December 2020: Palaeogeography, Palaoclimatology, Palaoecology
The common whelk, Buccinum undatum, is a commercially important gastropod found throughout the North Atlantic. One method of age and life history analysis for gastropod species is the use of oxygen isotope ratio (δ18O) measurements from their shells, which is a well-established technique for the reconstruction of historical seawater temperatures at the time of shell biomineralization. Palaeotemperature calibrations have been developed for different types of calcium carbonate as well as species-specific equations to produce the most accurate seawater temperature reconstructions. Here we investigate the four-layer internal structure of B. undatum shells and confirm an aragonite composition using Micro-Raman Spectroscopy (MRS). We then calibrate a species-specific palaeotemperature equation for this gastropod species. This was achieved through the isotopic analysis of shells from laboratory reared specimens of known provenance reared at specific seawater temperatures to produce the following:
t(°C) = 14.96 (± 0.15) – 4.94 (± 0.22) x (δ18Oshell – δ18Owater)
The calibrated equation differs significantly from previously published data derived from both aragonite and calcite. An offset of 1.04‰ (± 0.41‰) was discovered between observed δ18OShell values and those expected under equilibrium, suggesting a species-specific vital effect. The calibrated equation was used to reconstruct accurate, high resolution historical seawater temperatures from three sites across the U.K. (Shetland, the Menai Strait and Jersey). With this new accurate calibration, both modern and fossil B. undatum shells now have the potential to be employed as high-resolution archives of recent and historical seawater temperature.
Hollyman, P R, Leng, M J, Chenery, S R N, Sloane, H J, and Richardson, C A. 2020. Calibration of shell δ18O from the common whelk Buccinum undatum highlights potential for palaeoenvironmental reconstruction. Palaeogeography, Palaeoclimatology, Palaeoecology.
October 2020: Palaeogeography, Palaoclimatology, Palaoecology
The shell δ18O of young modern Aequipecten opercularis from the southern North Sea provides an essentially faithful record of seasonal variation in seafloor temperature. In this well-mixed setting, A. opercularis shell δ18O also serves as a proxy for seasonal variation in surface temperature. Individuals from less agitated (e.g. deeper) settings in a warm climate would not be expected to record the full seasonal range in surface temperature because of thermal stratification in summer. Such circumstances have been invoked to explain cool isotopic summer temperatures from early Pliocene A. opercularis of eastern England. Support for a sub-thermocline setting derives from high-amplitude variation in microgrowth-increment size, which resembles the pattern in sub-thermocline A. opercularis from the southern Mediterranean Sea. Here, we present isotope and increment profiles from further sub-thermocline individuals, live-collected from a location in the Adriatic Sea for which we provide modelled values of expected shell δ18O. We also present data from supra-thermocline shells from the English Channel and French Mediterranean coast. The great majority of sub-thermocline A. opercularis show high-amplitude variation in increment size, and winter and summer δ18O values are generally quite close to expectation. However, the relatively warm summer conditions of 2015 are not recorded, in most cases due to a break in growth, perhaps caused by hypoxia. The supra-thermocline shells show subdued increment variation and yield isotopic winter and summer temperatures quite close to the local directly measured values. A. opercularis shells therefore provide a fairly good isotopic record of ambient temperature (if not always of relatively warm summer conditions below the thermocline) and their hydrographic setting can be determined from increment data. Early Pliocene examples from eastern England can be interpreted as having lived in a setting below the thermocline, with a higher seasonal range in surface temperature than now in the adjacent southern North Sea.
Johnson, A L A, Valentine, A-M M, Schöne, B R, Leng, M J, Sloane, H J, and Janekoviće, I. 2020. Growth-increment characteristics and isotopic (δ18O) temperature record of sub-thermocline Aequipecten opercularis (Mollusca:Bivalvia): Evidence from modern Adriatic forms and an application to early Pliocene examples from eastern England. Palaeogeography, Palaeoclimatology, Palaeoecology.
