Sedimentary Proxies Research Articles

Disentangling the Evidence of Milankovitch Forcing From Tree-Ring and Sedimentary Records

Tree-ring records constitute excellent high-resolution data and provide valuable information for climate science and paleoclimatology. Tree-ring reconstructions of past temperature variations agree to show evidence for annual-to-centennial anomalies in past climate and place the industrial-era warming in the context of the late Holocene climate patterns and regimes. Despite their wide use in paleoclimate research, however, tree rings have also been deemed unsuitable as low-frequency indicators of past climate. The arising debate concerns whether the millennia-long tree-ring records show signals of orbital forcing due to the Milankovitch cycles. Here, we produce a summer-temperature reconstruction from tree-ring chronology running through mid- and late-Holocene times (since 5486 BCE) comprising minimum blue channel light intensity (BI). The BI reconstruction correlates with existing and new tree-ring chronologies built from maximum latewood density (MXD) and, unlike the MXD data, shows temperature trends on Milankovitch scales comparable to various types of sedimentary proxy across the circumpolar Arctic. Our results demonstrate an unrevealed potential of novel, unconventional tree-ring variables to contribute to geoscience and climate research by their capability to provide paleoclimate estimates from inter-annual scales up to those relevant to orbital forcing.

Nitrogen burial characteristics of Quaternary sediments and its controls on high ammonium groundwater in the Central Yangtze River Basin

As the strata sedimentary process proceeds, considerable amounts of nitrogen (N) is buried in sediments, which controls the sources and fate of N in the “groundwater-sediment” system. However, there is little concern regarding N burial characteristics in continuous sediment profiles from surface layer to deep aquifer thus far. In this study, lithology, grain size, geochronology, exchangeable N contents and geochemical proxies of sediments were analyzed to reveal the controlling mechanisms of N burial characteristics in Quaternary sediments and to interpret the enrichment of N in groundwater of central Yangtze River Basin. The results demonstrated a similar distribution trend for buried N in two sedimentary cores, which were high in the surface layer and decreased to stable in the deep aquifer. Excessive exchangeable N (EX-N) contents in sediments were mainly attributed to geologic origin. The N burial characteristics were controlled by the evolution of depositional environment: sedimentary facies determined the concentrations of total organic nitrogen (TON), further affecting the mineralization capacity of sediments; while paleoclimate regulated the intensity of the N transformation processes, ultimately influencing the actual concentrations of EX-N in sediments. In addition, due to the fast accumulation of alluvial deposits after Last Glacial Maximum and rapid development of Jianghan Lake Groups during Holocene, abundant organic matter (with high TON contents) was buried in sediments, which were still able to produce more ammonium or nitrate, and further posing continuous threats to groundwater quality. This study provided a new interpretation for the formation of high-ammonium aquifer in terms of depositional evolution.

Plant Macrofossils Reveal Aquatic Macrophyte Successions of a Typical Shallow Lake (Huanggai Lake, China) in the Past Century.

Aquatic macrophytes are one of the important biotic components of shallow lake ecosystems. Understanding the long-term evolution of the macrophyte community is crucial for lake management. Huanggai Lake, a typical shallow lake in the middle reach of the Yangtze River, was selected as the research site for this study. Based on 210Pb/137Cs dating, aquatic plant macrofossils were used to reconstruct the succession of aquatic macrophytes in the past century. Our results show that the lake maintained a consistent natural state before 1940, with a relatively low abundance of aquatic plants dominated by species such as Najas minor. From 1940 to 1974, human activities gradually intensified in the lake leading to the emergence of eutrophic species such as Potamogeton maackianus, along with the increasing abundance of other emergent and floating aquatic macrophytes. Since 1974, more pollution-resistant, emergent species such as Potamogeton maackianus and Potamogeton crispus have become dominant. The abundance of aquatic macrophytes reached its maximum in the early 1990s. Combined with macrofossil succession and other multiple sedimentary proxy analyses, driving mechanisms for aquatic macrophytes are discussed. Both the nearby Liangzi Lake and Huanggai Lake share many common features of aquatic plant evolution. This study is the first of its kind to use plant macrofossils (with identifiable images) as a proxy for aquatic macrophyte succession in a shallow Yangtze lake. In absence of long-term monitoring records, this study highlights the increased application of plant macrofossils for reconstructing the vegetation dynamics and restoration of degraded lakes exposed to severe anthropogenic impacts over the past century.

