Isotope Record Research Articles

Insights Into the Terminal Ediacaran Marine Carbonate Record From Shale-Hosted Carbonate Carbon Isotopes

The marine carbon isotope record (δ13C) used for chemostratigraphy and reconstruction of carbon cycle dynamics is commonly assembled using carbonate rocks. There is, however, evidence that carbonate cements hosted within fine-grained clastics (shales and mudstones) in some settings may also express δ13C trends that covary with the record from carbonates. We present new carbon and oxygen isotopic data from shale-hosted carbonate cements (herein termed δ13Ccarb-sh and δ18Ocarb-sh, n = 107, <16 wt% CaCO3) of the terminal Ediacaran Nama Group, Namibia (≥550.5 to <539.6 Million years ago; Ma). These data are compared with the published carbon and oxygen isotopic record from coeval carbonates (δ13Ccarb and δ18Ocarb, n = 1611) and total organic carbon (TOC) concentrations. We show that, in the Nama Group, δ13Ccarb-sh compositions in samples of intermediate to high CaCO3/TOC (>0.4) can approximate contemporaneous δ13Ccarb in open marine mixed carbonate-clastic settings. By contrast, δ13Ccarb-sh values in samples with low CaCO3/TOC (<0.4) that were deposited in clastic settings distant from the locus of carbonate deposition are more negative than contemporaneous δ13Ccarb. These data suggest that δ13Ccarb-sh may approach seawater composition in samples with low TOC when deposited in settings characterized by high CO32- concentration, where carbonate can rapidly precipitate from seawater during early diagenesis. However, the use of δ13Ccarb-sh to infill gaps in the existing δ13Ccarb record remains uncertain, even when these criteria are fulfilled. Intervals of δ13C-δ18O co-variability in the Nama Group succession appear to correlate with units where seawater mixing with meteoric fluids was more likely during early diagenesis, such as clastic-dominated settings, which also show significant decreasing δ18O through time with gradual sub-basin infill. We further consider uncertainties in lithostratigraphic correlation of the upper Urusis Formation of the Nama Group that enable three new possible correlations to be proposed for δ13Ccarb-sh data within the terminal Ediacaran to lower Cambrian (<542.65 Ma to >532 Ma) regional and global δ13Ccarb records.

The strength, position, and width changes of the intertropical convergence zone since the Last Glacial Maximum.

The intertropical convergence zone (ITCZ) plays a key role in regulating tropical hydroclimate and global water cycle through changes in its convection strength, latitudinal position, and width. The long-term variability of the ITCZ, along with the corresponding driving mechanisms, however, remains obscure, mainly because it is difficult to separate different ITCZ variables in paleoclimate proxy records. Here, we report a speleothem oxygen isotope (δ18O) record from southwestern Sulawesi, Indonesia, and compile it with other speleothem records from the Maritime Continent. Using the spatial gradient of speleothem δ18O along a transect across the ITCZ, we constrain ITCZ variabilities over the Maritime Continent during the past 30,000 y. We find that ITCZ convection strength overall intensified from the last glacial period to the Holocene, following changes in climate boundary conditions. The mean position of the regional ITCZ has moved latitudinally no more than 3° in the past 30,000 y, consistent with the deduction from the atmospheric energy framework. However, different from modern observations and model simulations for future warming, the ITCZ appeared narrower during both the late Holocene and most part of the last glacial period, and its expansion occurred during Heinrich stadials and the early-to-mid Holocene. We also find that during the last glacial and deglacial period, prominent millennial-scale ITCZ changes were closely tied to the variability of the Atlantic meridional overturning circulation (AMOC), whereas during the Holocene, they were predominantly modulated by the long-term variability of the Walker circulation.

Mediterranean water in the Atlantic Iberian margin reveals early isolation events during the Messinian Salinity Crisis