October 2020: Palaeogeography, Palaoclimatology, Palaoecology
Reduction in atmospheric pCO2 has been hypothesised as a causal mechanism for the Mid-Pleistocene Transition (MPT), which saw global cooling and increased duration of glacials between 0.6 and 1.2 Ma. Sea ice-modulated high latitude upwelling and ocean-atmospheric CO2 flux is considered a potential mechanism for pCO2 decline, although there are no long-term nutrient upwelling records from high latitude regions to test this hypothesis. Using nitrogen isotopes and opal mass accumulation rates from 0 to 1.2 Ma, we calculate a continuous high resolution nutrient upwelling index for the Bering Sea and assess possible changes to regional CO2 fluxes and to the relative control of sea ice, sea level and glacial North Pacific Intermediate Water (GNPIW) on deep mixing and nutrient upwelling in the region. We find nutrient upwelling in the Bering Sea correlates with global ice volume and air temperature throughout the study interval. From ~1 Ma, and particularly during the 900 ka event, suppressed nutrient upwelling would have lowered oceanic fluxes of CO2 to the atmosphere supporting a reduction in global pCO2 during the MPT. This timing is consistent with a pronounced increase in sea ice during the early Pleistocene and restriction of flow through the Bering Strait during glacials after ~900 ka, both of which would have acted to suppress upwelling. We suggest that sea-level modulated GNPIW expansion during glacials after 900 ka was the dominant control on subarctic Pacific upwelling strength during the mid-late Pleistocene, while sea ice variability played a secondary role.
Worne, S, Kender, S, Swann, G E A Leng, M J, and Ravelo, A C. 2020. Reduced upwelling of nutrient and carbon-rich water in the subarctic Pacific during the Mid-Pleistocene Transition. Palaeogeography, Palaeoclimatology, Palaeoecology, 555, 109845.
September 2020: Science Advances
The scarcity of high-resolution empirical data directly tracking diversity over time limits our understanding of speciation and extinction dynamics and the drivers of rate changes. Here, we analyze a continuous species-level fossil record of endemic diatoms from ancient Lake Ohrid, along with environmental and climate indicator time series since lake formation 1.36 million years (Ma) ago. We show that speciation and extinction rates nearly simultaneously decreased in the environmentally dynamic phase after ecosystem formation and stabilized after deep-water conditions established in Lake Ohrid. As the lake deepens, we also see a switch in the macroevolutionary trade-off, resulting in a transition from a volatile assemblage of short-lived endemic species to a stable community of long-lived species. Our results emphasize the importance of the interplay between environmental/climate change, ecosystem stability, and environmental limits to diversity for diversification processes. The study also provides a new understanding of evolutionary dynamics in long-lived ecosystems.
Wilke, T, Hauffe, T, Jovanovska, E, Cvetkoska, A, Donders, T, Ekschmitt, K, Francke, A, Lacey, J H, Levkov, Z, Marshall, C R, Neubauer, T A, Silvestro, D, Stelbrink, B, Vogel, H, Albrecht, C, Holtvoeth, J, Krastel, S, Leicher, N, Leng, M J, Lindhorst, K, Masi, A, Ognjanova-Rumenova, N, Panagiotopoulos, K, Reed, J M, Sadori, L, Tofilovska, S, Van Bocxlaer, B, Wagner-Cremer, F, Wesselingh, F P, Wolters, V, Zanchetta, G, Zhang, X, and Wagner, B. 2020. Deep drilling reveals massive shifts in evolutionary dynamics after formation of ancient ecosystem. Science Advances, 6.
September 2020: Marine Micropaleontology
Stable isotope analyses of ostracod shells are a commonly-used proxy for palaeoenvironmental reconstruction. Although the fundamental controls on isotope composition of ostracod shells are well understood and, in some instances, quantifiable, the paleoclimatic and palaeoenvironmental interpretation of records from lake sediments depends strongly on the characteristics of individual lakes including the climatic setting, depth, volume, hydrology, aquatic vegetation and catchment properties. This is particularly important for coastal lakes where physio-chemical variations may occur on diurnal timescales. Here, we combine variations in δ18Owater, δ18Oostracod and δ13Costracod, hourly water temperature, and Mg/Caostracod inferred water temperatures (constraining calcification temperature) to improve palaeoenvironmental interpretation and provide insights into lake carbon cycle. The dataset improves understanding of complex coastal lake site systematics and downcore interpretation of stable isotopes from C. torosa, a geographically widespread brackish water ostracod. The δ18Oostracod values show a complex relationship with temperature and suggest, in most circumstances, that δ18Owater is the dominant control on δ18Oostracod. During times of fresher water, δ13Costracod increases, suggesting increasing aquatic productivity. Above a certain δ18Owater threshold however, aquatic productivity begins to decline. The interpretation of δ13Costracod in some coastal lakes, may therefore be dependent on understanding of the range of expected δ18Owater. Due to short-term (diurnal to seasonal) variations that cause large ranges in δ18Owater and δ18Oostracod, stable isotope analyses of C. torosa should be: (1) undertaken on multiple single shells (2) where carapaces are preserved, paired with trace-element/Ca analyses on the same individual; and (3) undertaken alongside a study of the modern lake system.
