Proxy Record Research Articles

Reef-building Pacific oysters record seasonal variations in water mass-properties of tidal basins from the Central Wadden Sea (North Sea)

High-resolution proxy records are needed to evaluate models simulating past, present and future climatic conditions. Before such records can be reliably employed, calibration studies have to be conducted to assess the confidence intervals of the proxies. Here, we use shells of the fast growing Pacific oyster Magallana gigas from the Central Wadden Sea, North Sea, a temperate barrier island-backbarrier tidal flat-salt marsh system with large seasonal changes of water mass-properties, for the calibration of geochemical proxies. M. gigas represents a non-native invasive species that rapidly develops oyster reefs. Calcite shells of two specimens from the intertidal and subtidal zones were sampled in high resolution yielding sub-monthly data sets. The time period represented in the shell, based on δ18O age modeling, was estimated at 8–10 years and the growth of the shells was restricted from (late) spring to (early) autumn of each year. Mg/Ca, Mn/Ca and Sr/Ca ratios of the intertidal and subtidal specimens show similar seasonal patterns. Mg/Ca and Sr/Ca ratios are investigated as high-resolution sea surface temperature (SST) proxies. Important ontogenetic effects (i.e., increasing time-averaging with increasing age) as well as inter-specimen variability are discussed as limiting factors for the proxy development. Intertidal Mg/Ca ratios show only a significant correlation to the high-resolution SST record of the Central Wadden Sea when the early ontogenetic stage is considered. Sr/Ca ratios were comparable in terms of absolute values and amplitudes to those of M. gigas in the Northern Wadden Sea, but amplitudes were decreasing with increasing age. These findings seriously hamper the application of Mg/Ca and Sr/Ca for reliable palaeotemperature reconstructions regardless of ontogenetic stage. The Mn/Ca ratios were investigated as proxy for Mn cycling in tidal basins, where it is interrelated with seasonal changes in primary production. In addition to the generally observed seasonal variability of the Mn/Ca oyster records, the subtidal Mn/Ca is significantly elevated compared to intertidal Mn/Ca. The subtidal Mn/Ca offset likely reflects differences in Mn cycling in tidal settings and could, therefore, serve in the palaeorecord as indicator to differentiate inter- and subtidal habitats in the same embedding sedimentary facies. This habitat effect has to be considered as an important factor besides environmental change when interpreting the high-resolution proxy record of fossil oysters.

A Detailed Paleoclimate Proxy Record for the Middle Danube Basin Over the Last 430 kyr: A Rock Magnetic and Colorimetric Study of the Zemun Loess-Paleosol Sequence

In mid-latitude Eurasia, loess-paleosol sequences (LPS) provide the most widespread sedimentary records of Quaternary paleoenvironmental evolution. In the Middle Danube Basin (MDB), these archives cover at least the last million years of climate history, and occasionally contain archeological findings. The studied Zemun LPS is located on the right bank of the Danube in Northern Serbia. The site was declared as a protected site, based on Paleolithic artifacts found on the riverbank and stemming from unknown stratigraphic levels of the loess cliffs exposed along the Danube. The present study aims to provide a stratigraphic, paleoenvironmental, and temporal context for the Zemun LPS by means of environmental magnetic and colorimetric methods. Our investigations result in a chronostratigraphic scheme allowing direct comparison with other well-established reference records in the MDB and elsewhere. Two potential tephra layers tentatively assigned to the so-called L2 and Bag tephras, which are both widespread in the MDB and beyond were investigated for their bulk magnetic properties. The resulting integrated age model suggests that the Zemun LPS records a detailed history of a quasi-continuous accumulation of mineral dust from Marine Oxygen Isotope Stage (MIS) 11–5a (c. 430–60 ka). The outcome of our integrative approach indicates a continuous aridification over the last four interglacial/glacial cycles and we discuss potential changes in seasonality over time.

