Holocene Warm Period Research Articles

The Relative Contributions of Internal Variability and External Forcing to Pacific Walker Circulation over the Last Millennium

Abstract Pacific Walker circulation (PWC) is an important component of tropical atmospheric circulation. Current studies have mainly focused on PWC changes over recent decades and the near future, while less effort has been devoted to long-term PWC variations. In this study, we investigate PWC changes over the last millennium (LM) based on the Community Earth System Model LM Ensemble (CESM-LME). The simulated PWC variations show no significant trend but do reveal decadal fluctuations during the LM, which underestimate the strengthened Little Ice Age (LIA)–Medieval Climate Anomaly (MCA) PWC differences indicated by proxy-based reconstructions. A quantitative estimation of the contributions made to PWC variability from internal variability and external forcing is conducted by using multiple linear regression (MLR) analysis. The internal variabilities contribute approximately 80% to the changes in PWC during the LM, among which the interdecadal Pacific oscillation (IPO) has the largest contribution. In the positive phase of the IPO, the Indo-Pacific sea level pressure (SLP) gradient decreases, and anomalous westerlies occur in the tropical western Pacific, corresponding to a weakened PWC. The relationships between the IPO and PWC show multidecadal-to-centennial fluctuations, suggesting that other internal modes or external forcings may have modulated the IPO–PWC relationship. Volcanic forcing is also an important contributor to PWC variability during the LM. The simulated PWC significantly weakens and lasts for 1–2 years after large volcanic eruptions. The El Niño–like SST pattern and the corresponding zonal SLP gradient lead to the PWC weakening following large volcanic eruptions. Significance Statement As one of the most active components of tropical atmospheric circulation, Pacific Walker circulation (PWC) can significantly modulate global climate through atmospheric teleconnections. Previous studies have mainly focused on PWC variability over recent warm periods and future changes. However, understanding of PWC changes is currently lacking due to limited instrumental observations. The last millennium (LM) has a rich archive of proxy records that give us a longer perspective on climate variability and change, which is an excellent period for understanding the long-term mechanisms and changes in PWC. In this study, we use simulations from the CESM-LME to quantify the relative importance of internal variability and external forcings to PWC variations during the LM. The simulated PWC variations show no significant trend but decadal fluctuations, which underestimates the reconstructed PWC variations during the LM. The internal variabilities contribute approximately 80% to the changes in PWC, among which the interdecadal Pacific oscillation (IPO) has the largest contribution. External forcing, especially the contribution made by volcanic forcing, is also nonnegligible. Both the positive phase of the IPO and large tropical volcanic eruptions could trigger El Niño–like SST anomalies, accompanied by a decreased Indo-Pacific SLP gradient and a weakened PWC.

Uncertainty of the Simulated Mid-Pliocene Changes of Sahel Summer Rainfall in the PlioMIP2 Multimodel Ensemble

Abstract The mid-Pliocene (approximately 3.3–3.0 Ma) was one of the past geological warm periods. Since this past warmer climate was in many respects comparable to near future warming projections, how the regional monsoonal rainfall changed during the mid-Pliocene is an important scientific and socioeconomic concern. Based on phase 2 of the Pliocene Model Intercomparison Project (PlioMIP2), this work examines the simulated changes of Sahel summer rainfall during the mid-Pliocene. The results show the considerable intermodel uncertainty of the simulated mid-Pliocene Sahel rainfall changes in the PlioMIP2 multimodel ensemble, which is due to the uncertainties of both the simulated dynamic and thermodynamic responses to this past warmer climate. In particular, we find that the intermodel spread in the simulated northern North American warming is a major source of the uncertainty of the mid-Pliocene Sahel summer rainfall changes through two processes. One is a direct dynamic process: a stronger warming over northern North America could enhance the meridional temperature gradient between the extratropical and tropical regions, inducing an interhemisphere energy imbalance of the atmosphere. This could lead to a northward shift of the intertropical convergence zone, strengthening the Western African summer monsoon (WASM) circulation and Sahel summer rainfall. Another is an indirect thermodynamic process: the strengthened WASM circulation could further induce anomalous moisture convergence over the Sahel region, increasing local atmospheric moisture at the low-level troposphere, in favor of a wetter Sahel. Our results suggest that an improved warming simulation over northern North America is essential for the hydrological cycle simulation around the Sahel in the mid-Pliocene warmer climate. Significance Statement The Sahel summer rainfall change in a global warmer climate is a widespread scientific and socioeconomic issue because of its important impacts on regional agriculture, ecosystems, food security, water resources, and even cultural environment. However, the present study identifies a nonnegligible intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene (one of the most recent geological warm periods in Earth’s history) in the Pliocene Model Intercomparison Project phase 2 (PlioMIP2) multimodel ensemble. In particular, such an intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene is attributed to that of the simulated northern North America warming via both the direct dynamic process and the indirect thermodynamic process. This is quite different from the uncertainty source of near-future projections of Sahel summer rainfall changes. The present results improve our understanding of the underlying physics of the hydrological cycle change around the Sahel region in the mid-Pliocene warmer climate, with implications for the future projections of regional monsoonal rainfall.