Investigation of Modern n-Alkanes in Daihai Lake Basin, Northern China: Implications for the Interpretation of Paleoclimate Research

n-Alkanes are one of the most used proxies in lake sediments to reconstruct past climate change. However, the distribution and concentration of n-alkanes are controlled by multiple factors, and their interpretation across northern China has revealed obvious discrepancies. It is therefore important to investigate the controlling factors of n-alkane proxies before using them for paleoclimate reconstruction. In this study, we collected fresh plant leaves, basin surface soils, lake surface sediments, and a short sediment core (DH20B) in the Daihai Lake basin to analyze the paleoclimate implications of n-alkanes. Our results show that long-chain (C27–C35) n-alkanes in Daihai Lake are dominated by allochthonous sources. The average chain length of long-chain n-alkanes (ACL27–35) and total long-chain n-alkane concentration (∑alklong-chain) of DH20B are significantly correlated with regional summer temperature (r = 0.54, p < 0.01) and summer precipitation (r = 0.41, p < 0.05) over the past 60 years. These results indicate that ACL27–35 and ∑alklong-chain from Daihai Lake sediments have the potential to reconstruct past summer temperature and summer precipitation, respectively, because higher summer temperature promotes the synthesis of longer-chain n-alkanes to reduce water loss (leading to higher ACL27–35) and increased summer precipitation promotes plant growth (leading to higher ∑alklong-chain). Moreover, we found that human activity significantly affected ∑alklong-chain through cultivation and grazing after 2005. Our findings may have broad significance for paleoclimate reconstruction of other hydrologically closed lakes, highlighting the importance of proxy validation studies.

Quantitative Link Between Sedimentary Chlorin and Sea-Surface Chlorophyll-a.

Primary productivity in the ocean plays a major role in the global carbon cycle. To estimate its changes through geological time, different sedimentary proxies are used. However, the relative weights of the various processes driving the sedimentary accumulation of organic matter are not fully constrained or represent the flux of specific algal classes. Here, we compare sea‐surface chlorophyll‐a (SSchla) abundance estimated from remote sensing data over the last 20 years with the sedimentary concentration of its derivatives (i.e., chlorin) on a suite of 140 core‐top sediments from different biogeochemical regions. We estimate with field data that only 0.33% of SSchla in tropical and subtropical regions is transferred to surface sediments in the form of chlorin. Despite the small fraction of chlorin that arrive to the sea‐floor, the sedimentary spatial distribution of chlorin is driven primarily by SSchla concentration in high and moderate productivity locations (SSchla > 0.20 mg·m−3). Our calibration paves the way for the use of chlorin as quantitative proxies of primary productivity in paleoreconstructions and cautions on their use in low primary productivity settings.

Isotopic and traits-based trophic diversity of Canadian Beaufort Sea benthic communities in relation to food supply

In the Arctic, climate-driven alterations to the quality and quantity of organic matter reaching the seafloor will likely affect benthic food web function. We used biomass-weighted diversity measures based on trophic traits (those related to feeding behaviours, e.g., mobility, size, food source), stable isotope ratios (δ15N and δ13C), and taxonomy to assess linkages between benthic food web structure and indicators of food supply in the shelf and slope ecosystems of the Canadian Beaufort Sea and Amundsen Gulf (down to 500 m). Benthic communities across both regions used a similar diversity of sedimentary organic matter sources according to stable isotopes, despite known differences between regions in organic matter input and benthic–pelagic coupling. Shelf-edge and upper-slope communities exhibited relatively high trophic trait diversity, which likely reflected the exploitation of pulsed food inputs associated with dynamic processes at the shelf break that affect the production and advection of organic matter to the benthos. Pairwise relationships between trophic traits and sedimentary proxies of benthic food supply were not significant at the regional scale. However, cluster analyses supported the notion that trophic trait composition was influenced by proximity to the shelf edge, where food supply is influenced by episodic events that may not be reflected by in situ sedimentary proxies. Our findings add to growing evidence that evaluating trophic traits composition may provide more information regarding functional responses to changes in benthic food supply than either isotopic or taxonomic diversity indices alone.

Eutrophication and Deoxygenation Forcing of Marginal Marine Organic Carbon Burial During the PETM.