Recent studies highlight the role of the Mediterranean Outflow Water (MOW), in the intensification of the Atlantic Meridional Overturning Circulation and as source of heat and salty water to high latitudes. During the Late Miocene the MOW suffered major changes and likely a total collapse during the Messinian Salinity Crisis (MSC). In order to study the MOW evolution in the Atlantic margin during the Tortonian-Messinian interval we completed a new high resolution geochemical and stable isotope record for the corresponding interval of the Montemayor-1 and Huelva-1 cores. Both sites are located in the Guadalquivir Basin on the former Atlantic side of the Mediterranean – Atlantic gateways (Iberian Atlantic margin) during the late Miocene. The tuning of this isotope record with astronomical solutions and other global isotope curves has allowed the establishment of an improved chronology and, consequently, to precisely date environmental changes happening on the Atlantic margin of the Iberian peninsula and their link to Mediterranean and global events. At 7.17 Ma, in concomitance with a shallowing of the basin, the residence time, temperature and salinity of the bottom waters increased. These changes were related to a reduction of the MOW reaching the Atlantic side as a consequence of the restriction of the last strand of the Betic corridor that connected the Mediterranean and the Atlantic. This hypothesis is in line with the analogous changes observed in several Mediterranean Sea locations, where from 7.17 Ma onward a reduced Mediterranean – Atlantic connection is observable. Furthermore, the new isotope chronology sheds light, through comparison with other records, on the age of Messinian geomagnetic reversals.

Evolution of the alpine Critical Zone since the Last Glacial Period using Li isotopes from lake sediments

Comprehending and predicting the way humans affect the Earth's Critical Zone remains a challenge. An understanding of the past changes resulting from human and non-human influences in the dynamics of the Critical Zone is crucial. Here, we use a retrospective approach to address this question based on a new lithium (Li) isotope record from the Late Glacial Period to the present from a pre-Alpine lake sediment sequence (Lake La Thuile, France). Coupled with the lake sediment archive, the investigation of present-day soils in the lake catchment suggests that lake sediments are not necessarily recording the erosoin of topsoil in the catchment. Our findings indicate that soil particles can be sorted during transportation to the lake, with finer particles being preferentially mobilized, highlighting the influence of fine particle transport on the Li isotope signature of soils and lake sediments. Characterized by low Li isotope signatures, changes in weathering signatures in lake sediments can be amplified by the combined effect of soil development and selective transport. In the La Thuile catchment, soil development was limited during the Late Glacial Period, whereas it became a dominant process during the Holocene climatic optimum together with enhanced selective transport of fine particles. Human activities since 3,000–4,000 yr cal BP induced a strong perturbation hindering both soil formation and selective transport by reinforcing erosion rates. After a period of topsoil destruction caused by intense deforestation and agriculture, lake Li isotopes record the evolution of soil profiles associated with changes in agricultural practices.

Variations in Mid-Pleistocene glacial cycles: New insights from osmium isotopes

Reconstruction of Pleistocene glacial cycles from oxygen isotope records identifies dramatic changes in both global climate state and ice sheet size. The Mid-Pleistocene Transition (MPT) is an interval during which global ice sheets underwent a major transition from temporally symmetrical 41-thousand-year (kyr) cycles in the Early to Middle Pleistocene to asymmetrical 100-kyr cycles in the Middle to Late Pleistocene. The proposed timing of this transition and potential triggers remains contentious with possible mechanisms ranging from cryosphere dynamics to variations in Earth's energy balance. One suggested driver of the MPT involves the “Regolith Hypothesis”, which proposes that the Laurentide Ice Sheet (LIS) underwent fluctuations in ice sheet stability due to changes in subglacial substrate lithology. To empirically investigate this hypothesis, we use variations in seawater osmium (Os) isotope values from IODP sites U1302 and U1308 over an 800 kyr interval to evaluate changes in basal substrate across this transition. Our Os data identify a lithological shift beneath the LIS during the MPT that resulted in a marked change in the chemical weathering flux delivered to the ocean. We identify a pulse of glacially weathered material reaching the central North Atlantic, likely during the first Heinrich event at Marine Isotope Stage (MIS) 16, in response to increased ice sheet size post MPT. After MIS 16, the Os signal becomes less radiogenic which may correspond to a less extensive “missing glacial” which would have reduced overall weathering fluxes. Our Pleistocene Os isotope record (at ∼15 kyr resolution) contributes to our understanding of subglacial weathering of the LIS across the MPT and helps further constrain the flux of Os to the ocean during glacial-interglacial cycles.

