Age Model Uncertainties Research Articles

Revising chronological uncertainties in marine archives using global anthropogenic signals: a case study on the oceanic 13C Suess effect

Abstract. Marine sediments are excellent archives for reconstructing past changes in climate and ocean circulation. Overlapping with instrumental records, they hold the potential to elucidate natural variability and contextualize current changes. Yet, dating uncertainties of traditional approaches (e.g., up to ± 30–50 years for the last 2 centuries) pose major challenges for integrating the shorter instrumental records with these extended marine archives. Hence, robust sediment chronologies are crucial, and most existing age model constraints do not provide sufficient age control, particularly for the 20th century, which is the most critical period for comparing proxy records to historical changes. Here we propose a novel chronostratigraphic approach that uses anthropogenic signals such as the oceanic 13C Suess effect and spheroidal carbonaceous fly-ash particles to reduce age model uncertainties in high-resolution marine archives. As a test, we apply this new approach to a marine sediment core located at the Gardar Drift, in the subpolar North Atlantic, and revise the previously published age model for this site. We further provide a refined estimate of regional reservoir corrections and uncertainties for Gardar Drift.

Evaluating manual versus automated benthic foraminiferal δ18O alignment techniques for developing chronostratigraphies in marine sediment records

Abstract. Paleoceanographic interpretations of Plio-Pleistocene climate variability over the past 5 million years rely on the evaluation of event timing of proxy changes in sparse records across multiple ocean basins. In turn, orbital-scale chronostratigraphic controls for these records are often built from stratigraphic alignment of benthic foraminiferal stable oxygen isotope (δ18O) records to a preferred dated target stack or composite. This chronostratigraphic age model approach yields age model uncertainties associated with alignment method, target selection, the assumption that the undated record and target experienced synchronous changes in benthic foraminiferal δ18O values, and the assumption that any possible stratigraphic discontinuities within the undated record have been appropriately identified. However, these age model uncertainties and their impact on paleoceanographic interpretations are seldom reported or discussed. Here, we investigate and discuss these uncertainties for conventional manual and automated tuning techniques based on benthic foraminiferal δ18O records and evaluate their impact on sedimentary age models over the past 3.5 Myr using three sedimentary benthic foraminiferal δ18O records as case studies. In one case study, we present a new benthic foraminiferal δ18O record for International Ocean Discovery Program (IODP) Site U1541 (54°13′ S, 125°25′ W), recently recovered from the South Pacific on IODP Expedition 383. The other two case studies examine published benthic foraminiferal δ18O records of Ocean Drilling Program (ODP) Site 1090 and the ODP Site 980/981 composite. Our analysis suggests average age uncertainties of 3 to 5 kyr associated with manually derived versus automated alignment, 1 to 3 kyr associated with automated probabilistic alignment itself, and 2 to 6 kyr associated with the choice of tuning target. Age uncertainties are higher near stratigraphic segment ends and where local benthic foraminiferal δ18O stratigraphy differs from the tuning target. We conclude with recommendations for community best practices for the development and characterization of age uncertainty of sediment core chronostratigraphies based on benthic foraminiferal δ18O records.

Tropical decadal variability in nutrient supply and phytoplankton community in the Central Equatorial Pacific during the late Holocene

We have reconstructed baseline δ15N and δ13C of export production at Kingman Reef in the Central Equatorial Pacific (CEP) at sub-decadal resolution, nearly continuously over the last 2000 years. The changes in δ15N reflects the strength of the North Equatorial Counter Current (NECC) relative to the South Equatorial Current (SEC), and to a lesser extent, the North Equatorial Current (NEC). Seasonal to multi-decadal variation in the strength of these currents, through the redistribution of heat, have global climate impacts and influence marine and terrestrial ecosystems. We use modern El Niño-La Nina dynamics and the Tropical Pacific Decadal Variability (TPDV) pattern, which is defined in the CEP, as a framework for analyzing the isotopic data. The CEP δ15N and δ13C records exhibit multi-decadal (50–60 year) variability consistent with TPDV. A large multi-centennial feature in the CEP δ15N data, within age-model uncertainties, is consistent with one of the prolonged dry-pluvial sequences in the American west at the end of the Medieval Climate Anomaly, where low TPDV is correlated with drier conditions. This unique record shows that the strength of the NECC, as reflected in baseline δ15N and δ13C, has at quasi-predictable intervals throughout the late Holocene, toggled the phytoplankton community between prokaryotes and picoplankton versus eukaryotes.

