Glacier response to Holocene warmth inferred from in situ ¹⁰Be and ¹⁴C bedrock analyses in Steingletscher's forefield (central Swiss Alps)

Mid-latitude mountain glaciers are sensitive to local summer temperature changes. Chronologies of past glacier fluctuations based on the investigation of glacial landforms therefore allow for a better understanding of natural climate variability at local scale, which is relevant for the assessment of the ongoing anthropogenic climate warming. In this study, we focus on the Holocene, the current interglacial of the last 11 700 years, which remains a matter of dispute regarding its temperature evolution and underlying driving mechanisms. In particular, the nature and significance of the transition from the early to mid-Holocene and of the Holocene Thermal Maximum (HTM) are still debated. Here, we apply an emerging approach by combining in situ cosmogenic 10Be moraine and 10Be–14C bedrock dating from the same site, the forefield of Steingletscher (European Alps), and reconstruct the glacier's millennial recession and advance periods. The results suggest that, subsequent to the final deglaciation at ∼10 ka, the glacier was similar to or smaller than its 2000 CE extent for ∼7 kyr. At ∼3 ka, Steingletscher advanced to an extent slightly outside the maximum Little Ice Age (LIA) position and until the 19th century experienced sizes that were mainly confined between the LIA and 2000 CE extents. These findings agree with existing Holocene glacier chronologies and proxy records of summer temperatures in the Alps, suggesting that glaciers throughout the region were similar to or even smaller than their 2000 CE extent for most of the early and mid-Holocene. Although glaciers in the Alps are currently far from equilibrium with the accelerating anthropogenic warming, thus hindering a simple comparison of summer temperatures associated with modern and paleo-glacier sizes, our findings imply that the summer temperatures during most of the Holocene, including the HTM, were similar to those at the end of the 20th century. Further investigations are necessary to refine the magnitude of warming and the potential HTM seasonality.

Climate change since the Last Interglacial in northern New Zealand inferred from pollen and chironomid records of the Auckland Maars