Neogene hyperaridity in Arabia drove the directions of mammalian dispersal between Africa and Eurasia

The evolution of the present-day African savannah fauna has been substantially influenced by the dispersal of Eurasian ancestors into Africa. The ancestors evolved endemically, together with the autochthonous taxa, into extant Afrotropical clades during the last 5 million years. However, it is unclear why Eurasian ancestors moved into Africa. Here we use sedimentological observations and soluble salt geochemical analyses of samples from a sedimentary sequence in Western Iran to develop a 10-million-year long proxy record of Arabian climate. We identify transient periods of Arabian hyperaridity centred 8.75, 7.78, 7.50 and 6.25 million years ago, out-of-phase with Northern African aridity. We propose that this relationship promoted unidirectional mammalian dispersals into Africa. This was followed by a sustained hyperarid period between 5.6 and 3.3 million years ago which impeded dispersals and allowed African mammalian faunas to endemically diversify into present-day clades. After this, the mid-Piacenzian warmth enabled bi-directional fauna exchange between Africa and Eurasia, which continued during the Pleistocene.

Detecting tropical cyclones from climate- and oscillation-free tree-ring width chronology of longleaf pine in south-central Georgia

The knowledge of past tropical cyclone (TC) activity is vital to understanding patterns of current and future TCs and how they will impact society, infrastructure, and the natural system. Various historical, biological, and geological proxies are commonly used to reconstruct TC behavior; however, these records vary significantly in their temporal resolution. Tree rings are known to provide high-resolution data and have been proven as a valuable proxy record of past TC activity. Here we aim to produce a TC reconstruction based on tree-ring width data of longleaf pine (Pinus palustris Mill.) located in Lake Louise near Valdosta, Georgia. Results of growth-climate analysis showed that all climate and oscillation variables influence the tree growth, and removing this climate influence may be necessary to unmask the TC signal. For this purpose, we used stepwise linear regression to iteratively model tree growth with monthly climate factors (precipitation, temperature, drought) and monthly oscillation indices (ENSO, AMO, NAO, PDO) to obtain residual chronologies. Tree-ring chronologies were compared to TC data from the National Hurricane Center's North Atlantic Hurricane Database (HURDAT2) for a 150-km buffer zone around Lake Louise between 1894 and 1999. Of the storms that entered the buffer, 13 of the 17 ≤−0.3 growth rings and 8 of the 10 ≤−0.4 growth rings occurred in the year directly after a TC event. Our results revealed the strong climate and oscillation influence in the raw chronology and no significant TC signal. However, the removal of temperature and precipitation influence from raw chronology unmasked the TC signal (p < 0.05). The TC signal was even more pronounced if influence of oscillation indices on growth had been also removed. The most pronounced TC signal was identified between most intense TCs (≥ 33 ms−1) and the climate and oscillation signal-free chronology. We emphasize that future research is needed for validation and elimination of the influence of other external growth drivers, as well as replication studies at multiple sites using similar statistical applications.

Centennial hydroclimatic and anthropogenic processes of South East South America modulate interannual and decadal river discharge

The Río de la Plata (RdlP) mud depocenter is an outstanding centennial ultra-high-resolution information source of hydroclimatic variability and terrigenous export because sedimentation attains values of 13 mmyr−1. Thus, our main objective was to inter-compare and correlate the direct hydroclimatic observations to the proxy record from the RdlP mud depocenter. We correlated continental input and marine productivity proxies to the instrumental historical hydroclimatic observations, and determined the associated periodicity and coherences of such environmental processes by means of wavelet analyses. We discuss the results in the context of both natural climate variability and anthropogenic impacts within the RdlP drainage basin. To this end, we performed a 210Pb and 14C mixed age-depth model and μXRF scanning (every 2 mm), and used Ti/Al and Ba/Al as proxies for continental input and marine productivity changes. We incorporated instrumental observations, i.e., rainfall, river discharges and climate variability modes (CVM) such as: El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). The statistical correlation analyses indicated a consistent correspondence between both proxies and rainfall anomalies for the warm season within the lower Paraná drainage basin. Such rainfall anomalies are dominated at inter-annual, decadal and multi-decadal scales by ENSO, PDO and AMO modes of variability, respectively, which together modulated river flow. Thus, centennial hydrological and proxy variability appear to be mainly modulated by the CVM. In this sense the present study is in close agreement and complementary to previous worldwide studies by showing similar results for RdlP drainage basin. We further identified a strong change in the proxy data after 1970s; i.e., the highest continental input and marine productivity record, and we related it to the combined effect of climatic variability and anthropogenic impact. Such a period is related to a dominance of positive ENSO (El Niño events) and PDO, and negative AMO, leading to increased rainfall and runoff over the drainage basin, together with intensified agricultural practices. The strong intensification of agricultural activities, mostly soy beans, together with increasing rainfall and runoff led to increased soil erosion and sediment transport, whose final fate is the RdlP mud depocenter.