Critical periods in the marine life history of juvenile western Alaska chum salmon in a changing climate

Recent precipitous declines in western Alaska chum salmon Oncorhynchus keta returns followed unprecedented warming in the northern Bering Sea ecosystem. To better understand the role of anomalous events on the early marine ecology of juvenile chum salmon in the northern Bering Sea, we utilized time-series observations over a 17 yr period (2003-2019) of sea surface temperature (SST) and juvenile chum salmon size (length and weight), diet, energy density, and relative abundance. Particular attention was paid to more recent (2014-2019) years in which there was unprecedented loss of sea ice in the northern Being Sea in comparison to previous warm (2003-2005) and cold (2006-2013) periods. Our findings indicate significant correlations between SST and juvenile chum salmon relative biomass (positive) and energy density (negative). We found that juvenile chum salmon were larger during warm periods than during cold periods; however, there was no significant difference in their length and weight between the warm periods. Juvenile chum salmon fed on lower quality prey during warm periods than during cold periods, with an increase in the proportion of lower quality prey during the recent warm period. Consequently, the energy density of juvenile chum salmon was also lower during warm periods than during cold periods, with the lowest values occurring during the recent warm period (2014-2019). These results identify a shift in energy allocation and/or prey quality of juvenile chum salmon with temperature and illustrate how marine ecosystems have altered the nutritional condition of juvenile chum salmon prior to winter, when energy reserves are considered critical to survival.

Spring phytoplankton bloom phenology during recent climate warming on the Bering Sea shelf

High-latitude ecosystems commonly experience large phytoplankton blooms in spring, which provide basal resources for a range of grazers including zooplankton, benthic consumers and fishes. Variation of the timing and intensity of the spring phytoplankton bloom influences the degree of spatial and temporal overlap with consuming organisms. In the Bering Sea, blooms occur in association with ice retreat or as pelagic open-water blooms. In the last few years, the Bering Sea shelf experienced unprecedented and widespread warming. Understanding how those climatic changes subsequently influenced phytoplankton bloom dynamics is critical for evaluating Bering Sea food web responses. We estimate spring bloom timing and type (ice-associated, open-water) across the Bering Sea shelf using a combination of data from the long-running oceanographic moorings on the eastern shelf (M2, M4, M5, M8) and satellite ocean color data from 1998 to 2022. We assess 1) if the Bering Sea shelf experienced noticeable changes in spring bloom timing or type in the last two decades, 2) whether bloom phenology was accentuated by the recent warm period (2018–2019), and 3) what influences do winds and sea surface temperatures have on spring bloom timing and where are these variables influential? Our spatial analyses reinforce the conclusion that ice retreat is the dominant forcing factor of bloom timing for the Bering Sea shelf with some influence of wind for open-water blooms. Overall, bloom timing has not shifted seasonally with climate warming in the last two decades for most of the Bering Sea shelf, except for nearshore areas and mainly in the northern Bering Sea. In warm years when ice retreats early prior to the last week of March, blooms form in open waters and bloom timing on the middle and outer shelf is delayed when wind mixing is prevalent in ice-free springs. In recent years, open-water blooms were more widespread than previously experienced, and even occurred in the northern Bering Sea during 2018–2019. A progression to more open water blooms in a future warmer climate will influence the availability of basal resources for pelagic and benthic consumers.