The Paleocene‐Eocene Thermal Maximum (PETM) is recognized globally by a negative excursion in stable carbon isotope ratios (δ13C) in sedimentary records, termed the carbon isotope excursion (CIE). Based on the CIE, the cause, duration, and mechanisms of recovery of the event have been assessed. Here, we focus on the role of increased organic carbon burial on continental margins as a key driver of CO2 drawdown and global exogenic δ13C during the recovery phase. Using new and previously published sediment proxy data, we show evidence for widespread enhanced primary production, low oxygen waters, and high organic carbon (Corg) burial in marginal and restricted environments throughout the δ13C excursion. With a new biogeochemical box model for deep and marginal environments, we show that increased phosphorus availability and water column stratification on continental margins can explain the increased Corg burial during the PETM. Deoxygenation and recycling of phosphorus relative to Corg were relatively mild, compared to modern day anoxic marine systems. Our model reproduces the conditions reconstructed by field data, resulting in a burial of 6,000 Pg across the PETM, in excess of late Paleocene burial, and ∼3,300 Pg C for the critical first 40 kyr of the recovery, primarily located on continental margins. This value is consistent with prior data and model estimates (∼2,000–3,000 Pg C). To reproduce global exogenic δ13C patterns, this Corg burial implies an injection of 5,000–10,000 Pg C during the first ∼100–150 kyr of the PETM, depending on the source's δ13C (−11‰ to −55‰).

Multi-proxy characterization of storm deposits on Sanibel Island, Florida: A modern analog for paleotempestology

Hurricanes have serious impacts on human lives and infrastructure, especially as a result of flooding caused by storm surge and precipitation. To better prepare coastal populations for future hurricanes due to an increasingly warming world, a better understanding of hurricane storm surge is key. Southwest Florida is particularly vulnerable to hurricane storm surge where most of the coastline is within 2.5 m of sea level and population is on the rise (2.5% between 2019 and 2020 in Lee County). This study presents a geologic record of intense hurricane strikes from Sanibel Island dating back to approximately ca. 1920. Based on sedimentary proxies such as grain size, organics, moisture content, and calcium carbonate, three tempestites were identified in cores from the Sanibel Island Marsh. Radiometric dating constrained Tempestite 1 as Hurricane Donna (1960) and Tempestites 2 and 3 as the 1926 Great Miami Hurricane and the Tampa Bay hurricane of 1921. Of the eight intense hurricanes (category 3–5) taking place between ~1920–2016, only three hurricanes were recorded in the geologic record. Tempestites 1, 2, and 3 were deposited by intense, large radius storms with significant storm surge, which would indicate that at least three storms made significant impact on the Southwest Florida region between ~1920–1960. Since 1960 no storms have breached the Sanibel Island barrier. It is important for the public to recognize that even though this region has not experienced significant storm surge in the last ~60 years, several hurricanes occurring between 1920 and 1960 produced storms surges greater than ~10 ft. that were able to breach a major barrier island (Sanibel Island). Human-induced climate change is predicted to increase storminess and sea level rise in Southwest Florida with concomitant risk for future storm surge.

Arctic and sub-Arctic lake water δ2H and δ18O along a coastal-inland transect: Implications for interpreting water isotope proxy records

High-latitude lakes are sensitive to climate change and store information about large-scale circulation changes and catchment-integrated processes. Lakes are mainly recharged by meteoric water, meaning that some lake sediment proxies may indirectly archive the stable isotopic composition of hydrogen (δ2H) and oxygen (δ18O) of past precipitation. Yet, despite similar precipitation input, lakes within a region may exhibit a wide range of isotopic values due to the varying influence of inflow seasonality and evaporation. Moreover, the relative sensitivity of each lake to these controls may vary through time, something that is difficult to account for. Here, we evaluate the impact of variable inflow δ2H and evaporation on the lake water isotopic composition across northern Fennoscandia (Norway, Finland, and Sweden). We measured lake water δ2H and δ18O of 135 lakes spanning from the north Norwegian coast along a 460 km transect to the Bothnian Bay, sampled from 2018 to 2020. Our data show that both coastal and inland lakes are sensitive to distillation during moisture transport, and that lakes farther from the Atlantic Ocean are additionally impacted by evaporation. We estimated the isotopic composition of lake water inflow values for evapo-concentrated transect lakes (δ2HI) using a Bayesian method. Resampled transect lakes had more depleted δ2HI in 2020 than in 2019, indicating either that precipitation was 2H-depleted or that more winter precipitation contributed inflow to the lakes in 2020 compared to in 2019. We suggest that the more 2H-depleted values in 2020 were a response to a snow-rich winter, associated with extremely positive Arctic Oscillation (AO+) conditions and increased moisture supply from the North Atlantic. We find evidence that lake water isotopic variability in this region reflects a combination of seasonal precipitation changes associated with atmospheric circulation changes, and catchment-integrated evaporation. Careful consideration of the variable sensitivity to these processes is essential when making inferences about past climate based on lake water isotope proxies.