Quantifying Benthic δ18O Lags Across Termination 1: A Probabilistic Approach Based on Radiocarbon and Benthic δ18O Chronologies

AbstractTemporal offsets (“lags”) between benthic δ18O (δ18Ob) signals in different locations are not only a source of age model uncertainty during δ18Ob stratigraphic alignment but also provide an opportunity to improve reconstructions of deep ocean circulation change during Termination 1 (T1). While methods based on the visual identification of identical δ18Ob features have previously been used to estimate lags, here we present a novel method to calculate δ18Ob lags and their statistical uncertainties by subtracting a core's radiocarbon age model from an age model based on its δ18Ob alignment to a target stack. This method produces lag estimates as a function of time with statistical uncertainties. As an example of this approach, we calculate lags for a depth transect of 12 cores in the Brazil Margin, whose lags were described qualitatively by Lund et al. (2015, https://doi.org/10.1002/2014PA002657). Our new methodology supports a previously described lag between mid‐depth (1,802–2,296 m) and deep (2,500–2,951 m) water; specifically, we find a statistically significant lag across this boundary throughout T1, with a maximum of 2.08 kyr (95% credible interval: 1.36–2.85) at 14 ka BP. We also identify statistically significant lags between intermediate (1,105–1,627 m) and mid‐depth cores that were not previously described. Furthermore, we demonstrate that the Brazil Margin δ18Ob lags during T1 are inconsistent with a tracer transport model under modern circulation pathways (Gebbie, 2012, https://doi.org/10.1029/2011PA002273) that fits an Atlantic‐Pacific δ18Ob lag.

North American pollen records provide evidence for macroscale ecological changes in the Anthropocene

Recent global changes associated with anthropogenic activities are impacting ecological systems globally, giving rise to the Anthropocene. Critical reorganization of biological communities and biodiversity loss are expected to accelerate as anthropogenic global change continues. Long-term records offer context for understanding baseline conditions and those trajectories that are beyond the range of normal fluctuation seen over recent millennia: Are we causing changes that are fundamentally different from changes in the past? Using a rich dataset of late Quaternary pollen records, stored in the open-access and community-curated Neotoma database, we analyzed changes in biodiversity and community composition since the end Pleistocene in North America. We measured taxonomic richness, short-term taxonomic loss and gain, first/last appearances (FAD/LAD), and abrupt community change. For all analyses, we incorporated age-model uncertainty and accounted for differences in sample size to generate conservative estimates. The most prominent signals of elevated vegetation change were seen during the Pleistocene-Holocene transition and since 200 calendar years before present (cal YBP). During the Pleistocene-Holocene transition, abrupt changes and FADs were elevated, and from 200 to -50 cal YBP, we found increases in short-term taxonomic loss, FADs, LADs, and abrupt changes. Taxonomic richness declined from ~13,000 cal YBP until about 6,000 cal YBP and then increased until the present, reaching levels seen during the end Pleistocene. Regionally, patterns were highly variable. These results show that recent changes associated with anthropogenic impacts are comparable to the landscape changes that took place as we moved from a glacial to interglacial world.

A Meta‐Analysis of Studies Attributing Significance to Solar Irradiance

AbstractThe relationship between solar irradiance and climate is greatly debated. This inferred relationship is often characterized via the statistical analysis of paleoclimate data. REDFIT is a commonly used statistical tool that overcomes uneven sampling to identify significant periodicities of variability in proxy data. We critically examine the use of REDFIT to identify solar signals in these data. By conducting a literature review, we show the REDFIT significance thresholds used by researchers to analyze paleoclimate data vary considerably. As there is some subjectivity and practicality involved in any statistical analysis, some variability is to be expected. However, we observe that the bulk of the significance thresholds used in the literature are less stringent than the critical false‐alarm level outlined by REDFIT's creators. We reexamine periodicities deemed “significant” in a published data set to show that using this more stringent, more objective critical false‐alarm threshold likely eliminates the previously inferred significance of solar signals in proxy data. Likewise, we address a lack of consideration of age model uncertainty on REDFIT's reliability in identifying solar periodicities. Overall, we show that the relationship between solar irradiance and climate, as identified by REDFIT analyses, may not be as robust as previous work might suggest.