Drought-heatwave compound events are stronger in drylands

Climate change is exacerbating the occurrence of compound droughts and heatwaves (CDHWs), which pose a serious threat to human health and socio-economic development. Using daily maximum temperature (Tmax) and monthly self-calibrating Palmer drought severity index (sc-PDSI) dataset, the evolution patterns of CDHWs and compound wet-heatwave events, the dominant drivers and the relative contributions of droughts and heatwaves in the drylands and humid areas from 1961 to 2020 were compared and analyzed. The results show that the CDHWs are stronger in drylands than in humid areas, the growth rate of CDHWs in drylands was almost twice that of the humid areas, the CDHWs are greater than the multi-year average intensity of compound wet-heatwave events by up to 2.4 times. Moreover, CDHWs has increased significantly from the past period (1961–1990) to the recent warm period (1991–2020), and the heatwave threshold has increased by about 5 °C. In most drylands, the contribution of heatwaves to CDHWs dominates, whereas in humid areas, the droughts contribution to CDHWs does. The compounding effects of droughts and heatwaves may exacerbate the severity of CDHWs regionally and are most pronounced in drylands, taking into account optimal lags. The study findings could provide scientific and technological support to actively address global climate change risks.

Observed mechanisms activating the recent subpolar North Atlantic Warming since 2016.

The overturning circulation of the subpolar North Atlantic (SPNA) plays a fundamental role in Earth's climate variability and change. Here, we show from observations that the recent warming period since about 2016 in the eastern SPNA involves increased western boundary density at the intergyre boundary, likely due to enhanced buoyancy forcing as a response to the strong increase in the North Atlantic Oscillation since the early 2010s. As these deep positive density anomalies spread southward along the western boundary, they enhance the North Atlantic Current and associated meridional heat transport at the intergyre region, leading to increased influx of subtropical heat into the eastern SPNA. Based on the timing of this chain of events, we conclude that this recent warming phase since about 2016 is primarily associated with this observed mechanism of changes in deep western boundary density, an essential element in these interactions. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

North American Hydroclimate During Past Warms States: A Proxy Compilation‐Model Comparison for the Last Interglacial and the Mid‐Holocene

AbstractDuring the mid‐Holocene (MH: ∼6,000 years Before Present) and Last Interglacial LIG (LIG: ∼129,000–116,000 years Before Present) differences in the seasonal and latitudinal distribution of insolation drove Northern Hemisphere high‐latitude warming comparable to that projected for the end of the 21st century in low emissions scenarios. Paleoclimate proxy records point to distinct but regionally variable hydroclimatic changes during these past warm intervals. However, model simulations have generally disagreed on North American regional moisture patterns during the MH and LIG. To investigate how closely the latest generation of models associated with the Paleoclimate Model Intercomparison Project (PMIP4) reproduces proxy‐inferred moisture patterns during recent warm periods, we compare hydroclimate output from 17 PMIP4 models with newly updated compilations of moisture‐sensitive North American proxy records during the MH and LIG. Agreement is lower for the MH, with models producing wet anomalies across the western United States (US) where most proxies indicate increased aridity relative to the preindustrial period. The models that agree most closely with the LIG proxy compilation display relative wetness in the eastern US and Alaska, and dryness in the northwest and central US. An assessment of atmospheric dynamics using an ensemble of the three LIG simulations that best agree with the proxies suggests that weaker winter North Pacific pressure gradients and steeper summer North Pacific and Atlantic gradients drive LIG precipitation patterns. Our updated compilations and proxy‐model comparisons offer a tool for benchmarking climate models and their performance in simulating climate states that are warmer than present.

Integrated Reconstruction of Late Quaternary Geomorphology and Sediment Dynamics of Prokljan Lake and Krka River Estuary, Croatia

The upper part of the Krka River estuary and Prokljan Lake are a specific example of a well-stratified estuarine environment in a submerged river canyon. Here, we reconstructed the geomorphological evolution of the area and classified the data gathered in the study, integrating multibeam echosounder data, backscatter echosounder data, side-scan sonar morpho-bathymetric surveys, and acoustic sub-bottom profiling, with the addition of ground-truthing and sediment analyses. This led to the successful classification of the bottom sediments using the object-based image analysis method. Additional inputs to the multibeam echosounder data improved the segmentation of the seafloor classification, geology, and morphology of the surveyed area. This study uncovered and precisely defined distinct geomorphological features, specifically submerged tufa barriers and carbonate mounds active during the Holocene warm periods, analogous to recent tufa barriers that still exist and grow in the upstream part of the Krka River. Fine-grained sediments, classified as estuarine sediments, hold more organic carbon than coarse-grained sediments sampled on barriers. A good correlation of organic carbon with silt sediments allowed the construction of a prediction map for marine sedimentary carbon in this estuarine/lake environment using multibeam echosounder data. Our findings highlight the importance of additional inputs to multibeam echosounder data to achieve the most accurate results.