Roberts, L R, Holmes, J A, Sloane, H J, Arrowsmith, C, Leng, M J, and Horne, D J. 2020. δ18O and δ13C of Cyprideis torosa from coastal lakes: Modern systematics and down-core interpretation. Marine Micropaleontology. 160.
September 2020: Earth and Planetary Science Letters
The Li/Mg, Sr/Ca and oxygen isotopic (O) compositions of many marine biogenic carbonates are sensitive to seawater temperature. Corals, as cosmopolitan marine taxa with carbonate skeletons that can be precisely dated, represent ideal hosts for these geochemical proxies. However, efforts to calibrate and refine temperature proxies in cold-water corals (<20 °C) remain limited. Here we present skeletal Li/Mg, Sr/Ca, O and carbon isotope (C) data from live-collected specimens of aragonitic scleractinian corals (Balanophyllia, Caryophyllia, Desmophyllum, Enallopsammia, Flabellum, Lophelia, and Vaughanella), both aragonitic and high-Mg calcitic stylasterid genera (Stylaster and Errina), and shallow-water high-Mg calcite crustose coralline algae (Lithophyllum, Hydrolithon, and Neogoniolithon). We interpret these data in conjunction with results from previously explored taxa including aragonitic zooxanthellate scleractinia and foraminifera, and high-Mg calcite octocorals. We show that Li/Mg ratios covary most strongly with seawater temperature, both for aragonitic and high-Mg calcitic taxa, making for reliable and universal seawater temperature proxies. Combining all of our biogenic aragonitic Li/Mg data with previous calibration efforts we report a refined relationship to temperature: Li/MgAll Aragonite = (). This calibration now permits paleo-temperature reconstruction to better than ±3.4 °C (95% prediction intervals) across biogenic aragonites, regardless of taxon, from 0 to 30 °C. For taxa in this study, aragonitic stylasterid Li/Mg offers the most robust temperature proxy (Li/MgStylasterid (Arag) = ()) with a reproducibility of ±2.3 °C. For the first time, we show that high-Mg calcites have a similar exponential relationship with temperature, but with a lower intercept value (Li/Mg = ()). This calibration opens the possibility of temperature reconstruction using high-Mg calcite corals and coralline algae. The commonality in the relationship between Li/Mg and temperature transcends phylogeny and suggests a similar abiogenic trace metal incorporation mechanism.
Stewart, J A, Robinson, L F, Day, R D, Strawson, I, Burke, A, Rae, J W B, Spooner, P T, Samperiz, A, Etnoyer, P J, Williams, B, Paytan, A, Leng, M J, Häussermann, V, Wickes, L N, Bratt, R, and Pryer, H. 2020. Refining trace metal temperature proxies in cold-water scleractinian and stylasterid corals. Earth and Planetary Science Letters, 545, 116412.
September 2020: Earth and Planetary Science Letters
Stylasterids are a ubiquitous deep-sea coral taxon that build their skeletons from either calcite, aragonite, or both. Yet, robust geochemical proxy data from these corals are limited. In this study, 95 modern stylasterids, spanning a wide range of depths (63 to 2894 m) and ambient seawater temperatures (0 to 17 °C), were tested for their potential use as paleoceanographic archives. Stable oxygen and carbon isotopic composition (O and C) were measured from the main trunk of all specimens and five specimens were further sub-sampled to assess internal chemical variability. The isotope data show non-equilibrium precipitation from seawater for both O and C, with the growing tips of colonies yielding the isotopically lowest values. Overall, the calcitic corals showed lower isotope values for O and C than aragonitic specimens. Within the aragonite corals, we present a O:temperature calibration that exhibits a significant linear relationship with the equation Ocoral-seawater = −0.22(°C) + 3.33(±0.06) across a temperature range of 0 to 30 °C, using samples from this study and published data. This work highlights the potential application of stylasterid coral O data to reconstruct paleo seawater temperature.
Samperiz, A, Robinson, L F, Stewart, J A, Strawson, I, Leng, M J, Rosenheim, B E, Ciscato, E R, Hendry, K R, and Santodomingo, N. 2020. Stylasterid corals: A new paleotemperature archive. Earth and Planetary Science Letters, 545, 116407.