Methane Source Attribution in the UK Using Multi‐Year Records of CH4 and δ13C

AbstractIsotopic measurements of atmospheric methane are valuable for the verification of bottom‐up atmospheric emissions inventories. The balance of sources in emissions inventories must be consistent with the δ13C‐CH4 isotopic record in the air. Long‐term records of both methane mole fraction and δ13C from five sites across the UK are presented, showing post‐2007 growth in CH4 and negative trend in δ13C, consistent with global background sites. Miller‐Tans analyses of atmospheric measurements identified that the δ13C signature of the methane source mix varied between −50.1 and −56.1‰, with less depleted δ13C signatures at sites receiving air from urban areas, consistent with an increased proportion of thermogenic sources. Isotopic signatures calculated for all sites are more enriched than those expected from the bottom‐up emissions inventory, suggesting that inventories for the UK either underestimate contributions of thermogenic/pyrogenic emissions or overestimate biogenic sources.

Estimating Plio‐Pleistocene North African Monsoon Runoff Into the Mediterranean Sea and Temperature Impacts

AbstractSapropels are dark, organic‐rich layers found in Mediterranean sediments that formed during periods of bottom water anoxia. While various mechanisms have been proposed to have caused anoxic conditions, a primary factor is considered to be water column stratification induced by freshwater runoff related to intensified North African monsoon precipitation during precession minima. Monsoon intensification also induced Green Sahara Periods that may have impacted North African hominin dispersal. In this study, we present a novel regression‐based deconvolution of a high‐resolution planktonic foraminiferal oxygen isotope record to estimate the combination of freshwater runoff reaching the eastern Mediterranean and associated surface warming of the water column over the past 5 million years. Sapropels are known to occur in clusters associated with periods of high orbital eccentricity. Our analysis reveals a consistent influence of orbital eccentricity in modulating the North African monsoon, and a possible shift in runoff source area induced by the initiation of Northern Hemisphere ice sheets. Our findings provide important insights into the role of the North African monsoon in shaping Mediterranean environmental changes over the past 5 million years.

Assessing earlywood-latewood proportion influence on tree-ring stable isotopes

Tree-ring stable isotopes are typically measured in latewood cellulose to mitigate potential carry-over effects from previous year storage pools. The isotopic composition of individual tree-ring segments is thought to include considerable intra-annual variability. This sampling strategy may be complicated by steep intra-annual isotope gradients that can rival the inter-annual variability, however. Consistent sampling of latewood material may not always be possible due to low sample availability or high prevalence of narrow rings or low amounts of latewood because of species-specific changes in ring width. Therefore, years that contain samples with higher portions of non-latewood (earlywood) material may influence the final chronology of isotopic variability. Here, we analyze the potential influence that changing earlywood and latewood components of individual tree rings can have on stable carbon and oxygen records from Quercus spp. and Pinus heldreichii chronologies. Analysis of stable isotopes in oak tree rings with varying amounts of latewood show no statistically significant differences in the range of isotopic composition, nor any major differences when considering the same calendric year. Similar results were found for the pine data, when comparing stable isotope measurements with earlywood-to-latewood ratio and maximum density. We argue that this simple approach should be applied to any long-term tree-ring stable isotope record in order to provide a better understanding of the potential biases that could arise from previously recorded intra-annual variability in the wood.

Mid-Miocene terrestrial carbon isotope shift driven by atmospheric CO2 in the Xining Basin, NE Tibetan Plateau

The Mid-Miocene Climatic Optimum (MMCO; ~17–15 Ma) was a relatively recent warming event unrelated to human activity that provides an ideal analogue for our global warming future. However, the terrestrial ecosystem response to the MMCO remains unclear because of limited records. Continuous sedimentation from the Tashan Borehole (TSB) in the Xining Basin, northeastern Tibetan Plateau (NETP) spans the entire MMCO, with dates of 18.5–13.5 Ma from high-resolution paleomagnetic studies. Here, we explore the organic carbon isotope (δ13CTOC) record of TSB to investigate the NETP terrestrial ecosystem response to the MMCO. The sediments are typical floodplain deposits with δ13CTOC values ranging from −28.03‰ to −25.16‰, indicating terrestrial C3 plants were the dominant NETP terrestrial vegetation during MMCO. Further, an obviously negative shift of 0.6‰ occurred in TSB δ13CTOC during ~17.6–14.2 Ma. TSB δ13CTOC values show the opposite trend to global benthic δ13C values, both in the long-term Monterey Excursion event and short-term carbon maxima events. Increased atmospheric CO2 during the MMCO enhanced the carbon isotope fractionation through photosynthesis by terrestrial plants and resulted in more 12C-enriched organic matter. Similar processes on a global scale subsequently accounted for part of the Monterey carbon isotope excursion. Therefore, in warm and humid conditions, the carbon isotope cycle of terrestrial C3 plants in the Xining Basin was mainly controlled by atmospheric CO2 concentration, with superimposition of effects of enhanced East Asian summer monsoon precipitation.