Separating Anthropogenically‐ and Naturally‐Caused Temperature Trends: A Systematic Approach Based On Climate Memory Analysis

AbstractSeparating anthropogenic and natural contributions to global warming is crucial for understanding and predicting climate change, but it is challenging due to the complexity of the climate system. In this study, we try to address this issue by estimating the anthropogenic forcing impacts with its long‐term persistence features properly considered. In this way, a data‐driven approach is proposed to decompose the observed trends from global and regional scales into anthropogenically‐ and naturally‐caused trends. We find that there is a continuous anthropogenic global warming trend since the beginning of the last century, even during the recent global warming hiatus period. On regional scales, the anthropogenically‐forced trends among regions are found at a similar level, while their unevenly distributed warming trends among regions may be attributed to natural causes. Our findings provide new insight into climate warming, and our approach based on persistence analysis provides a new perspective for attribution studies.

The northern Bering Sea zooplankton community response to variability in sea ice: evidence from a series of warm and cold periods

Recent, unprecedented losses of sea ice have resulted in widespread changes in the northern Bering Sea ecosystem, and this study explores the zooplankton community response. Time-series observations were used to identify zooplankton community changes in the northern (>60°N) Bering Sea (NBS) over a 17 yr period (2002-2018). The overall objective was to determine if the changes in zooplankton populations previously described for the southeastern Bering Sea shelf (<60°N) were also observed in the NBS over alternating warm and cold periods. Particular attention was paid to more recent (2014-2018) years that showed significant losses of sea ice in the NBS (2017/2018) in comparison to a prior warm period (2003-2005) and an intervening cold period (2006-2013). A multivariate framework (redundancy analysis) was used to explore correlations with environmental conditions, and differences in mean abundance across the differing warm and cold periods were tested. The NBS zooplankton community had different responses across each warm and cold period, and the primary driver for the differences in response was sea ice. Redundancy analysis demonstrated that the zooplankton community during the second warm period experienced greater variability compared to the prior warm period. The zooplankton community had higher abundances of small copepods and meroplankton and reduced abundances of Calanus spp. and chaetognaths during the most recent warm period. This suggests that the NBS zooplankton will not be impacted by reduced sea ice when the ice coverage extends south of 60°N, but show community change once a minimum threshold in ice extent and timing of retreat is reached. Shifts in the zooplankton community may have had cascading effects on higher trophic levels that were evident during the latter warm period.

Status of Skate Populations in the Barents Sea in the Recent Warm Period

Status of Skate Populations in the Barents Sea in the Recent Warm Period

Evolution of the groundwater flow system driven by the sedimentary environment since the Last Glacial Maximum in the central Yangtze River Basin

• Fluctuations in the Yangtze River and sedimentary environment control GFS evolution. • Sedimentary evolution caused unconformable distribution of groundwater age in a GFS. • Continuous uplift of discharge base level favors preservation of ancient groundwater. • Holocene fine-grained sedimentary strata led to the stagnation of regional GFS. Understanding the circulation and evolution of groundwater and its control mechanism in the Jianghan Plain, which exhibits the complex sedimentary evolution and a progressively deteriorating groundwater environment, is highly imperative and challenging to promote the practical application of groundwater flow system (GFS) and protect the groundwater resources in the region. The aim of this work was to elucidate the evolution pattern of the GFS in the Jianghan Plain and its underlying mechanism. To this end, the grain size characteristics of sediments, geochronological information, stable isotopes in clay porewater, paleoclimate indicators, and existing groundwater age were analyzed to reveal the sedimentary environment of aquifer systems at the basin scale in the study area. It was found that the sedimentary environment changed from one of deep downcutting during the Last Glacial Maximum (LGM) to a fluvial facies that was rapidly infilled with coarse-grained sediments during the last deglaciation period (LDP), and then to a stable lacustrine facies with fine-grained sediments during the Holocene warm period (HWP). These changes were closely linked to the fluctuations of the Yangtze River. Consequently, the existing GFS pattern in the basin presents an unconformable distribution of groundwater age, indicating that it results from the temporal superposition of groundwater flow controlled by the historical sedimentary environment. Dramatic sea level fluctuations since the LGM have resulted in significant changes in the water level of the Yangtze River. This process has impacted the driving force of groundwater, resulting in the evolution of the GFS from a pattern of a fully developed regional GFS during the LGM to the present pattern of a multi-hierarchy nested GFS.