<p>In light of contemporary climate change it is more important than ever to understand past shifts in climate, especially past warm phases, and their effects on ecosystems and societies. From compilations of global climate reconstructions, several periods have been identified that might have been warmer than today, the two most recent of which are the Holocene Thermal Maximum (~11 – 5 kyr BP) and the Last Interglacial (~129 – 116 kyr BP). However, spatio-temporal complexities are typically smoothed out in global climate reconstructions and we do not have a good understanding of the regional differences in past climate. The southern mid-latitudes especially are underrepresented in palaeoclimate research. For this thesis I analyse the sediments from two maars within the Auckland Volcanic Field: Orakei Basin, which erupted ~126.0 kyr BP and accumulated sediments until ~9 – 8.5 kyr BP; and Lake Pupuke, which still contains a lake today and therefore covers the Holocene. Quantitative climate reconstructions are necessary to put the Orakei Basin and Lake Pupuke records in a broad context and to enable comparisons of past and future climates. For this study I focus on biological proxies preserved by lake sediments, namely pollen, which primarily responds to mean annual air temperatures (MAAT), and chironomids, a surrogate for summer air temperatures (SmT). Together, MAAT and SmT reconstructions from the same site can provide insight into changing seasonality over time, an underexplored dimension of proxy-based reconstructions. The chironomid record covers just the last ~14 cal kyr BP however, because of low head capsule abundances in older sediment sections. The Orakei Basin pollen record and associated MAAT reconstruction cover ~85 to 9 cal kyr BP and show five distinct phases comparable to Marine Isotope Stages (MIS) 5 to 1. This association is confirmed by the preliminary tephrochronology of the core. The broad similarity of the Orakei MAAT trend to the MIS and other records from New Zealand implies all were driven by northern high-latitude summer insolation, consistent with the Milankovitch orbital forcing hypothesis. Several patterns superimposed on the general trend stand out: first, MIS 4 is a brief cool period, which is inconsistent with the observation that glacier advances equivalent to those of the late last glacial maximum occurred ~65 kyr BP in the Southern Alps, possibly due to the seasonal distribution of energy from solar insolation. Second, MIS 3 displays an earlier warm phase followed by a progressive cooling trend which might be correlated to decreasing local summer insolation intensity. Third, glacial conditions of MIS 2 appear consistent with the early onset of the last glacial maximum in the southern mid latitudes, which was likely driven by regional insolation intensity. The Lake Pupuke pollen and chironomid records, covering the last ~14 cal kyr BP, show no evidence of a past warm period equivalent to the Holocene Thermal Maximum. MAAT is stable throughout the Holocene, whereas SmT increases between 10 and 3 cal kyr BP. The latter shows a strong relationship with integrated local summer insolation. The temperature reconstructions lead to the conclusion, first, that seasonality was low during the Early Holocene (12 to 9.3 cal kyr BP), and second, that during mid-to-late Holocene (after ~7 cal kyr BP) summers were hot and dry, allowing the tall conifer kauri to expand throughout northern New Zealand. The Lake Pupuke chironomid-SmT reconstruction highlighted an issue with the transfer function model, namely, that it was not able to reconstruct values close to modern day (18.9°C). Therefore, I explore an extended training set which encompasses a longer temperature gradient. New models are fitted using both traditional techniques and modern machine learning methods. The new model improves the SmT reconstruction from Lake Pupuke, in the sense that reconstructed temperatures now reach modern day values. However, the SmT trend is the same as the original trend, substantiating the previously drawn conclusions. During the course of this research, I discovered that density separation during pollen preparation can lead to varying relative abundances, depending on the specific gravity used. After some experimentation I found that using a low specific gravity (2.0; recommended value in the literature) can result in the overrepresentation of buoyant pollen grains, leading to erroneous interpretations. Together, these results point out the importance of considering regional-to-local drivers of climate changes superimposed on global reconstructions. Multi-proxy records can help disentangle the different aspects of the climate system, where especially chironomids can be helpful to elucidate the role of SmT and local summer insolation. Finally, this thesis shows the importance of questioning the appropriateness of conventional methodologies and where possible, addressing their limitations.</p>

Climate change since the Last Interglacial in northern New Zealand inferred from pollen and chironomid records of the Auckland Maars