Lake environmental changes over the past 5000 years recorded by multiple proxies of sediments in Lake Fuxian, southwest China

Lake environmental changes over the past 5000 years recorded by multiple proxies of sediments in Lake Fuxian, southwest China

A 1900 Year Sediment Record Suggests Recent Establishment of Black Mangrove (Avicennia Germinans) Stands within a Salt Marsh in St. Augustine, Florida, USA

Salt marshes and mangroves are currently being affected by rising temperatures. Mangroves thrive below −29° N latitude in Florida, USA, and have a low tolerance for extreme cold events, whereas salt marshes dominate further north. One potential effect of climate change is a reduction in the frequency of extreme cold events, which may lead to mangrove expansion into salt marsh systems. Our research identified sediment proxy indicators of salt marsh and mangrove environments. These indicators were applied to soil cores from intertidal wetlands near the current northern limit of mangrove presence on the east coast of Florida, to determine if mangrove expansion into salt marsh environments has precedence in the deeper past. Our findings suggest that mangrove and salt marsh sediments can be distinguished using a combination of stable carbon isotope ratios of sedimentary organic matter and macroscopic plant fragments, and our results showed that a mangrove stand that we cored established only recently. This result is consistent with other work in the southeastern United States that suggests that mangroves established at the current boreal limit only recently after the end of the Little Ice Age, and that the current mangrove expansion may be fueled by anthropogenic climate change.

Marine Paleoseismic Evidence for Seismic and Aseismic Slip Along the Hayward‐Rodgers Creek Fault System in Northern San Pablo Bay

AbstractDistinguishing between seismic and aseismic fault slip in the geologic record is difficult, yet fundamental to estimating the seismic potential of faults and the likelihood of multi‐fault ruptures. We integrated chirp sub‐bottom imaging with targeted cross‐fault coring and core analyses of sedimentary proxy data to characterize vertical deformation and slip behavior within an extensional fault bend along the Hayward‐Rodgers Creek fault system in northern San Pablo Bay. We identified and traced four key seismic horizons (R1–R4), all younger than approximately 1400 CE, that cross the fault and extend throughout the basin. A stratigraphic age model was developed using detailed down‐core radiocarbon and radioisotope dating combined with measurements of anthropogenic metal concentrations. The onset of hydraulic mining within the Sierra Nevada in 1852 CE left a clear geochemical and magnetic signature within core samples. This key time horizon was used to calculate a local reservoir correction and reduce uncertainty in radiocarbon age calibration and models. Vertical fault offset of strata younger than the most recent surface‐rupturing earthquake on the Hayward fault in 1868 CE suggest near‐surface vertical creep is occurring along the fault in northern San Pablo Bay at a rate of approximately 0.4 mm/yr. In addition, we present evidence of at least one and possibly two coseismic events associated with growth strata above horizons R1 and R2, with median event ages estimated to be 1400 CE and 1800 CE, respectively. The timing of both these events overlaps with paleoseismic events on adjacent fault sections, suggesting the possibility of multi‐fault rupture.

Evolution of the inter-reef Halimeda carbonate factory in response to Holocene sea-level and environmental change in the Great Barrier Reef

The inter-reef Halimeda bioherms of the northern Great Barrier Reef (GBR) have accumulated up to 25 m of positive relief and up to four times greater volume of calcium carbonate sediment than the nearby coral reefs during the Holocene. Covering >6000 km2, the Halimeda bioherms represent a significant contribution to the development of the northeast Australian continental shelf geomorphology, neritic carbonate factory, and sedimentary archive of post-glacial environmental changes. However, the geochronological record of initiation and development of the Halimeda bioherm carbonate factory was poorly constrained and based on very few datapoints. A comprehensive age dataset is presented, comprising sixty-three new AMS radiocarbon measurements of Halimeda and foraminifera grains, mollusc shells and bulk soil from twelve inter-reef sediment cores, and ten previously published Halimeda ages. Facies transitions and environmental changes are recorded from lithological and palynological analyses spanning Last Interglacial Halimeda deposits, Last Glacial Maximum terrestrial palaeosols, transgressive mangrove vegetated shorelines, and the turn-on of the Halimeda bioherm carbonate factory. Mangrove pollen and coral records from the study area provide additional spatial, temporal and environmental context.Halimeda had established by 11.1 cal kyr BP, just 450 years after transgressive shelf inundation and approximately 1000 years earlier than previous inferred estimates. The outer-shelf carbonate factory was initially dominated by benthic foraminifera, then Halimeda was productive for at least 2100 years prior to the turn-on of Holocene coral reefs in the study area at 8.9 cal kyr BP from Boulder Reef (15.4°S, 145.4°E). Inter-reef Halimeda bioherms play a major role in carbon and nutrient cycling and potentially preserve a near-continuous geochemical record of northeast Australian Holocene oceanographic and climatic changes, filling spatial and temporal gaps not covered by coral and other marine sediment proxies.