Comparing age model techniques for the last glacial cycle: A case study of ten Iberian Margin sediment cores

Because interpretations of paleoclimate records from ocean sediment cores rely on age models to estimate age as a function of core depth, age model uncertainty can affect the conclusions drawn from paleoclimate data. To assess the age model uncertainty associated with three methods, we compare the age models they generate over the last glacial cycle for high-resolution data from ten western and southern Iberian Margin cores. The first method uses radiocarbon data and the Bayesian statistical software Bacon [Blaauw and Christen, 2011]. The second method aligns benthic δ18O to a target core using the probabilistic alignment algorithm HMM-Match [Lin et al., 2014]. The third method is alignment of planktonic δ18O or sea surface temperature (SST) using the deterministic alignment software Match [Lisiecki and Lisiecki, 2002]. Where three or more age model types overlap in a core, the average range of age estimates is 1.7 kyr, and 90% of age ranges are less than 3 kyr. For the two probabilistic methods, HMM-Match and Bacon, age estimates agree to within uncertainty, and the highest resolution records yield 95% confidence interval widths of 1–2 kyr. However, HMM-Match appears to underestimate alignment uncertainty near gaps in benthic δ18O data in one core, as indicated by disagreement with the core's planktonic δ18O and SST alignments. Planktonic δ18O appears to provide the most precise age models for Iberian Margin cores due to strong millennial-scale variability and good correlation between aligned cores. Probabilistic alignment algorithms should be developed for planktonic δ18O and SST proxies to improve estimates of the age uncertainty for these alignments.

Strong Southern African Monsoon and weak Mozambique Channel throughflow during Heinrich events: Implication for Agulhas leakage

The Mozambique Channel is a conduit of trade wind-driven throughflow that is a key component of the Agulhas Current and Agulhas leakage, a flux of warm and salty water from the tropical Indo-Pacific to the Atlantic Ocean. Agulhas leakage is thought to modulate Atlantic meridional overturning circulation variability. Previous studies from the Cape Basin suggest that enhanced Agulhas leakage played an important role in accelerating glacial terminations. The southern African monsoon response to abrupt climate changes associated with meltwater-induced reorganizations of the North Atlantic meridional overturning circulation, and its impact on the Mozambique Channel throughflow and, by extension, on the Agulhas leakage is not well understood. Here we present a high-resolution 26,000 year-long hydroclimate record of northern Madagascar, a core region of the southern hemisphere monsoon domain, and a mixed layer temperature reconstruction using sediment cores collected from the runoff-influenced eastern Mozambique Channel. The record indicates precipitation increases centered at 11.7-12.5 thousand years before present (kyr BP), 14.5-19 kyr BP, 23-24 kyr BP, 25-26 kyr BP. Considering age model uncertainties, this is the first strong evidence for southern African monsoon strengthening in response to meltwater-induced northern high latitude climate instabilities during the Younger Dryas (YD), Heinrich Stadial 1 (HS1), HS2 and the HS-like event prior to HS2, in agreement with the results of transient climate simulations. Furthermore, our study shows a reversal of the mixed layer temperature gradient between the western and eastern Mozambique Channel during Heinrich event 1 (HE1). We posit that the gradient reversal indicates a weakening of the trade wind-driven South Equatorial Current and Mozambique Channel throughflow that likely weakened the Agulhas leakage, potentially creating a feedback that may have contributed to the sustained weakening of the AMOC during HE1 by reducing the amount of heat and salt leakage into the Atlantic.