<p>In light of contemporary climate change it is more important than ever to understand past shifts in climate, especially past warm phases, and their effects on ecosystems and societies. From compilations of global climate reconstructions, several periods have been identified that might have been warmer than today, the two most recent of which are the Holocene Thermal Maximum (~11 – 5 kyr BP) and the Last Interglacial (~129 – 116 kyr BP). However, spatio-temporal complexities are typically smoothed out in global climate reconstructions and we do not have a good understanding of the regional differences in past climate. The southern mid-latitudes especially are underrepresented in palaeoclimate research. For this thesis I analyse the sediments from two maars within the Auckland Volcanic Field: Orakei Basin, which erupted ~126.0 kyr BP and accumulated sediments until ~9 – 8.5 kyr BP; and Lake Pupuke, which still contains a lake today and therefore covers the Holocene. Quantitative climate reconstructions are necessary to put the Orakei Basin and Lake Pupuke records in a broad context and to enable comparisons of past and future climates. For this study I focus on biological proxies preserved by lake sediments, namely pollen, which primarily responds to mean annual air temperatures (MAAT), and chironomids, a surrogate for summer air temperatures (SmT). Together, MAAT and SmT reconstructions from the same site can provide insight into changing seasonality over time, an underexplored dimension of proxy-based reconstructions. The chironomid record covers just the last ~14 cal kyr BP however, because of low head capsule abundances in older sediment sections. The Orakei Basin pollen record and associated MAAT reconstruction cover ~85 to 9 cal kyr BP and show five distinct phases comparable to Marine Isotope Stages (MIS) 5 to 1. This association is confirmed by the preliminary tephrochronology of the core. The broad similarity of the Orakei MAAT trend to the MIS and other records from New Zealand implies all were driven by northern high-latitude summer insolation, consistent with the Milankovitch orbital forcing hypothesis. Several patterns superimposed on the general trend stand out: first, MIS 4 is a brief cool period, which is inconsistent with the observation that glacier advances equivalent to those of the late last glacial maximum occurred ~65 kyr BP in the Southern Alps, possibly due to the seasonal distribution of energy from solar insolation. Second, MIS 3 displays an earlier warm phase followed by a progressive cooling trend which might be correlated to decreasing local summer insolation intensity. Third, glacial conditions of MIS 2 appear consistent with the early onset of the last glacial maximum in the southern mid latitudes, which was likely driven by regional insolation intensity. The Lake Pupuke pollen and chironomid records, covering the last ~14 cal kyr BP, show no evidence of a past warm period equivalent to the Holocene Thermal Maximum. MAAT is stable throughout the Holocene, whereas SmT increases between 10 and 3 cal kyr BP. The latter shows a strong relationship with integrated local summer insolation. The temperature reconstructions lead to the conclusion, first, that seasonality was low during the Early Holocene (12 to 9.3 cal kyr BP), and second, that during mid-to-late Holocene (after ~7 cal kyr BP) summers were hot and dry, allowing the tall conifer kauri to expand throughout northern New Zealand. The Lake Pupuke chironomid-SmT reconstruction highlighted an issue with the transfer function model, namely, that it was not able to reconstruct values close to modern day (18.9°C). Therefore, I explore an extended training set which encompasses a longer temperature gradient. New models are fitted using both traditional techniques and modern machine learning methods. The new model improves the SmT reconstruction from Lake Pupuke, in the sense that reconstructed temperatures now reach modern day values. However, the SmT trend is the same as the original trend, substantiating the previously drawn conclusions. During the course of this research, I discovered that density separation during pollen preparation can lead to varying relative abundances, depending on the specific gravity used. After some experimentation I found that using a low specific gravity (2.0; recommended value in the literature) can result in the overrepresentation of buoyant pollen grains, leading to erroneous interpretations. Together, these results point out the importance of considering regional-to-local drivers of climate changes superimposed on global reconstructions. Multi-proxy records can help disentangle the different aspects of the climate system, where especially chironomids can be helpful to elucidate the role of SmT and local summer insolation. Finally, this thesis shows the importance of questioning the appropriateness of conventional methodologies and where possible, addressing their limitations.</p>

Long-Chain Alkenones in the Shimosa Group Reveal Palaeotemperatures of the Pleistocene Interglacial Palaeo-Tokyo Bays

The Shimosa Group, middle- to late-Pleistocene sedimentary succession, has been the focus of stratigraphic attention because it is beneath the Tokyo metropolitan area of central Japan. It is also of palaeoclimatic significance because it contains important interglacial marine strata of the past 450,000 years. Since the marine strata of the Shimosa Group were formed in the fluvial, estuary, and shallow inner bay known as Palaeo-Tokyo Bay, few occurrences of marine microfossils, make it difficult to quantitatively reconstruct the palaeotemperatures. Here, we extracted long-chain alkenones from the core GS-UR-1 penetrating the Shimosa Group to Marine Isotope Stage (MIS) 11. We found that the alkenone unsaturation ratio appears to reflect the sea surface temperatures (SSTs) of Palaeo-Tokyo Bays formed during MIS 5e, 7e, 9, and 11, which might be recorded around the peak of each interglacial period. The palaeo-SSTs during each interglacial period were 2–3 ℃ higher than the palaeo-SSTs of Tokyo Bay in the pre-industrial era, seemed to reach the similar level as the Holocene thermal maximum. We suggest that the LCA-based proxy, which has not been utilized hitherto in studies on the Shimosa Group, demonstrates its potential to provide palaeoclimatic and stratigraphic information.