Anthropogenic warming reduces the carbon accumulation of Tibetan Plateau peatlands

Peatland carbon accumulation generally increased during past intervals of natural warming. With recent anthropogenically-dominated warming being unprecedented over the past ∼2000 years, however, it is unclear how peatland carbon dynamics may operate compared to those under historical natural warmings. Here we examine the impacts of the recent warming from 1850 CE to present (Recent Warm Period; RWP) and the historical warming during 1000–1300 CE (Medieval Warm Period; MWP) on peatland carbon accumulation of the Tibetan Plateau using two continuous peat core records. We observed major seasonal difference between the two warming periods, with growing season (summer) warming dominating the MWP, while greater non-growing season warming characterizing the RWP. Consequently, we found a high net carbon balance during the MWP, suggesting that warming increased plant production more than decomposition of organic matter. In contrast, a low net carbon balance was observed during the RWP because non-growing season warming greatly enhanced soil carbon decomposition that could have completely offset plant carbon uptake. Therefore, recent anthropogenic warming might have induced a different ecosystem response with lower net carbon balance over the past millennium than that during historical natural warmings. As other regions of the globe have similar growing season versus non-growing season warming patterns, the risk of carbon loss from global peatlands may have been underestimated, implying that there is an even greater challenge for managing ecosystem carbon sinks under future climate change.

Ommastrephid squid paralarvae potential nursery habitat in the tropical-subtropical convergence off Mexico

The Gulf of California and adjacent Pacific have passed through a prolonged warming period that affects the Humboldt squid D. gigas life cycle, which was the third largest fishery in Mexico until its total collapse after 2010. The aims of the present study were (i) to characterize the nursery habitat of paralarvae of the complex Sthenoteuthis oualaniensis–Dosidicus gigas (SD complex) on the basis of 1112 paralarvae collected from zooplankton trawls carried out during ten oceanographic cruises off northwestern Mexico between 2010 and 2017; and (ii) to model the seasonal and interannual variability of the nursery habitat, applying a generalized additive model (GAM) using a regional long-term (1922–2017) hydrographic database. The GAM showed significant relationships of paralarvae abundance with the seasonal average of thermocline depth, absolute salinity, and conservative temperature. The potential nursery habitat of the SD complex paralarvae showed its maximum expected abundance in the entrance of the Gulf of California during spring, when the thermocline depth was shallowest (<30 m). During summer the nursery habitat moved northward along the Pacific west coast of the peninsula. Autumn and winter were less suitable seasons for paralarvae nursery because the thermocline was deep throughout the area of study. Potential nursery habitat did not change interannually in location during La Niña conditions, but did increase the expected abundance in its southern extension. El Niño events contracted the nursery habitat near Cabo San Lucas, and caused the habitat to move northwards over both coasts of the Baja California peninsula. Despite seasonal and interannual climatic variability, two regions remained suitable habitats almost throughout the year. One was associated with coastal upwelling regions (Cabo Corrientes) and the second comprised the oceanic region of high mesoscale kinetic energy located south of Cabo San Lucas. We conclude that the potential nursery habitat of the SD complex is more coastal than oceanic, and is associated with a shallow thermocline in the tropical-subtropical convergence off Mexico. The recent anomalous warming period in this region could drive the changes in the morphology of D. gigas in the Gulf of California, given the effects of environmental variability over early stages of development throughout the adult ontogenesis.

Holocene sedimentary history of South Danamandıra Lake: a peatland in west of İstanbul, Çatalca Peninsula, NW Turkey