Biogenic and anthropogenic sources of sedimentary organic matter in marine coastal areas: A multi-proxy approach based on bulk and molecular markers

The investigation of organic matter sources is an especially challenging issue for marine coastal areas influenced by both natural and anthropogenic inputs. This study focuses on the provenance of sedimentary organic matter deposited in the Gulf of Gdańsk (southern Baltic) and Oslofjord/Drammensfjord (southern Norway) based on the analysis of multiple specific proxies in recent sediments (0–5 cm layer) sampled at 12 sites in 2014. Bulk markers (total organic carbon/total nitrogen [TOC/TN] and stable organic carbon isotopes [δ13CTOC]) and various molecular geochemical markers of organic matter origin (n-alkanes, branched alkanes, unresolved complex mixture [UCM], polycyclic aromatic hydrocarbons [PAHs], hopanes, steranes, and polar steroids including sterols and stanols) were used to comprehensively assess the origin of sedimentary organic matter in the studied basins. The results of this study indicate that the combined use of TOC/TN and δ13CTOC may lead to ambiguous outcomes, and anthropogenic input may affect the sedimentary TOC/TN ratio. The analysis of molecular proxies allowed the identification of locations most impacted by anthropogenic organic matter derived from petrogenic sources and/or sewage effluents and the classification of the studied sites based on the contribution of marine/terrestrial biogenic matter. This study demonstrates that key issues, such as the forms of sedimentary chemical markers, post-depositional changes, and specific diagenetic products, although neglected by some researchers, must be considered for the reliable assessment of organic matter sources. To the best of our knowledge, this is the first such extensive study on surface sediments from the Gulf of Gdańsk and Oslofjord/Drammensfjord in terms of the use of both bulk and molecular markers to investigate organic matter sources.

Robust periodic signals in proxy records with chronological uncertainty and variable temporal resolution

The detection of periodic fluctuations is important in the quest for a deeper understanding of the drivers of past climates in the interests of being better able to understand the climate changes which are likely in the decades to come. Paleoclimatological information derived from natural archives is typically accompanied by chronological uncertainty and variable temporal resolution, both of which complicate the analysis of their time series and both are often ignored. This has, however, changed in recent years with the development of statistical tools supporting, e.g. spectral analysis tasks, which aim to take these problems into account. In cases where the original data is no longer available, it may not be possible to assess the reliability of published reports of periods detected. In this study we aim to test/model whether or not a signal for a given period can be robustly detected from a sedimentary proxy record considering its mean sampling resolution and degree of chronological uncertainty. To achieve this aim, annually sampled time series free of gaps and timescale error were modeled with white or red noise, resampled in a controlled way to simulate different time resolutions with timescale uncertainty, ultimately mimicking a real-life sedimentary record. In fact, an ensemble of potential timescales was retrieved, and their spectral characteristics explored. It was found that: (i) although sampling frequency (i.e. temporal spacing) is limiting from the side of the smallest-period, (ii) at higher mean sampling resolutions, it can ameliorate the detectability of periodic signals even in the presence of timescale error; furthermore, (iii) the increase in mean sampling resolution is less influential in an autocorrelated time series, since more information is retained due to the phenomenon of persistence. An online tool called CUSP was also developed, which gives a suggestion whether a given period can be considered to be present in a robust way utilizing our test results.