Glacier response to Holocene warmth inferred from in situ ¹⁰Be and ¹⁴C bedrock analyses in Steingletscher’s forefield (central Swiss Alps)

Mid-latitude mountain glaciers sensitively respond to local summer temperature changes. Chronologies of past glacier fluctuations based on the investigation of glacial landforms therefore allows for a better understanding of warm-season climate variability at local scale. In this study, we focus on the Holocene, the current interglacial of the last 11,700 years, which remains matter of dispute regarding its temperature evolution and underlying driving mechanisms. In particular, the nature and significance of the transition from the early to mid-Holocene and of the Holocene Thermal Maximum (HTM) are still debated. Here, we apply a new approach by combining in situ cosmogenic 10Be moraine and 10Be-14C bedrock dating from the same site, the forefield of Steingletscher (European Alps), and reconstruct the glacier’s millennial recession and advance periods. The results suggest that subsequent to the final deglaciation at ~10 ka, the glacier was mostly smaller than its 2000 CE extent until ~3 ka, followed by the predominant occurrence of glacier advances until the end of the Little Ice Age in the 19th century. These findings agree with existing proxy records of Holocene summer temperature and glacier evolution in the Alps, showing that glaciers throughout the region retreated beyond modern extents for most of the Early and mid-Holocene. This implies that at least the summer climate of the HTM was warmer than that of the end of the 20th century for several millennia. Further investigations are necessary to refine the magnitude of warming and the potential HTM seasonality.

Glacial kettles as archives of early human settlement along the Northern Rocky Mountain Front

The Northern Rocky Mountain Front (hereafter Northern Front) is a prominent geographic feature in archaeological models of human dispersal in the terminal Pleistocene and Early Holocene of North America. Testing those models has been arduous because of local geomorphological factors that tend to obliterate or otherwise limit access to archaeological finds of relevant age. In this paper, we present well-stratified archaeological and environmental records dating back to 14,000–13,000 cal yr BP from the site of Billy Big Spring (Blackfeet Indian Reservation, Montana), located on a glacial kettle, a type of landform that has been largely ignored by regional archaeological research to date. Findings from Billy Big Spring show the continuous use of the Northern Front foothills throughout the major climatic and environmental disturbances of the Early Holocene, and possibly the terminal Pleistocene as well. As such, Billy Big Spring contributes to refining several archaeological models of early settlement of the Northern Front, particularly those that posit differential use of foothills versus plains settings during the midst of the Holocene Thermal Maximum. The record at Billy Big Spring also suggests that kettles, regardless of physiographic setting, provide a yet unsuspected and unsampled potential for preserving high-quality and easily accessible early archaeological and paleoenvironmental records.

Climate and the latitudinal limits of subtropical reef development

Climate plays a central role in coral-reef development, especially in marginal environments. The high-latitude reefs of southeast Florida are currently non-accreting, relict systems with low coral cover. This region also did not support the extensive Late Pleistocene reef development observed in many other locations around the world; however, there is evidence of significant reef building in southeast Florida during the Holocene. Using 146 radiometric ages from reefs extending ~ 120 km along Florida’s southeast coast, we test the hypothesis that the latitudinal extent of Holocene reef development in this region was modulated by climatic variability. We demonstrate that although sea-level changes impacted rates of reef accretion and allowed reefs to backstep inshore as new habitats were flooded, sea level was not the ultimate cause of reef demise. Instead, we conclude that climate was the primary driver of the expansion and contraction of Florida’s reefs during the Holocene. Reefs grew to 26.7° N in southeast Florida during the relatively warm, stable climate at the beginning of the Holocene Thermal Maximum (HTM) ~ 10,000 years ago, but subsequent cooling and increased frequency of winter cold fronts were associated with the equatorward contraction of reef building. By ~ 7800 years ago, actively accreting reefs only extended to 26.1° N. Reefs further contracted to 25.8° N after 5800 years ago, and by 3000 years ago reef development had terminated throughout southern Florida (24.5–26.7° N). Modern warming is unlikely to simply reverse this trend, however, because the climate of the Anthropocene will be fundamentally different from the HTM. By increasing the frequency and intensity of both warm and cold extreme-weather events, contemporary climate change will instead amplify conditions inimical to reef development in marginal reef environments such as southern Florida, making them more likely to continue to deteriorate than to resume accretion in the future.