Optimizing sampling strategies in high-resolution paleoclimate records

Abstract. The aim of paleoclimate studies resolving climate variability from noisy proxy records can in essence be reduced to a statistical problem. The challenge is to extract meaningful information about climate variability from these records by reducing measurement uncertainty through combining measurements for proxies while retaining the temporal resolution needed to assess the timing and duration of variations in climate parameters. In this study, we explore the limits of this compromise by testing different methods for combining proxy data (smoothing, binning, and sample size optimization) on a particularly challenging paleoclimate problem: resolving seasonal variability in stable isotope records. We test and evaluate the effects of changes in the seasonal temperature and the hydrological cycle as well as changes in the accretion rate of the archive and parameters such as sampling resolution and age model uncertainty in the reliability of seasonality reconstructions based on clumped and oxygen isotope analyses in 33 real and virtual datasets. Our results show that strategic combinations of clumped isotope analyses can significantly improve the accuracy of seasonality reconstructions compared to conventional stable oxygen isotope analyses, especially in settings in which the isotopic composition of the water is poorly constrained. Smoothing data using a moving average often leads to an apparent dampening of the seasonal cycle, significantly reducing the accuracy of reconstructions. A statistical sample size optimization protocol yields more precise results than smoothing. However, the most accurate results are obtained through monthly binning of proxy data, especially in cases in which growth rate or water composition cycles obscure the seasonal temperature cycle. Our analysis of a wide range of natural situations reveals that the effect of temperature seasonality on oxygen isotope records almost invariably exceeds that of changes in water composition. Thus, in most cases, oxygen isotope records allow reliable identification of growth seasonality as a basis for age modeling in the absence of independent chronological markers in the record. These specific findings allow us to formulate general recommendations for sampling and combining data in paleoclimate research and have implications beyond the reconstruction of seasonality. We briefly discuss the implications of our results for solving common problems in paleoclimatology and stratigraphy.

Global Ocean Sediment Composition and Burial Flux in the Deep Sea

AbstractQuantitative knowledge about the burial of sedimentary components at the seafloor has wide‐ranging implications in ocean science, from global climate to continental weathering. The use of 230Th‐normalized fluxes reduces uncertainties that many prior studies faced by accounting for the effects of sediment redistribution by bottom currents and minimizing the impact of age model uncertainty. Here we employ a recently compiled global data set of 230Th‐normalized fluxes with an updated database of seafloor surface sediment composition to derive atlases of the deep‐sea burial flux of calcium carbonate, biogenic opal, total organic carbon (TOC), nonbiogenic material, iron, mercury, and excess barium (Baxs). The spatial patterns of major component burial are mainly consistent with prior work, but the new quantitative estimates allow evaluations of deep‐sea budgets. Our integrated deep‐sea burial fluxes are 136 Tg C/yr CaCO3, 153 Tg Si/yr opal, 20Tg C/yr TOC, 220 Mg Hg/yr, and 2.6 Tg Baxs/yr. This opal flux is roughly a factor of 2 increase over previous estimates, with important implications for the global Si cycle. Sedimentary Fe fluxes reflect a mixture of sources including lithogenic material, hydrothermal inputs and authigenic phases. The fluxes of some commonly used paleo‐productivity proxies (TOC, biogenic opal, and Baxs) are not well‐correlated geographically with satellite‐based productivity estimates. Our new compilation of sedimentary fluxes provides detailed regional and global information, which will help refine the understanding of sediment preservation.

GeoChronR – an R package to model, analyze, and visualize age-uncertain data

Abstract. Chronological uncertainty is a hallmark of the paleoenvironmental sciences and geosciences. While many tools have been made available to researchers to quantify age uncertainties suitable for various settings and assumptions, disparate tools and output formats often discourage integrative approaches. In addition, associated tasks like propagating age-model uncertainties to subsequent analyses, and visualizing the results, have received comparatively little attention in the literature and available software. Here, we describe geoChronR, an open-source R package to facilitate these tasks. geoChronR is built around an emerging data standard (Linked PaleoData, or LiPD) and offers access to four popular age-modeling techniques (Bacon, BChron, OxCal, BAM). The output of these models is used to conduct ensemble data analysis, quantifying the impact of chronological uncertainties on common analyses like correlation, regression, principal component, and spectral analyses by repeating the analysis across a large collection of plausible age models. We present five real-world use cases to illustrate how geoChronR may be used to facilitate these tasks, visualize the results in intuitive ways, and store the results for further analysis, promoting transparency and reusability.