Carbonate clumped isotopes record the heterogenous onset of metamorphism in the Alta stock contact aureole, Utah

Carbonate clumped isotopes record the heterogenous onset of metamorphism in the Alta stock contact aureole, Utah

Winter hydroclimate contrasts between southern and northern Europe at ∼ 5.2 ka

Winter climate variations in midlatitude regions have far-reaching effects on both natural and human systems worldwide. However, most climate reconstructions focus on either annual or summer climate conditions. Here we present a stalagmite stable isotope record from Urşilor Cave (Romania) where the δ18O values are controlled by winter temperature. Because of its East-Central Europe (ECE) location, the cave site is well suited to capture variations in precipitation and temperature in response to changes in the North Atlantic region. Present day precipitation and temperature in this region are controlled by two teleconnection patterns, the Arctic Oscillation (AO) and the East Atlantic (EA). By analyzing composite maps for four different scenarios of AO and EA phase, we suggest that between 5.6 and 5.2 ka the positive phases of AO and EA became increasingly dominant. This shift led to generally warmer winters in northern Europe, but drier conditions in southern Europe and the Levant. Considering the compound effect of AO and EA on the hydroclimate of ECE, our study highlights the importance of examining the combined impact of teleconnection patterns on climate proxies influenced by various forcing mechanisms.

Archean to Paleoproterozoic magmatic and metamorphic evolution of the Lofoten – Vesterålen area in northern Norway through zircon U-Pb and Hf isotopic analysis

Neoarchean (2.8–2.5 Ga) and Paleoproterozoic (2.5–1.6 Ga) crust preserves a record of major crustal growth events, the oldest supercontinent assemblies, and the likely transition to modern plate tectonics. The Lofoten–Vesterålen area in Norway underwent a multistage tectono-magmatic history from the Neoarchean to the Paleoproterozoic. In this region three main components are distinguished: 1) a granulite facies zone (GFZ); 2) an amphibolite facies zone (AFZ); and 3) an anorthosite-mangerite-charnockite-granite (AMCG) suite. New zircon U-Pb and Lu-Hf isotope data are used to investigate the evolution of this 2.7–2.6 and 1.9–1.8 Ga crust and evaluate how the developments fit in the global context of Earth’s crustal evolution. In-situ LASS–MC-ICPMS analyses were conducted on zircons to simultaneously determine U-Pb ages and Hf isotope compositions. Solution Hf isotope compositions were additionally obtained on zircons previously dated by ID-TIMS. In general, there is a good agreement between solution and in-situ data. Samples from the GFZ and AFZ both yield Neoarchean (2.7–2.6 Ga) and Paleoproterozoic (1.9–1.8 Ga) populations. Neoarchean populations have εHf(i) of +0.9 to −0.3; Paleoproterozoic populations have εHf(i) around −16 and −7 at ∼1.80 and ∼1.87 Ga, respectively. The AMCG suite was emplaced in two magmatic pulses: the early one at 1.87–1.86 Ga characterized by εHf(∼1.87 Ga) ∼ −7, and the main pulse at 1.80–1.79 Ga has εHf(∼1.80 Ga) between −4.0 and −15.0. Metamorphic overprinting recorded within the GFZ and the AMCG at ∼1.80 Ga is absent in the AFZ, supporting the hypothesis of a tectonic boundary between these domains. The Hf isotope compositions suggest that Paleoproterozoic magmatism was sourced from a combination of juvenile material and incorporated ancient crust, probably in variable proportions, as suggested by the variation in the isotope composition of the AMCG suite. Broadly chondritic Hf isotope compositions in Neoarchean rocks indicate the absence of significantly older crust as a source component. Main crustal growth events recorded in the Lofoten–Vesterålen area coincide with global age peaks and zircon Hf isotope record. The AMCG suite was emplaced during the Columbia supercontinent assembly. Neoarchean and Paleoproterozoic crust in the Lofoten–Vesterålen area exemplifies the elevated preservation potential in collisional tectonic settings.