Coals peripheral to opening and closure of the northern Western Interior Seaway: depositional signatures recording interplay between Milankovitch orbital forcing and salt dissolution collapse-subsidence structures with responsiveness to craton deformation and eustatic sea-level

Aptian paralic and Paleocene intracratonic mires that bracket the northern reaches of the Western Interior Seaway preserve Milankovitch orbital forcing cycle signals and record the dynamic interplay with craton-linked salt dissolution tectonism in the Alberta Basin foreland and the adjoined intracratonic Williston Basin to the southeast. These mires, now lignite-bearing strata, along the northern Seaway provide a proxy record for Milankovitch orbital signals that would not have been otherwise recorded because of the voluminous clastic sediments accumulated along the western Canadian portion of the Cretaceous Seaway, in contrast to pervasive orbital forcing signals recorded by organic-rich cyclic carbonate deposits accumulated along the central and southern reaches of the Seaway. Prior to opening of the northern segment of the Seaway, the base levels of the paralic mires of the lower McMurray Formation (Aptian) were hydraulically linked to the rising sea-level fluctuations resulting from southward transgression of the Boreal Sea. In contrast, inland fluvial-margin peats of the Paleocene Ravenscrag and Fort Union formations accumulated following the closure of the Cretaceous Seaway. These mires were increasingly sensitive to aquifer-eustatic controls on the water table–mire relationship. Orbital forcing signals were preserved in the central Williston Basin mires in contrast to their masking by salt dissolution collapse-subsidence structures that exerted dominant control on peat mire configurations across the northern Williston Basin. Paralic mires accumulated at the top of the lower McMurray Formation record short-term orbital forcing, possibly representing an obliquity cycle recorded by lithotype sequencing in petrographic profiles. Mires accumulated along the coastal areas during the transition from a lowstand system tract to an early transgressive system tract. The linkage of sea level with coastal water table fluctuations permitted pervasive accumulations of paralic mires at the top of the lower McMurray Formation. Internal mire cycles resulted in coal lithotype profiles responsive to Milankovitch fifth-order and higher sub-cycles. In contrast, concurrent cataclysmic sinkhole collapses and differential fault block displacements were responsive to underlying salt removal patterns. The sea level/water table linkage was punctuated by the salt tectonism and orbital forcing signals were masked, resulting in sinkholes infilled with different coal lithotype sequences. Following the closure of the Western Interior Seaway, inland Paleocene fluvial-margin mires across the northern intracratonic Williston Basin were responsive to larger scale salt removal events in the subsurface. The collapse-subsidence structures were propagated several km up-section, diffused but sufficient to control mire configurations and mask orbital forcing signals. Southward into the central and southwestern Williston Basin, these strong salt dissolution tectonism patterns were out-of-phase with the repetitive mire accumulations, permitting the recognition of the markedly weaker signals of the 100 kyr orbital eccentricity cycle.