This study investigates the sedimentological evolution of the South Danamandıra Lake (SDL) lake in Çatalca Peninsula, 70 km west of İstanbul, using Georadar data and multiproxy analyses of five sediment cores. The lake is a 1.3-m deep, endorheic freshwater peatland, heavily colonised by common reed (Phragmites sp.). The multiproxy core investigations include a lithological core description and environmental magnetism, physical properties (gamma density and magnetic susceptibility), geochemical elemental, pollen and radiocarbon dating analyses. The lithological sequence in the lake consists of an upper peat unit and a lower sand-silt-clay unit. The peat unit is characterized by lower magnetic susceptibility, density and lithophile elements (K, Fe, Ti, and Zr) concentrations than the sand-silt-clay unit. Overall interpretation of the multiproxy data and the age-depth model suggest that the SDL was formed in a shallow depression of a fluvial channel at ca 10.9 cal kyr BP, and became a eutrophic lake at 8.1 cal kyr BP during the early Holocene warm period. Redox-sensitive element (i.e. Mn) distribution and mineral magnetic properties indicate that the peat unit has accumulated under anoxic conditions below a thin oxic surficial layer. Increase in the Taraxacum, Asteraceae, and Poaceae pollen percentages, together with high siliciclastic inputs in the lake, indicate that anthropogenic influence in the area started at 5.4 cal kyr BP.

Evaluating the large-scale hydrological cycle response within the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) ensemble

Abstract. The mid-Pliocene (∼3 Ma) is one of the most recent warm periods with high CO2 concentrations in the atmosphere and resulting high temperatures, and it is often cited as an analog for near-term future climate change. Here, we apply a moisture budget analysis to investigate the response of the large-scale hydrological cycle at low latitudes within a 13-model ensemble from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). The results show that increased atmospheric moisture content within the mid-Pliocene ensemble (due to the thermodynamic effect) results in wetter conditions over the deep tropics, i.e., the Pacific intertropical convergence zone (ITCZ) and the Maritime Continent, and drier conditions over the subtropics. Note that the dynamic effect plays a more important role than the thermodynamic effect in regional precipitation minus evaporation (PmE) changes (i.e., northward ITCZ shift and wetter northern Indian Ocean). The thermodynamic effect is offset to some extent by a dynamic effect involving a northward shift of the Hadley circulation that dries the deep tropics and moistens the subtropics in the Northern Hemisphere (i.e., the subtropical Pacific). From the perspective of Earth's energy budget, the enhanced southward cross-equatorial atmospheric transport (0.22 PW), induced by the hemispheric asymmetries of the atmospheric energy, favors an approximately 1∘ northward shift of the ITCZ. The shift of the ITCZ reorganizes atmospheric circulation, favoring a northward shift of the Hadley circulation. In addition, the Walker circulation consistently shifts westward within PlioMIP2 models, leading to wetter conditions over the northern Indian Ocean. The PlioMIP2 ensemble highlights that an imbalance of interhemispheric atmospheric energy during the mid-Pliocene could have led to changes in the dynamic effect, offsetting the thermodynamic effect and, hence, altering mid-Pliocene hydroclimate.

Changing Indian monsoon rainfall patterns under the recent warming period 2001–2018

Changing Indian monsoon rainfall patterns under the recent warming period 2001–2018

Commentary: Early Diagenetic Imprint on Temperature Proxies in Holocene Corals: A Case Study From French Polynesia

Coral-based reconstructions of sea surface temperatures (SSTs) using Sr/Ca, U/Ca and δ 18 O are important tools for quantitative analysis of past climate variabilities. However, post-depositional alteration of coral aragonite, particularly early diagenesis, restrict the accuracy of calibrated proxies even on young corals. Considering the diagenetic effects, we present new Mid to Late Holocene SST reconstructions on well-dated (U/Th: ∼70 yr to 5.4 ka) fossil Porites sp. collected from the Society Islands, French Polynesia. For few corals, quality pre-screening routines revealed the presence of secondary aragonite needles inside primary pore space, resulting in a mean increase in Sr/Ca ratios between 5-30%, in contrast to the massive skeletal parts. Characterized by a Sr/Ca above 10 mmol/mol, we interpret this value as the threshold between diagenetically altered and unaltered coral material. At a high-resolution, observed intra-skeletal variability of 5.4 to 9.9 mmol/mol probably reflects the physiological control of corals over their trace metal uptake, and individual variations controlled by CaCO 3 − precipitation rates. Overall, the Sr/Ca, U/Ca and δ 18 O trends are well correlated, but we observed a significant offset up to ± 7 • C among the proxies on derived palaeo-SST estimates. It appears that the related alteration process tends to amplify temperature extremes, resulting in increased SST-U/Ca and SST-Sr/Ca gradients, and consequently their apparent temperature sensitivities. A relative SST reconstruction is still feasible by normalizing our records to their individual mean value defined as SST. This approach shows that SST records derived from different proxies agree with an amplitudinal variability of up to ± 2 • C with respect to their Holocene mean value. Higher SST values than the mean SSTs (Holocene warm periods) were recorded from ∼1.8 to ∼2.8 ka (Interval I), ∼3.7 to 4.0 ka (Interval III) and before ∼5 ka, while lower SST values (Holocene cold periods, Interval II and IV) were recorded in between. The ensuing SST periodicity of ∼1.5 ka in the Society Islands record is in line with the solar activity reconstructed from 10 Be and 14 C production (Vonmoos et al., 2006), emphasizing the role of solar activity on climate variability during the Late Holocene.