Palaeo-productivity record from Norwegian Sea enables North Atlantic Oscillation (NAO) reconstruction for the last 8000 years

The North Atlantic Oscillation is the dominant atmospheric driver of North Atlantic climate variability with phases corresponding to droughts and cold spells in Europe. Here, we exploit a suggested anti-correlation of North Atlantic Oscillation-phase and north-eastern North Atlantic primary productivity by investigation of south-eastern Norwegian Sea sediment cores spanning the last 8000 years. Age model uncertainties between 2 and 13 years for the period 1992–1850 AD allows for the proxy to observational data calibration. Our data suggest that Ca/Fe core-scanning results reflect sedimentary CaCO3 variability in the region. Cross-correlating the Ca/Fe record with nearby phytoplankton counts and dissolved O2 data suggests that Ca/Fe can be used as a proxy for primary productivity variability in the region. Our data support an anti-correlation of primary productivity to the winter North Atlantic Oscillation index. Hence, we propose a sub-decadally resolved palaeo-North Atlantic Oscillation reconstruction based on an open-ocean record spanning the last 8000 years.

SISALv2: a comprehensive speleothem isotope database with multiple age–depth models

Abstract. Characterizing the temporal uncertainty in palaeoclimate records is crucial for analysing past climate change, correlating climate events between records, assessing climate periodicities, identifying potential triggers and evaluating climate model simulations. The first global compilation of speleothem isotope records by the SISAL (Speleothem Isotope Synthesis and Analysis) working group showed that age model uncertainties are not systematically reported in the published literature, and these are only available for a limited number of records (ca. 15 %, n=107/691). To improve the usefulness of the SISAL database, we have (i) improved the database's spatio-temporal coverage and (ii) created new chronologies using seven different approaches for age–depth modelling. We have applied these alternative chronologies to the records from the first version of the SISAL database (SISALv1) and to new records compiled since the release of SISALv1. This paper documents the necessary changes in the structure of the SISAL database to accommodate the inclusion of the new age models and their uncertainties as well as the expansion of the database to include new records and the quality-control measures applied. This paper also documents the age–depth model approaches used to calculate the new chronologies. The updated version of the SISAL database (SISALv2) contains isotopic data from 691 speleothem records from 294 cave sites and new age–depth models, including age–depth temporal uncertainties for 512 speleothems. SISALv2 is available at https://doi.org/10.17864/1947.256 (Comas-Bru et al., 2020a).

Deglacial temperature controls on no-analog community establishment in the Great Lakes Region

Understanding the drivers of vegetation dynamics and no-analog communities in eastern North America is hampered by a scarcity of independent temperature indicators. We present a new branched glycerol dialkyl glycerol tetraether (brGDGT) temperature record from Bonnet Lake, Ohio (18–8 ka) and report uncertainty estimates based on Bayesian linear regression and bootstrapping. We also reanalyze a previously published brGDGT record from Silver Lake, Ohio, using improved chromatographic methods. All pollen- and brGDGT-based temperature reconstructions showed qualitatively similar deglacial trends but varying magnitudes. Separating 5- and 6- methyl brGDGTs resulted in substantially lower estimates of deglacial temperature variations (6.4 °C) than inferred from earlier brGDGT methods and pollen (11.8 °C, 12.0 °C respectively). Similar trends among proxies suggest good fidelity of brGDGTs to temperature, despite calibration uncertainties. At both sites, the rise and decline of no-analog communities closely track brGDGT-inferred temperatures, with a lag of 0–150 years. The timing of temperature and ecological events varies between Bonnet and Silver Lakes, likely due to age model uncertainties. Climate sensitivity analyses indicate a linear sensitivity of vegetation composition to temperature variations, albeit noisy and significant only with a 500-year bin. The formation of no-analog plant communities in the upper Midwest is closely linked to late-glacial warming, but other factors, such as temperature seasonality or end-Pleistocene megafaunal extinctions, remain viable.

Astronomical time scale of the Turonian constrained by multiple paleoclimate proxies