Mitigating the potential contamination risk incurred in sampling herbarium specimens for phytolith reference collection

Phytoliths are emerging as a critical paleoenvironmental and archaeological proxy record in a variety of landscapes. These non-organic objects preserve well in both terrestrial and lacustrine sedimentary contexts. Creating a local reference collection is a crucial step for contending with the multiplicity and diversity of phytolith forms, but previous authors have noted a potential problem with one of the most common methods for extracting phytoliths from fresh and herbaria specimens, the dry-ashing method. This method incurs a contamination risk in the muffle furnace during ashing. The standard weight of plant tissue for extraction is relatively high (200 mg) when considering the amount of material accessible in herbaria collections. We devised the following experimental study to test the contamination potential of our muffle furnace, and the possibility of using a smaller weight of plant tissue (100 mg) to derive the same results and reduce potential risk. The results of this study indicate that reducing the initial plant material by half the weight will result in a slightly lower contamination risk and equivalent diversity of morphotypes in the final phytolith sample. Therefore, we argue that reducing the plant tissue will enable a less costly use of herbaria material and a lower chance of inter-sample contamination.

Paleomonsoonal shifts during ∼13700 to 3100 yr BP in the central Ganga Basin, India with a severe arid phase at ∼4.2 ka

We present a multiproxy record of grain size and stable carbon isotope from the Lilaur lake, Ganga Basin, India to understand the history of lake formation linked to changes in Indian monsoon precipitation during ~13714 to 3073 calibrated year before Present (cal yr BP). A comparison is made between Lilaur lake proxy record with those from other parts of South Asia as well as North Atlantic to understand extent and causes of monsoon variability and its influence on the fauna and flora of the region. Chronology of lake core was constrained using AMS 14C and optically stimulated luminescence (OSL) dates. This study suggests a transition from river to lake during ~5800 to 5189 cal yr BP which is supported by reduced precipitation with weakening of the Indian summer monsoon (ISM). This river to lake transition culminated in a severe arid phase during 4250 to 4050 cal yr BP (4.2 ka event), corresponding to enhanced El Niño activity, southward shift of the Inter-Tropical Convergence Zone, and aridification of India when the ISM was weak.

Reconciling bubble nucleation in explosive eruptions with geospeedometers

Magma from Plinian volcanic eruptions contains an extraordinarily large numbers of bubbles. Nucleation of those bubbles occurs because pressure decreases as magma rises to the surface. As a consequence, dissolved magmatic volatiles, such as water, become supersaturated and cause bubbles to nucleate. At the same time, diffusion of volatiles into existing bubbles reduces supersaturation, resulting in a dynamical feedback between rates of nucleation due to magma decompression and volatile diffusion. Because nucleation rate increases with supersaturation, bubble number density (BND) provides a proxy record of decompression rate, and hence the intensity of eruption dynamics. Using numerical modeling of bubble nucleation, we reconcile a long-standing discrepancy in decompression rate estimated from BND and independent geospeedometers. We demonstrate that BND provides a record of the time-averaged decompression rate that is consistent with independent geospeedometers, if bubble nucleation is heterogeneous and facilitated by magnetite crystals.

Large-scale features of Last Interglacial climate: results from evaluating the &amp;lt;i&amp;gt;lig127k&amp;lt;/i&amp;gt; simulations for the Coupled Model Intercomparison Project (CMIP6)–Paleoclimate Modeling Intercomparison Project (PMIP4)

Abstract. The modeling of paleoclimate, using physically based tools, is increasingly seen as a strong out-of-sample test of the models that are used for the projection of future climate changes. New to the Coupled Model Intercomparison Project (CMIP6) is the Tier 1 Last Interglacial experiment for 127 000 years ago (lig127k), designed to address the climate responses to stronger orbital forcing than the midHolocene experiment, using the same state-of-the-art models as for the future and following a common experimental protocol. Here we present a first analysis of a multi-model ensemble of 17 climate models, all of which have completed the CMIP6 DECK (Diagnostic, Evaluation and Characterization of Klima) experiments. The equilibrium climate sensitivity (ECS) of these models varies from 1.8 to 5.6 ∘C. The seasonal character of the insolation anomalies results in strong summer warming over the Northern Hemisphere continents in the lig127k ensemble as compared to the CMIP6 piControl and much-reduced minimum sea ice in the Arctic. The multi-model results indicate enhanced summer monsoonal precipitation in the Northern Hemisphere and reductions in the Southern Hemisphere. These responses are greater in the lig127k than the CMIP6 midHolocene simulations as expected from the larger insolation anomalies at 127 than 6 ka. New synthesis for surface temperature and precipitation, targeted for 127 ka, have been developed for comparison to the multi-model ensemble. The lig127k model ensemble and data reconstructions are in good agreement for summer temperature anomalies over Canada, Scandinavia, and the North Atlantic and for precipitation over the Northern Hemisphere continents. The model–data comparisons and mismatches point to further study of the sensitivity of the simulations to uncertainties in the boundary conditions and of the uncertainties and sparse coverage in current proxy reconstructions. The CMIP6–Paleoclimate Modeling Intercomparison Project (PMIP4) lig127k simulations, in combination with the proxy record, improve our confidence in future projections of monsoons, surface temperature, and Arctic sea ice, thus providing a key target for model evaluation and optimization.