Reconstructing 1200 years of Hydroclimate Variability in the Southern Margins of the Arabian Desert: Insights From a Paleo-Lake in Southern Yemen

The climate of the Arabian Desert is not well documented during the past two millennia due to the scarcity of continuous and well-dated terrestrial archives in the region. Reliable interpretation from the climatic records from this region are pivotal for identifying periodicities of inter-annual to multi-decadal variability and trends driven by shifts in position of the Intertropical Convergence Zone (ITCZ) and the strength of the monsoons. A high-resolution multiproxy approach is presented for a ∼3.3 m composite core, GBW, from a karst lake located in Ghayl ba Wazir, southern Yemen. Sedimentary proxies, including particle size distribution, coupled with magnetic susceptibility (MS) and geochemistry (XRF), provide a comprehensive picture of sediment depositional changes that may be linked to climate and environmental variability over the southern Arabian Desert. The chronology of the GBW core is provided by five radiocarbon (14C) dates from terrestrial macrofossils (wood and twigs) extracted from sediment samples and indicates the core extends to ∼900 CE. Our data indicates generally wetter conditions from 930 to 1400 CE corresponding to the “Medieval climate anomaly (MCA)” followed by arid phases during 1,410–1700 CE coinciding with the “Little Ice Age (LIA)”. Evidence for a drier LIA include high authigenic calcium precipitation [Ca/(Al + Fe + Ti)], decreased TOC/TIC values, and gypsum precipitation, whereas the wetter MCA is characterized by higher detrital element ratios (Ti/Al, K/Al, Rb/Sr), and increased TOC/TIC and deposition of finer sediments (EM1). Furthermore, end-member mixing analyses (EMMA) derived from the grain-size distribution (EM2 and EM3) corroborates the deposition of coarser silt sediment through wind erosion and production of carbonate sand during the LIA concurrently with low lake levels under generally dry conditions. Aridity during the LIA is consistent with evidence and theory for weakened boreal summer monsoons during intervals of northern hemisphere cooling.

Nitrogen isotope evidence for a redox-stratified ocean and eustasy-driven environmental evolution during the Ordovician–Silurian transition

The Ordovician-Silurian (O/S) transition was a critical geological interval, during which significant biological events and dramatic environmental changes occurred. Extensive research has proven that changes in the ocean environment are one of the main causes of events that affect the biosphere. However, the redox structure and related geochemical evolution of the ocean during the O/S transition remain under debate. N isotopes are thought to have significant potential for further elucidating the oceanic redox structure and related geochemical evolution during the O/S transition, and are an effective supplement to the former environmental proxies of bottom sediments. This study presents high-resolution N isotope data from a well-sequenced drill core across the O/S transition from the Lower Yangtze Platform, South China, showing an interesting isotopic variation with high δ 15 N values (+1‰ to +4‰) during the Late Katian to Hirnantian ages of the Ordovician and much lower values (−1‰ to +1‰) during the Rhuddanian Age of the earliest Silurian. This δ 15 N variation is mostly controlled by ocean redox conditions and the corresponding changes in the nitrate reservoir. Combined with previously reported δ 15 N data, a redox-stratified ocean with a relatively deep redox transition zone was reconstructed according to the distribution of δ 15 N profiles across different palaeogeographic locations. The lower δ 15 N values and more reducing conditions in deep water sections demonstrate that glacio–eustatic fall, rather than increased upwelling, was likely the main driver of the global redox change during the Late Ordovician. Furthermore, a global δ 15 N decrease during the Late Katian may be associated with the debated Boda warm event, and the up to six positive δ 15 N excursions observed during the Hirnantian might suggest the cyclicity of the glaciation process. • δ 15 N can be correlated globally during the Ordovician-Silurian (O/S) transition. • Oceanic δ 15 N was controlled by water redox conditions during the O/S transition. • Positive δ 15 N excursions with a low δ 15 N baseline support a redox-stratified ocean. • δ 15 N records indicate Hirnantian redox evolution driven by eustatic sea-level fall. • Global δ 15 N variations correspond with a Boda warm event and multistage Hirnantian glaciation.