August 2020: Geobiology
With annual precipitation less than 20 mm and extreme UV intensity, the Atacama Desert in northern Chile has long been utilized as an analogue for recent Mars. In these hyperarid environments, water and biomass are extremely limited, and thus, it becomes difficult to generate a full picture of biogeochemical phosphate‐water dynamics. To address this problem, we sampled soils from five Atacama study sites and conducted three main analyses—stable oxygen isotopes in phosphate, enzyme pathway predictions, and cell culture experiments. We found that high sedimentation rates decrease the relative size of the organic phosphorus pool, which appears to hinder extremophiles. Phosphoenzyme and pathway prediction analyses imply that inorganic pyrophosphatase is the most likely catalytic agent to cycle P in these environments, and this process will rapidly overtake other P utilization strategies. In these soils, the biogenic δ18O signatures of the soil phosphate (δ18OPO4) can slowly overprint lithogenic δ18OPO4 values over a timescale of tens to hundreds of millions of years when annual precipitation is more than 10 mm. The δ18OPO4 of calcium‐bound phosphate minerals seems to preserve the δ18O signature of the water used for biogeochemical P cycling, pointing toward sporadic rainfall and gypsum hydration water as key moisture sources. Where precipitation is less than 2 mm, biological cycling is restricted and bedrock δ18OPO4 values are preserved. This study demonstrates the utility of δ18OPO4 values as indicative of biogeochemical cycling and hydrodynamics in an extremely dry Mars‐analogue environment.
Shen, J, Smith, A C, Claire, M W, and Zerkle, A L. Unraveling biogeochemical phosphorus dynamics in hyperarid Mars‐analogue soils using stable oxygen isotopes in phosphate. Geobiology. 2020; 00: 1– 20.
August 2020: Quaternary Science Reviews
The Holocene hydroclimate of south-central Alaska has been studied extensively, but conflicting interpretations between oxygen isotope paleoclimate datasets are seemingly as common as converging reconstructions, in part due to the challenges of interpreting oxygen isotope ratios in terms of climate. Here, we present a new Holocene record of biogenic silica abundance (BSi), diatom flora, and diatom oxygen isotopes (δ18OBSi) analyzed in sediments from Sunken Island Lake (SIL) in the Kenai Peninsula lowlands, which we interpret in the context of previously published paleoclimate records, and use to understand regional changes in hydroclimate. Changes in lake level documented by aerial photography coupled with a survey of regional lake water isotopes indicate SIL is sensitive to changes in the balance of precipitation and evaporation (P-E). However, an analysis of SIL δ18OBSi over the instrumental period indicates that δ18OBSi is sensitive to both P-E and the isotope composition of precipitation (δ18Oprecip), which is driven by changes in the Aleutian Low atmospheric pressure cell (AL). We attribute a ∼2‰ increase in δ18OBSi from 5.5 to 4.5 ka cal BP to a stronger AL, which resulted in the delivery of isotopically heavier precipitation to the Kenai lowlands, and wetter conditions during the late Holocene. These interpretations are supported by late Holocene increases in the relative abundance of planktonic diatoms and BSi-inferred storminess, and by evidence for higher-than-present lake levels on the paleo-shorelines above SIL at ∼1.5–0.5 ka cal BP. Our dataset demonstrates that this region was characterized by relatively low lake levels and dry climate in the early Holocene, a strengthening of the AL in the late Holocene, and wetter climate during the late Holocene until recent decades.
Broadman, E, Kaufman, D S, Henderson, A C E, Berg, E E, Anderson, S, Leng, M J, Stahnke, S A, and Muñoz, S E. 2020. Multi-proxy evidence for millennial-scale changes in North Pacific Holocene hydroclimate from the Kenai Peninsula lowlands, south-central Alaska. Quaternary Science Reviews, 241, 106420.