Drivers of δ18O Variability Preserved in Ice Cores From Earth's Highest Tropical Mountain

AbstractIn 2019, four ice cores were recovered from the world's highest tropical mountain, Nevado Huascarán (Cordillera Blanca, Peru; 9.11°S, 77.61°W). Composite hydroclimate records of the two Col cores (6,050 masl) and the two Summit cores (6,768 masl) are compared to gridded gauge‐analysis and reanalysis climate data for the most recent 60‐year. Spatiotemporal correlation analyses suggest that the ice core oxygen stable isotope (δ18O) record largely reflects tropical Pacific climate variability, particularly in the NINO3.4 region. By extension, the δ18O record is strongly related to rainfall over the Amazon Basin, as teleconnections between the El Niño Southern Oscillation and hydrological behavior are the main drivers of the fractionation of water isotopes. However, on a local scale, modulation of the stable water isotopes appears to be more closely governed by upper atmospheric temperatures than by rainfall amount. Over the last 60 years, the statistical significance of the climate/δ18O relationship has been increasing contemporaneously with the atmospheric and oceanic warming rates and shifts in the Walker circulation. Isotopic records from the Summit appear to be more sensitive to large‐scale temperature changes than the records from the Col. These results may have substantial implications for modeling studies of the behavior of water isotopes at high elevations in the tropical Andes.

Rapid Expansion of Fixed Nitrogen Deficit in the Eastern Pacific Ocean Revealed by 50‐Year Time Series

AbstractClimate change is expected to increase the strength of ocean Oxygen Deficient Zones (ODZs), but we lack a detailed understanding of the temporal or spatial variability of these ODZs. A 50‐year time series in the Eastern Tropical North Pacific (ETNP) ODZ revealed that it has strengthened by 30% from 1994 to 2019. We subdivided the ODZ into a core and a deep layer based on potential density and revealed that different processes control the magnitude of fixed nitrogen loss between these regions. We postulate that the depth of the upper ETNP ODZ water mass, the 13°C Water, influences the organic carbon supply to the core ODZ and therefore its strength. We correlated the maximum fixed nitrogen loss in the core ODZ with a nearby sedimentary nitrogen isotope record and found that this recent increase in the magnitude of fixed nitrogen loss occurred only a few times over the last 1,200 years. Using this correlation, we derived the first confidence interval for the natural variability of the maximum fixed nitrogen loss within the ETNP ODZ, which has a range of 3.3 μmol kg−1 (p = 0.01). While the current increase is only comparable to two previous events, it is within the confidence interval for natural variability (p = 0.03). The deep ODZ also strengthened from 2016 to 2019 by approximately 30%, but this increase occurred more rapidly than the core ODZ, and this dramatic increase was not observed over the rest of the 40 years. Climate‐driven intensification could lead to unprecedented changes in the ETNP ODZ within the next decade.

Precisely constrained 134-ka strong monsoon event in the penultimate deglaciation by an annually laminated speleothem from the Asian monsoon domain

AbstractThe penultimate deglaciation was characterized by a sub-millennial-scale warm event in the Heinrich Stadial 11(HS11), termed the 134-ka event. However, its precise timing and structure remain poorly constrained due to the lack of high-resolution and precisely dated records. We present an oxygen isotope record of a speleothem with well-developed annual lamina from Zhangjia Cave, located on the north margin of the Sichuan Basin, characterizing Asian summer monsoon (ASM) changes in the 134-ka event, which included an increase excursion of ca. 149 years and decrease excursion of ca. 200 years, inferred from 3.3‰ δ18O variations. This event also divided the weak ASM interval-II (WMI-II), corresponding to HS11, into two stages, the WMI-IIa 132.8–134.1 ka and WMI-IIb 134.4–136.4 ka. With a comparable climatic pattern globally, the 134-ka event is essentially similar to the millennial-scale events in last glacial–deglacial period. Particularly, the observed weak-strong-weak ASM sequence (138.8–132.8 ka) is largely controlled by changes in the Atlantic Meridional Overturning Circulation (AMOC) forced by the meltwater of northern high-latitude ice sheets. Moreover, our results underpin that AMOC, rather than the global ice volume, is more critical to ASM variations during the last two deglaciations.