Atmospherically produced beryllium-10 in annually laminated late-glacial sediments of the North American Varve Chronology

Abstract. We attempt to synchronize the North American Varve Chronology (NAVC) with ice core and calendar year timescales by comparing records of atmospherically produced 10Be fallout in the NAVC and in ice cores. The North American Varve Chronology (NAVC) is a sequence of 5659 varves deposited in a series of proglacial lakes adjacent to the southeast margin of the retreating Laurentide Ice Sheet between approximately 18 200 and 12 500 years before present. Because properties of NAVC varves are related to climate, the NAVC is also a climate proxy record with annual resolution, and our overall goal is to place the NAVC and ice core records on the same timescale to facilitate high-resolution correlation of climate proxy variations in both. Total 10Be concentrations in NAVC sediments are within the range of those observed in other lacustrine records of 10Be fallout, but 9Be and 10Be concentrations considered together show that the majority of 10Be is present in glacial sediment when it enters the lake, and only a minority of total 10Be derives from atmospheric fallout at the time of sediment deposition. Because of this, an initial experiment to determine whether or not 10Be fallout variations were recorded in NAVC sediments by attempting to observe the characteristic 11-year solar cycle in short varve sections sampled at high resolution was inconclusive: short-period variations at the expected magnitude of this cycle were not distinguishable from measurement scatter. On the other hand, longer varve sequences sampled at decadal resolution display centennial-period variations in reconstructed 10Be fallout that have similar properties as coeval 10Be fallout variations recorded in ice core records. These are most prominent in glacial sections of the NAVC that were deposited in proglacial lakes and are suppressed in paraglacial sections of the NAVC that were deposited in lakes lacking direct glacial sediment input. We attribute this difference to the fact that buffering of 10Be fallout by soil adsorption can filter out short-period variations in an entirely deglaciated watershed, but such buffering cannot occur in the ablation zone of an ice sheet. This implies that proglacial lakes whose watershed is mostly glacial may effectively record 10Be fallout variations. We attempted to match centennial-period variations in reconstructed 10Be fallout flux from two segments of the NAVC with ice core fallout records. For both records, it is possible to obtain matches that result in acceptable correlation between NAVC and ice core 10Be fallout records, but the best-fitting matches for the two segments disagree, and only one of them is consistent with independent calendar year calibrations of the NAVC and therefore potentially valid. This leaves several remaining ambiguities in whether or not 10Be fallout variations can, in fact, be used for synchronizing NAVC and ice core timescales, but these could most likely be resolved by higher-resolution and replicate 10Be measurements on targeted sections of the NAVC.