One of the clocks that record the Earth history is (quasi-) periodic astronomical cycles. These cycles influence the climate that can be ultimately stored in sedimentary rocks. By cracking these (quasi-) periodic sedimentation signals, high resolution astronomical time scale (ATS) can be obtained. Paleoclimate proxies are widely used to extract astronomical cycles. However different proxies may respond differently to astronomical signals and non-astronomical noises including tectonics, diagenesis, and measurement error among others. Astronomical time scale constructed based on a single proxy where its signal-to-noise ratio is low may have uncertainty that is difficult to evaluate but can be revealed by utilizing other proxies. Here, we test eight astronomical age models using two astrochronological methods from four paleoclimate proxies (i.e., color reflection L* and b*, natural gamma radiation, and bulk density) from the Turonian to the Coniacian of the Cretaceous Period at the Demerara Rise in the equatorial Atlantic. The two astrochronological methods are time calibration using long eccentricity bandpass filtering (E1 bandpass) and tracking the long eccentricity from evolutive harmonic analysis (tracking EHA). The statistical mean and standard deviation of four age models from the four proxies are calculated to construct one integrated age model with age uncertainty in each method. Results demonstrate that extracting astronomical signals from multiple paleoclimate proxies is a valid method to estimate age model uncertainties. Anchored at the Cenomanian/Turonian boundary with an age of 93.9 ​± ​0.15 ​Ma from biostratigraphy, the ages for CC11/CC12 (calcareous nannofossil zones), Turonian/Coniacian (CC12/CC13), CC13/CC14, and Coniacian/Santonian boundaries are 91.25 ​± ​0.20 ​Ma, 89.87 ​± ​0.20 ​Ma, 86.36 ​± ​0.33 ​Ma, and 86.03 ​± ​0.32 ​Ma in E1 bandpass method, compared with 91.17 ​± ​0.36 ​Ma, 89.74 ​± ​0.38 ​Ma, 86.13 ​± ​1.31 ​Ma, and 85.80 ​± ​1.33 ​Ma respectively in tracking EHA method. These results are consistent with previous studies within error and provide a reliable estimation of uncertainties of the ages.

The response of a dune succession from Lençóis Maranhenses, NE Brazil, to climate changes between MIS 3 and MIS 2

In tropical Southern Hemisphere South America, late Quaternary marine core and speleothem records provide different proxies and accurate chronologies correlating millennial-scale intervals of increased precipitation with southward shifts of the Intertropical Convergence Zone (ITCZ). However, the climatic effect of these intervals on coastal landforms is poorly understood. Here we investigated the response of abrupt and long-lasting climate events during the Marine Isotope Stages 3 and 2 in a 13.8-m thick sand succession located in a large-scale coastal eolian system, the Lençóis Maranhenses Dunefield, NE Brazil, where winds and precipitation are anti-phased and controlled by the single action of the ITCZ. A chronology of the sediment overburden determined by optically stimulated luminescence (OSL) dating of 14 sediment samples obtained at 1-m intervals showed ages between 132.2 ± 7.0 and 12.9 ± 0.6 thousand years ago in stratigraphic order. A multi-proxy approach based on grain size, surface grain texture, heavy minerals, thermoluminescence (TL) sensitivity, inorganic geochemistry, reflectance, and magnetic parameters from 268 sediment samples collected at 5-cm intervals indicated periods of dune building and stabilization. Significant stabilization periods are synchronous with Heinrich Stadials 6, 4, 3, 2, and 1, and Greenland Stadial 4, within age model uncertainties. Heavy mineral analysis indicated a steady sand source to the dune succession, while TL sensitivity analysis of quartz grains showed the input allochthonous sediments during stadials. The preservation of peak dune activity during the Last Glacial Maximum is attributed to the enduring precipitation brought by the subsequent Heinrich Stadial 1 event.

Detection of Strong Precession Cycles from the Late Pliocene Sedimentary Records of Northeastern Tibetan Plateau

AbstractRecent studies demonstrate that aridification is sensitive to eccentricity forcing based on the northeastern Tibetan Plateau Cenozoic fluvio‐lacustrine stratigraphic records. However, it is unknown whether this is a bias associated with the fact that higher frequency periods normally have larger age model uncertainties, which tend to decrease spectral power of higher frequency orbital cycles (precession and obliquity) and enlarge lower frequency ones (eccentricity). Here we detect strong and well‐resolved (~23 and ~19 kyr) precession cycles in the magnetic susceptibility record of late Pliocene fluvio‐lacustrine sediments from the Qaidam Basin, northwestern China. We interpret the magnetic susceptibility as a proxy for the degree of chemical weathering in this region, based on the positive correlation between low‐frequency magnetic susceptibility and frequency‐dependent magnetic susceptibility. The different responses of magnetic susceptibility and a halite content proxy to orbital forcing suggest that aridification is more sensitive to eccentricity forcing than precipitation, but precipitation is more sensitive to precession forcing. This study indicates that different climatic aspects have different sensitivity to orbital forcing. Furthermore, a comparison of the magnetic susceptibility variations in the western Qaidam Basin and global ice volume proxy records suggests ice volume control of Qaidam Basin precipitation and chemical weathering at million‐year timescales.