Calibrating the triple oxygen isotope signature of cultured diatoms

AbstractThe 18O/16O of precipitated silica in natural environments is posited to record the temperature of the ambient environment. Measurements of 17O/16O in silica (SiO2), when paired with 18O/16O, provide an opportunity to further constrain the environmental conditions at the time of mineral precipitation and ripening. On Cenozoic timescales, siliceous frustule precipitation by diatoms has served as the primary sink of silica from the ocean. Frustule precipitation is a biologically catalyzed process from an ocean undersaturated in SiO2. Here, we measure the 17O/16O and 18O/16O in diatom frustules from semicontinuous cultures with two different diatom species and across a range of growth rates and temperatures. To further enable these measurements, we developed a laser fluorination method to reproducibly measure the triple oxygen isotope composition of silica frustules. While the δ13C of the organic matter scaled predictably with growth rate, neither the δ18O or Δ′17O of the frustule silica varied significantly with respect to variations in growth rate or species. Temperature, however, does carry a control on the product δ18O composition. Conversely, the measured Δ′17O compositions were systematically negative with respect to the corresponding theoretical equilibrium isotope prediction, consistent with other empirical work on the SiO2 system and only weakly temperature‐dependent. This isotopic offset may be caused by the silica concentrating mechanisms diatoms use to precipitate their frustules. These results are consistent with literature arguments regarding the utility of diatom δ18O in reconstructing temperature, but suggest that Δ′17O may not be a suitable temperature proxy for diatomaceous sediment.

Data-constrained assessment of ocean circulation changes since the middle Miocene in an Earth system model

Abstract. Since the middle Miocene (15 Ma, million years ago), the Earth's climate has undergone a long-term cooling trend, characterised by a reduction in ocean temperatures of up to 7–8 ∘C. The causes of this cooling are primarily thought to be due to tectonic plate movements driving changes in large-scale ocean circulation patterns, and hence heat redistribution, in conjunction with a drop in atmospheric greenhouse gas forcing (and attendant ice-sheet growth and feedback). In this study, we assess the potential to constrain the evolving patterns of global ocean circulation and cooling over the last 15 Ma by assimilating a variety of marine sediment proxy data in an Earth system model. We do this by first compiling surface and benthic ocean temperature and benthic carbon-13 (δ13C) data in a series of seven time slices spaced at approximately 2.5 Myr intervals. We then pair this with a corresponding series of tectonic and climate boundary condition reconstructions in the cGENIE (“muffin” release) Earth system model, including alternative possibilities for an open vs. closed Central American Seaway (CAS) from 10 Ma onwards. In the cGENIE model, we explore uncertainty in greenhouse gas forcing and the magnitude of North Pacific to North Atlantic salinity flux adjustment required in the model to create an Atlantic Meridional Overturning Circulation (AMOC) of a specific strength, via a series of 12 (one for each tectonic reconstruction) 2D parameter ensembles. Each ensemble member is then tested against the observed global temperature and benthic δ13C patterns. We identify that a relatively high CO2 equivalent forcing of 1120 ppm is required at 15 Ma in cGENIE to reproduce proxy temperature estimates in the model, noting that this CO2 forcing is dependent on the cGENIE model's climate sensitivity and that it incorporates the effects of all greenhouse gases. We find that reproducing the observed long-term cooling trend requires a progressively declining greenhouse gas forcing in the model. In parallel to this, the strength of the AMOC increases with time despite a reduction in the salinity of the surface North Atlantic over the cooling period, attributable to falling intensity of the hydrological cycle and to lowering polar temperatures, both caused by CO2-driven global cooling. We also find that a closed CAS from 10 Ma to present shows better agreement between benthic δ13C patterns and our particular series of model configurations and data. A final outcome of our analysis is a pronounced ca. 1.5 ‰ decline occurring in atmospheric (and ca. 1 ‰ ocean surface) δ13C that could be used to inform future δ13C-based proxy reconstructions.

Late Holocene Climate Changes in the Altai Region Based on a First High‐Resolution Biomarker Isotope Record From Lake Khar Nuur

AbstractThe Late Holocene marks a substantial cultural and economic transition in the eastern Eurasian Steppe and Altai Region with the dispersal of nomadic pastoralism. So far, paleoclimate conditions during this time remain unclear and controversial. Here, we present a high‐resolution 4.2 ka paleoclimate record from Lake Khar Nuur in the Mongolian Altai that is based on lake sediment proxies and biomarker compound‐specific δ2H analyses. Our results document increased aridity before ∼3.7 cal. ka BP, followed by two pronounced phases of warm and wet conditions from ∼3.5–2.8 to ∼2.3–1.5 cal. ka BP, and a strong increase in aridity since ∼1.5 cal. ka BP. Phases of warmer and wetter conditions coincide with a negative North Atlantic Oscillation, which has been responsible for advecting moisture into the region by more southerly‐displaced Westerlies and possibly favored the expansion of mobile nomadic pastoralism in the region.