Near‐Future pCO2 During the Hot Miocene Climatic Optimum

AbstractTo improve future predictions of anthropogenic climate change, a better understanding of the relationship between global temperature and atmospheric concentrations of CO2 (pCO2), or climate sensitivity, is urgently required. Analyzing proxy data from climate change episodes in the past is necessary to achieve this goal, with certain geologic periods, such as the Miocene climatic optimum (MCO), a transient period of global warming with global temperatures up to ~7°C higher than today, increasingly viewed as good analogues to future climate under present emission scenarios. However, a problem remains that climate models cannot reproduce MCO temperatures with less than ~800 ppm pCO2, while most previously published proxies record pCO2 &lt; 450 ppm. Here, we reconstructed MCO pCO2 with a multitaxon fossil leaf database from the well‐dated MCO Lagerstätte deposits of Clarkia, Idaho, USA, using four current methods of pCO2 reconstructions. The methods are principally based on either stomatal densities, carbon isotopes, or a combination of both—thus offering independent results. The total of six reconstructions mostly record pCO2 of ~450–550 ppm. Although slightly higher than previously reconstructed pCO2, the discrepancy with the ~800 ppm required by climate models remains. We conclude that climate sensitivity was heightened during MCO, indicating that highly elevated temperatures can occur at relatively moderate pCO2. Ever higher climate sensitivity with rising temperatures should be very seriously considered in future predictions of climate change.

Channel Profile Response to Abrupt Increases in Mountain Uplift Rates: Implications for Late Miocene to Pliocene Acceleration of Intracontinental Extension in the Northern Qinling Range-Weihe Graben, Central China

Abstract Cenozoic extension of the Qinling range-Weihe Graben system has occurred in response to the uplift and growth of the Tibetan Plateau. Rapid exhumation of the northern Qinling range since the late Miocene is also regarded as resulting from the eastward expansion of the northeast part of Tibet. Tectonic evidence of this in the landscape remains unclear, but the fluvial system can provide a sensitive proxy record of tectonic forcing through space and over time scales of 105–107 a. Here, we present a study of channel profiles in the northern Qinling range, which forms a footwall highland separated from the southern Weihe Graben by active normal faults. We identify a population of knickpoints that separate river profiles with a gentle upstream gradient from steeper downstream reaches. Above the knickpoints, steepness indices increase from the central part towards the west and east, whereas channel steepness shows its highest values in the Huaxian-Huayin section. We observed no systematic changes of channel steepness pattern as a function of rock resistance, drainage area, or channel concavity. Correlation analysis between channel steepness and basin elevation and relief documents the control of tectonic forcing on regional topography. While bearing no relation to geological outcrop boundaries, the knickpoints show a strong correlation between retreat distance, catchment area, and river length. We infer that the knickpoints formed in response to an increase in mountain uplift rates and retreated as a kinematic wave. Under linear slope exponent n, we calibrated channel erodibility K~1.00±0.44×10−6 m0.1/a and derived knickpoint ages of 5.59±1.80 Ma. Combining the ages of onset of active faulting and mountain growth in the NE Tibetan Plateau (8–10 Ma, e.g., Liupan Shan, Jishi Shan, and eastern segments of the Haiyuan and Kunlun faults) and in the southwest Qinling range (9–4 Ma), we conclude that growth of the NE Tibetan Plateau began in the mid-Miocene time and expanded eastwards to the Qinling range-Weihe Graben during the late Miocene and early Pliocene.

Decreasing Atmospheric CO2 During the Late Miocene Cooling

AbstractA pronounced late Miocene cooling (LMC) from ~7 to 5.7 Ma has been documented in extratropical and tropical sea surface temperature records, but to date, available proxy evidence has not revealed a significant pCO2 decline over this event. Here, we provide a new, high‐resolution pCO2 proxy record over the LMC based on alkenone carbon isotopic fractionation (εp) measured in sediments from the South Atlantic at Ocean Drilling Program (ODP) Site 1088. We apply a recent proxy calibration derived from a compilation of laboratory cultures, which more accurately reflects the proxy sensitivity to pCO2 changes during late Quaternary glacial‐interglacial cycles, together with new micropaleontological proxies to reconstruct past variations in algal growth rate, an important secondary influence on the εp. Our resulting pCO2 record suggests an approximately twofold to threefold decline over the LMC and confirms a strong coupling between climate and pCO2 through the late Miocene. Within this long‐term trend are pCO2 variations on sub‐myr timescales that may reflect 400‐kyr long‐eccentricity cycles, in which pCO2 minima coincide with several orbital‐scale maxima in published high‐resolution benthic δ18O records. These may correspond to ephemeral glaciations, potentially in the Northern Hemisphere. Our temperature and planktonic δ18O records from Site 1088 are consistent with substantial equatorward movement of Southern Ocean frontal systems during the LMC. This suggests that potential feedbacks between cooling, ocean circulation and deep ocean CO2 storage may warrant further investigation during the LMC.