Enhanced Himalayan Glacial Melting During YD and H1 Recorded in the Northern Bay of Bengal

AbstractOcean‐land thermal feedback mechanisms in the Indian Summer Monsoon (ISM) domain are an important but not well understood component of regional climate dynamics. Here we present a δ18O record analyzed in the mixed‐layer dwelling planktonic foraminifer Globigerinoides ruber (sensu stricto) from the northernmost Bay of Bengal (BoB). The δ18O time series provides a spatially integrated measure of monsoonal precipitation and Himalayan meltwater runoff into the northern BoB and reveals two brief episodes of anomalously low δ18O values between 16.3±0.4 and 16±0.5 and 12.6±0.4 and 12.3±0.4 thousand years before present. The timing of these events is centered at Heinrich event 1 and the Younger Dryas, well‐known phases of weak northern hemisphere monsoon systems. Numerical climate model experiments, simulating Heinrich event‐like conditions, suggest a surface warming over the monsoon‐dominated Himalaya and foreland in response to ISM weakening. Corroborating the simulation results, our analysis of published moraine exposure ages in the monsoon‐dominated Himalaya indicates enhanced glacier retreats that, considering age model uncertainties, coincide and overlap with the episodes of anomalously low δ18O values in the northernmost BoB. Our climate proxy and simulation results provide insights into past regional climate dynamics, suggesting reduced cloud cover, increased solar radiation, and air warming of the Himalaya and foreland areas and, as a result, glacier mass losses in response to weakened ISM.

Correlating paleoclimate time series: Sources of uncertainty and potential pitfalls

Comparing paleoclimate time series is complicated by a variety of typical features, including irregular sampling, age model uncertainty (e.g., errors due to interpolation between radiocarbon sampling points) and time uncertainty (uncertainty in calibration), which—taken together—result in unequal and uncertain observation times of the individual time series to be correlated. Several methods have been proposed to approximate the joint probability distribution needed to estimate correlations, most of which rely either on interpolation or temporal downsampling.Here, we compare the performance of some popular approximation methods using synthetic data resembling common properties of real world marine sediment records. Correlations are determined by estimating the parameters of a bivariate Gaussian model from the data using Markov Chain Monte Carlo sampling. We complement our pseudoproxy experiments by applying the same methodology to a pair of marine benthic δ18 O records from the Atlantic Ocean.We find that methods based upon interpolation yield better results in terms of precision and accuracy than those which reduce the number of observations. In all cases, the specific characteristics of the studied time series are, however, more important than the choice of a particular interpolation method. Relevant features include the number of observations, the persistence of each record, and the imposed coupling strength between the paired series. In most of our pseudoproxy experiments, uncertainty in observation times introduces less additional uncertainty than unequal sampling and errors in observation times do. Thus, it can be reasonable to rely on published time scales as long as calibration uncertainties are not known.

The Role of Uncertainty in Estimating Lead/Lag Relationships in Marine Sedimentary Archives: A Case Study From the Tropical Pacific

AbstractUnderstanding the mechanisms behind any changes in the climate system often requires establishing the timing of events imprinted on the geological record. However, these proxy records are prone to large uncertainties, which may preclude meaningful conclusions about the relative timing of events. In this study, we put forth a framework to estimate the uncertainty in phase relationships inferred from marine sedimentary records. The novelty of our method lies in the accounting of the various sources of uncertainty inherent to paleoclimate reconstruction and timing analysis. Specifically, we use a Monte Carlo process allowing sampling of possible realizations of the time series as functions of uncertainties in time, the climate proxy, and the identification of the termination timing. We then apply this technique to 15 published sea surface temperature records from the equatorial Pacific to evaluate whether we observed any significant changes in the termination timing between the east and the west. We find that the uncertainty on the relative timing estimates is on the order of several thousand years and mainly stems from age model uncertainty (90%). However, even small differences in mean termination timings can be detected with a sufficiently large number of samples. Improvements in the dating of sediment records provide an opportunity to reduce uncertainty in studies of this kind.