Preferential export of permafrost-derived organic matter as retrogressive thaw slumping intensifies

Enhanced warming of the Northern high latitudes has intensified thermokarst processes throughout the permafrost zone. Retrogressive thaw slumps (RTS), where thaw-driven erosion caused by ground ice melt creates terrain disturbances extending over tens of hectares, represent particularly dynamic thermokarst features. Biogeochemical transformation of the mobilized substrate may release CO2 to the atmosphere and impact downstream ecosystems, yet its fate remains unclear. The Peel Plateau in northwestern Canada hosts some of the largest RTS features in the Arctic. Here, thick deposits of Pleistocene-aged glacial tills are overlain by a thinner layer of relatively organic-rich Holocene-aged permafrost that aggraded upward following deeper thaw and soil development during the early Holocene warm period. In this study, we characterize exposed soil layers and the mobilized material by analysing sediment properties and organic matter composition in active layer, Holocene and Pleistocene permafrost, recently thawed debris deposits and fresh deposits of slump outflow from four separate RTS features. We found that organic matter content, radiocarbon age and biomarker concentrations in debris and outflow deposits from all four sites were most similar to permafrost soils, with a lesser influence of the organic-rich active layer. Lipid biomarkers suggested a significant contribution of petrogenic carbon especially in Pleistocene permafrost. Active layer samples contained abundant intrinsically labile macromolecular components (polysaccharides, lignin markers, phenolic and N-containing compounds). All other samples were dominated by degraded organic constituents. Active layer soils, although heterogeneous, also had the highest median grain sizes, whereas debris and runoff deposits consisted of finer mineral grains and were generally more homogeneous, similar to permafrost. We thus infer that both organic matter degradation and hydrodynamic sorting during transport affect the mobilized material. Determining the relative magnitude of these two processes will be crucial to better assess the role of intensifying RTS activity in CO2 release and ecosystem carbon fluxes.

Deciphering the evolution of the Bleis Marscha rock glacier (Val d'Err, eastern Switzerland) with cosmogenic nuclide exposure dating, aerial image correlation, and finite element modeling

Abstract. We constrain the Holocene development of the active Bleis Marscha rock glacier (Err–Julier area, eastern Swiss Alps) with 15 cosmogenic nuclide exposure ages (10Be, 36Cl), horizontal surface creep rate quantification by correlating two orthophotos from 2003 and 2012, and finite element modeling. We used the latter to separate the control on surface movement exerted by topography and material properties. Bleis Marscha is a stack of three overriding lobes whose formation phases are separated by time gaps expressed morphologically as over-steepened terrain steps and kinematically as a sharp downslope decrease in surface movement. The three discrete formation phases appear to be correlated to major Holocene climate shifts: Early Holocene low-elevation lobes (∼8.9–8.0 ka, after the Younger Dryas), Middle Holocene lobe (∼5.2–4.8 ka, after the Middle Holocene warm period), and Late Holocene high-elevation lobes (active since ∼2.8 ka, intermittently coexisting with oscillating Bleis Marscha cirque glacierets). The formation phases appear to be controlled in the source area by the climate-sensitive accumulation of an ice-debris mixture in proportions susceptible to rock glacier creep. The ongoing cohesive movement of the older generations requires ice at a depth which is possibly as old as its Early–Middle Holocene debris mantle. Permafrost degradation is attenuated by “thermal filtering” of the coarse debris boulder mantle and implies that the dynamics of the Bleis Marscha lobes that once formed persisted over millennia are less sensitive to climate. The cosmogenic radionuclide inventories of boulders on a moving rock glacier ideally record time since deposition on the rock glacier root but are stochastically altered by boulder instabilities and erosional processes. This work contributes to deciphering the long-term development and the past to quasi-present climate sensitivity of rock glaciers.