Tracking the formation and evolution of the Ordos Block basement, North China Craton: U-Pb age and Lu-Hf isotope record of detrital zircons from the early Mesoproterozoic sandstones

The Ordos Block (OB) is generally considered to be an important micro-block concerning the early geological evolution of the North China Craton (NCC). Previous geophysical and limited geological researches for the OB basement have provided a preliminary understanding of the lithological assemblages and geological structure of the basement, but little was discussed about its formation time and early Precambrian evolution. In this contribution, to better understand the formation and evolution of the OB basement, the new LA-ICP-MS U-Pb ages, Lu-Hf isotope, and trace element composition have been carried for the detrital zircons from the sandstones of the Changcheng System overlying the OB basement. The youngest concordant U-Pb age constrains the Changcheng System to be deposited after ∼1.7 Ga, consistent with the Changcheng System in the other regions of the NCC. The 3.6–3.3 Ga detrital zircons preserved in the sandstones from the Changcheng System, associated with the 3.4 Ga inherited zircon in basement rocks, indicate the potential presence of Paleoarchean crustal materials in the OB. The 2.8–2.6 Ga zircons with positive εHf(t) values (+0.6 to +6.4) have the two-stage model ages (TDMC = 3.23–2.75 Ga) close to their formation ages, combined with the crustal thickening during this period, suggesting a major crustal growth in the early Neoarchean. In addition, the main age clusters of the 2.5–2.4 Ga, 2.2–2.0 Ga, and 1.95–1.8 Ga yield εHf(t) values of −6.9 to +7.7 with TDMC of 3.39–2.47 Ga, −13.4 to +7.5 with TDMC of 3.49–2.19 Ga, and −14.0 to +2.2 with TDMC of 3.41–2.40 Ga, respectively, indicative of three main stages of crustal reworking associated with two stage of minor juvenile crustal growth around 2.5–2.4 Ga and 2.2–2.0 Ga. They were corresponding to partial melting of the continental crust in the late Neoarchean, magmatism resulting from the plate subduction in the mid-Paleoproterozoic, and the collision and amalgamation between eastern and western blocks in the late Paleoproterozoic, respectively. All demonstrate that the OB was an ancient block formed mainly in the early Neoarchean and then subjected to three strong tectonothermal events of 2.5–2.4 Ga, 2.2–2.0 Ga, and 1.95–1.8 Ga, which occurred in the peripheral orogenic belts.

Temperature variability over the past three centuries in the coastal region of Princess Elizabeth Land, Antarctica, reconstructed from ice core stable isotope records

High-resolution stable isotope records obtained from Antarctic ice cores can be used as proxies to investigate past climatic changes in Antarctica, overcoming the spatiotemporal limitations of observational and instrumental records. Here, we used a new high-resolution ice core stable isotope record (1709–2001 AD) obtained from the Lambert Glacier Basin 69 (LGB69) site and a published stable isotope record (1757–1987 AD) from coastal Princess Elizabeth Land (PEL) to reconstruct temperature variability in the coastal region of PEL over the past three centuries. The dominant moisture source region for coastal PEL is mid-high latitudes of the South Indian Ocean (SIO) and this has remained stable over the past three centuries. Owing to non-climatic noise, no statistically significant isotope‒temperature relationship can be evidenced at annual-to-decadal scales. However, the ice core δD records do enable us to reconstruct multi-decadal temperature variability in the coastal region of PEL. The temperature reconstructions showed a consistent warming trend during the 20th century, whereas the cold periods prior to the 20th century may be related to the Little Ice Age (LIA). Our reconstructions exhibit high reliability, evidenced by their agreement with previous reconstructions from PEL. We also found that annual temperature variability is related to the Southern Annular Mode (SAM), whereas multi-decadal temperature variability is related to the Indian Ocean Dipole (IOD). However, the relationship between the SAM and temperature is unclear before the 1970s. The results of this study are helpful in understanding long-term temperature changes in Antarctica and provide an important reference for the attribution of temperature changes.

Seawater carbon and strontium isotope variations in the eastern Tarim Basin during the Cambrian: Implications for connection with the open ocean