External forcing mechanisms controlling the North Atlantic coastal upwelling regime during the mid-Holocene

Abstract Nearshore upwelling along the eastern North Atlantic margin regulates regional marine ecosystem productivity and thus impacts blue economies. While most global circulation models show an increase in the intensity and duration of seasonal upwelling at high latitudes under future human-induced warmer conditions, projections for the North Atlantic are still ambiguous. Due to the low temporal resolution of coastal upwelling records, little is known about the impact of natural forcing mechanisms on upwelling variability. Here, we present a microfossil-based proxy record and modeling simulations for the warmest period of the Holocene (ca. 9–5 ka) to estimate the contribution of the natural variability in North Atlantic upwelling via atmospheric and oceanic dynamics. We found that more frequent high-pressure conditions in the eastern North Atlantic associated with solar activity and orbital parameters triggered upwelling variations at multidecadal and millennial time scales, respectively. Our new findings offer insights into the role of external forcing mechanisms in upwelling changes before the Anthropocene, which must be considered when producing future projections of midlatitude upwelling activity.

Climate Change in the Breadbasket of the Roman Empire—Explaining the Decline of the Fayum Villages in the Third Century CE*

The paper focuses on one of the most productive wheat-growing regions in the entire Roman Empire, the Arsinoite nome (modern Fayum) in Egypt. Towards the end of the third century CE, multiple formerly thriving farming villages at the edges of the district went into decline and were eventually abandoned. This paper presents a new perspective on causes of this abandonment by synthesizing existing research. The papyri as well as the archaeological record imply that irrigation problems arising simultaneously from the third century CE lay at the heart of the problem and led to the progressive desertification of formerly agricultural land. The surviving documentation allows us to trace what increasing water stress meant on the ground for the local population and what adaption strategies they undertook to deal with the degradation and desertification of their farmlands. While socio-economic factors certainly played a role in the decline of these settlements, a change in environmental conditions should be considered as well. In fact, natural proxies record a general shift in East African Monsoon patterns at the source areas of the Nile and consecutively lower Nile flood levels from the beginning of the third century on.

A high‐resolution speleothem proxy record of the Late Glacial in the European Alps: extending the NALPS19 record until the beginning of the Holocene

ABSTRACTPrevious research has shown that speleothems from the northern rim of the European Alps captured submillennial‐scale climate change during the last glacial period with exceptional sensitivity and resolution, mimicking Greenland ice‐core records. Here we extend this so‐called NALPS19 record across the Late Glacial using two stalagmites which grew continuously into the Holocene. Both specimens show the same high‐amplitude δ18O signal as Greenland ice cores down to decadal resolution. The start of the warming at the onset of the equivalent of Greenland Interstadial (GI) GI‐1e at 14.66 ± 0.18 ka agrees with the North Greenland Ice Core Project (NGRIP) (14.64 ± 0.28 ka) and comprised a temperature rise of about 5–6 °C. The transition from the equivalent of GI‐1a into the equivalent of Greenland Stadial (GS) GS‐1 (broadly equivalent to the Younger Dryas) commenced at 13.02 ± 0.13 ka which is consistent with NGRIP (12.80 ± 0.26 ka) within errors. The onset of the Holocene started at 11.78 ± 0.14 ka (11.65 ± 0.10 ka at NGRIP) and involved a warming of about 4–5 °C. In contrast to δ18O, δ13C values show no response to (sub)millennial climate shifts due to strong rock‐buffering and only record a long‐term trend of soil development starting with the rapid warming at 14.7 ka.

