan Salt Lake, coexisting with halite, and its deposition indicates that the paleo-temperature of brine in this area once fluctuated obviously. Twenty core samples from two boreholes in the Bieletan area of the Qarhan Salt Lake were subjected to fluid inclusions thermometry, and homogenization temperatures reflecting the brine temperatures during the precipitation of evaporite minerals in the Salt Lake were obtained, ranging from 12.4 to 28.9 °C and 9.2 to 19.8 °C, respectively. By scanning electron microscopy, acicular polyhalite and pompom-like aggregate of polyhalite have been identified in the sampling layer, indicating that evaporite deposition have reached the stage of potassium salt deposition. The temperature fluctuations of the fluid inclusions and the elemental chemical data are compared vertically, and the sections with significant temperature fluctuations are close to the layers where polyhalite were deposited. It shows that the paleo-temperature fluctuation impact the sedimentary environment of brine, and then affected the polyhalite deposition of Qarhan Salt Lake.