Cambrian was a critical period in Earth's history, marked by rapid diversification of early animals and significant changes in oceanic conditions. Marine sediments, particularly carbonates, have been used to study variations in the global carbon cycle, seawater redox, weathering input, etc. In search of new archives of Cambrian paleo-seawater chemistry, we conducted an integrated study of Cambrian C and Sr isotope stratigraphy using the carbonates of the Xingdi section in the Kruqtagh region of the eastern Tarim Basin. Based on reported paleontological data, the C isotope record is comparable to the Cambrian global seawater C isotope variation, providing a feasible stratigraphic framework for the Cambrian strata in the eastern Tarim Basin. A sharp δ13C increase at the top of the Xidasan Formation probably corresponds to the recovery of the Redlichiid–Olenellid Extinction C isotope Excursion. Additionally, a negative C isotope excursion in the lower part of the Mohuosan Formation and a positive C isotope excursion at the bottom of the Tuersaketagh Formation likely correspond to DrumIan Carbon isotope Excursion and Steptoean Positive Carbon Isotope Excursion, respectively. However, the Sr isotope curve in the eastern Tarim Basin decoupled from global seawater Sr isotope variations during Cambrian Series 2–3 until Furongian, indicating that the eastern Tarim Basin was not well connected with the open ocean until Furongian. It also highlights a significant impact of paleogeographical connections of basins with the open ocean on their seawater Sr isotopic signal.

New Geochronological Age Constraint and Chemostratigraphy for Aptian Lacustrine Strata, Cedar Mountain Formation, Utah

AbstractThe Early Cretaceous is an important time of transition in Earth history, marked by a succession of oceanic anoxic events and carbon cycle perturbations that drove changes on land and in the ocean. The need for more precise geochronologic constraints in terrestrial sediments of Early Cretaceous age that record faunal and floral transitions is especially critical. The Cedar Mountain Formation (CMF) is a continental lithostratigraphic unit that hosts a trove of paleoclimate archives and important dinosaurian fossil localities. Determining the timing of deposition of CMF strata has been an ongoing effort for many years. Here, we present new lithostratigraphic and carbon isotope chemostratigraphic data along with high‐precision radiometric ages to further constrain the Ruby Ranch Member of the CMF at a unique locality referred to as “Lake Carpenter,” where a thick section of dominantly lacustrine strata overlies fluvial‐overbank to palustrine strata more typical of other Ruby Ranch Member outcrops. A bentonite bed near the base of the section provides one of the most precise ages yet determined within the Ruby Ranch Member of 115.92 ± 0.14 Ma via CA‐ID‐TIMS U‐Pb analysis of zircons. The age and the trends in the carbon isotope record indicate that the Lake Carpenter sediments were deposited entirely within the late Aptian Stage. These unique new data provide an important step toward improving our understanding of the timing of Early Cretaceous evolutionary and paleoclimate events.

Carbonate-associated sulfate as an archive of sulfur and oxygen stable isotope composition of seawater sulfate: Evidence from reef carbonate rocks of the southern South China Sea

Carbonate-associated sulfate (CAS) is a useful proxy for the reconstruction of the isotopic composition of ancient seawater sulfate, archiving information on the global sulfur cycle, Earth's surface redox evolution, and biological activity. The CAS proxy is possibly compromised by early diagenetic alteration, which may cause secondary inhomogeneities in the isotopic composition of pore water sulfate achieved in the form of CAS. However, the effects of early diagenesis on the retention of primary δ34S signals of CAS in reef carbonates are not well understood, and it is unknown whether reef carbonate deposits can faithfully record the coeval isotopic composition of marine sulfate. To provide new constraints on the resilience of CAS isotopic signatures of reef carbonate, we have measured and analyzed CAS content and CAS isotopic composition of reef carbonates from the Meiji Atoll (well NK-1) of the South China Sea, representing carbonates affected by meteoric and marine diagenesis including dolomitization. CAS contents were found to decrease rapidly during meteoric and marine diagenesis, with minimum CAS contents caused by prolonged subaerial exposure. Despite low contents, the corresponding δ34SCAS values were found to vary within a narrow range from 21.5 to 24.1‰, compared to values of 20.9 to 22.7‰ of coeval barite, suggesting that the original δ34SCAS values are preserved and largely unaffected by recrystallization. However, δ18OCAS values were found to vary widely, ranging from 7.1‰ to 15.0‰, only recording the same oxygen isotope signal as coeval marine barite for reef carbonates younger than 5 Ma. Our data reveal that reef carbonates can be used as an archive of paleoceanic change, faithfully tracking δ34S changes in coeval seawater regardless of whether reef carbonates underwent recrystallization, including dolomitization. In contrast to sulfur isotopes, the oxygen isotope composition of CAS is more susceptible to alteration during diagenesis. Overall, this study provides new insight for the future reconstruction of the sulfate sulfur and oxygen isotope record of ancient seawater.