Evaluating Bayesian Radiocarbon‐dated Event Count (REC) models for the study of long‐term human and environmental processes

ABSTRACTChronological uncertainty complicates attempts to use radiocarbon dates as proxies for processes such as human population growth/decline, forest fires and marine ingression. Established approaches involve turning databases of radiocarbon‐date densities into single summary proxies that cannot fully account for chronological uncertainty. Here, I use simulated data to explore an alternative Bayesian approach that instead models the data as what they are, namely radiocarbon‐dated event counts. The approach involves assessing possible event‐count sequences by sampling radiocarbon date densities and then applying a Markov Chain Monte Carlo method to estimate the parameters of an appropriate count‐based regression model. The regressions based on individual sampled sequences were placed in a multilevel framework, which allowed for the estimation of hyperparameters that account for chronological uncertainty in individual event times. Two processes were used to produce simulated data. One represented a simple monotonic change in event‐counts and the other was based on a real palaeoclimate proxy record. In both cases, the method produced estimates that had the correct sign and were consistently biased towards zero. These results indicate that the approach is widely applicable and could form the basis of a new class of quantitative models for use in exploring long‐term human and environmental processes.

Learning from the past: Impact of the Arctic Oscillation on sea ice and marine productivity off northwest Greenland over the last 9,000 years.

Climate warming is rapidly reshaping the Arctic cryosphere and ocean conditions, with consequences for sea ice and pelagic productivity patterns affecting the entire marine food web. To predict how ongoing changes will impact Arctic marine ecosystems, concerted effort from various disciplines is required. Here, we contribute multi‐decadal reconstructions of changes in diatom production and sea‐ice conditions in relation to Holocene climate and ocean conditions off northwest Greenland. Our multiproxy study includes diatoms, sea‐ice biomarkers (IP25 and HBI III) and geochemical tracers (TOC [total organic carbon], TOC:TN [total nitrogen], δ13C, δ15N) from a sediment core record spanning the last c. 9,000 years. Our results suggest that the balance between the outflow of polar water from the Arctic, and input of Atlantic water from the Irminger Current into the West Greenland Current is a key factor in controlling sea‐ice conditions, and both diatom phenology and production in northeastern Baffin Bay. Our proxy record notably shows that changes in sea‐surface conditions initially forced by Neoglacial cooling were dynamically amplified by the shift in the dominant phase of the Arctic Oscillation (AO) mode that occurred at c. 3,000 yr BP, and caused drastic changes in community composition and a decline in diatom production at the study site. In the future, with projected dominant‐positive AO conditions favored by Arctic warming, increased water column stratification may counteract the positive effect of a longer open‐water growth season and negatively impact diatom production.

A 2600-year high-resolution climate record from Lake Trichonida (SW Greece)

Abstract. This paper aims at reconstructing the palaeoclimatic changes during the last 2600 years in southern Greece based on a proxy record from Lake Trichonida. For the first time, we provide a reliable age-depth model and continuous geochemical data for the largest and deepest lake in Greece. We use X-ray fluorescence (XRF) geochemical data supported by discrete mineral analysis based on X-ray diffraction (XRD), grain size distribution, and organic matter content to investigate changes in the lake sedimentary system and identify the major forcing mechanisms. A principal component analysis based on the XRF geochemical composition identifies the variation between carbonate-rich material, precipitating predominantly under drier and/or warmer conditions, and terrigenous sediment input, with it being more prominent during wetter and/or colder conditions. The first principal component (PC1) shows a very strong correlation with the weathering proxy log (Rb∕Sr), and we interpret both proxies as depicting fluctuations in the hydrological conditions. A cluster analysis, conducted on the continuous geochemical and colour parameters, highlights the similarities in the sediment characteristics deposited during wetter phases, notably during 1850–1750, 1500–1400, ca. 1100, and ca. 100 cal BP. When comparing the PC1 Trichonida record to independent records from the Balkans, we find generally concurring patterns on a multi-decadal to centennial scale. We show that phases with wetter conditions at Lake Trichonida coincide with a more negative North Atlantic Oscillation (NAO) index, suggesting that the precipitation variability in southern Greece is linked to changes in the NAO atmospheric pattern, as one major driving force. The 2600-year-long sedimentary record of Lake Trichonida contributes to a better understanding of Late Holocene palaeohydrological changes in an important climatic transitional zone in the eastern Mediterranean.