August 2020: Earth-Science Reviews
Understanding and quantifying the processes and geochemical cycles associated with catchment erosion, the development of soils and weathering horizons, and terrestrial habitat change beyond the scales of modern observations remain challenging. Such research, however, has become increasingly important to help predict future landscape change in light of increasing land use and rapid global warming. We herein review organic and inorganic geochemical tools applied to depositional archives to better understand various aspects of landscape evolution on geological time scales. We highlight the potentials and limitations of inorganic geochemical analytical methods, such as major element geochemistry, metal and radiogenic isotopes, and in-situ cosmogenic nuclides, as qualitative, semi-quantitative, and quantitative proxies for the transformation of bedrock material via regolith and soils to sediments. We also show how stable isotope geochemistry applied to lacustrine endogenic carbonates can be used to infer rock-water interactions, vegetation change, and soil development in limestone-rich catchments. Proxies focusing on the silicilastic element of sediment formation, transport and deposition are ideally combined with organic geochemical proxies for vegetation change and soil organic matter evolution in a catchment to gain a comprehensive picture of the Critical Zone’s evolution over time. Multi-proxy and multidisciplinary research combining organic and inorganic geochemical techniques from several sedimentary archives in the same catchment have high potential to provide comprehensive information on Quaternary landscape evolution and thus improve the robustness of associated forecasting models.
Francke, A, Holtvoeth, J, Codilean, A, Lacey, J H, Bayon, G, and Dosseto, A. 2020. Geochemical methods to infer landscape response to Quaternary climate change and land use in depositional archives: A review. Earth-Science Reviews, 207, 103218.
July 2020: Hydrological Processes
The young water fraction of streamflow (Fyw), an important hydrological variable, has been calculated for the first time, for a monsoon‐fed coastal catchment in northern Vietnam. Oxygen stable isotopes (δ18O) from 6 river sites in the Day River Basin (DRB) were analysed monthly, between January 2015 and December 2018. River δ18O signatures showed sine wave variability, reflecting the amount effect and tropical (dry‐rainy) seasonality of the region. The δ18O composition of precipitation ranged from ‐12.67 to +1.68‰, with a mean value of ‐5.14‰, and in‐streamflow signatures ranged from ‐11.63 to ‐1.37‰ with a mean of ‐5.02‰. Fractions of young water (Fyw) were calculated from the unweighted and flow‐weighted δ18O composition of samples. Unweighted Fyw ranged between 29±8% and 82±21% with a mean value of 51±19%, and was not significantly different from flow‐weighted Fyw (range between 33±25% and 92±73%, mean 52±36%). Both unweighted and flow‐weighted Fyw were highest in the middle of stream and lowest in downstream sites, capturing the impacts of landuse changes, hydrology, and human activities in the catchment. Our calculations imply that more than a half of rainwater reaches the DRB river mainstream within the first 3 months. The Fyw is much higher than the global average (of one third) and insensitive to discharge due to the combination of a humid catchment with high rainfall, low storage capacity, flat landscape and an intensive drainage system in the DRB. Also the low discharge sensitivity of Fyw in the DRB implies that the regional hydrology is severely altered by humans.
Duc, T A, Nga, D T, Panizzo, V N, McGowan, S, and Leng, M J. 2020. Using stable isotopes to estimate young water fractions in a heavily‐regulated, tropical lowland river basin. Hydrological Processes.
July 2020: Quaternary Science Reviews
Andøya on the NW coast of Norway is a key site for understanding the Last Glacial Maximum (LGM) in northern Europe. Controversy has arisen concerning the local conditions, especially about the timing and extent of local glacial cover, maximum July temperatures and whether pine and/or spruce could have grown there. We reviewed all existing data and add newly analysed ancient sedimentary DNA (sedaDNA), pollen, macrofossils, geochemistry and stable isotopes from three lake sediment cores from Øvre Æråsvatnet. A total of 23 new dates and age-depth modelling suggests the lake has been ice-free since GI2 (<23.4 cal ka BP) and possibly GS3 (<26.7 cal ka BP). Pinus and Picea sedaDNA was found in all three cores but at such low frequencies that it could not be distinguished from background contamination. LGM samples have an exceptionally high organic matter content, with isotopic values indicating that carbon and nitrogen derive from a marine source. Along with finds of bones of the little auk (Alle alle), this indicates that the lake received guano from an adjacent bird colony. SedaDNA, pollen and macrofossil assemblages were dominated by Poaceae, Brassicaceae and Papaver, but scattered occurrence of species currently restricted to the Low Arctic Tundra Zone (July temperature of 8–9 °C) such as Apiaceae (sedaDNA, 8–9 °C), and Alchemilla alpina (macrofossil, 8–9 °C) were also recorded. The review of >14.7 cal ka BP data recorded 94 vascular plant taxa, of which 38% have a northern limit in Shrub Tundra or more southern vegetation zones. This unusual assemblage likely stems from a combination of proximity to ice-free water in summer, geographical isolation linked with stochastic long-distance dispersal events, and the presence of bird-fertilized habitats. The environmental reconstruction based on all records from the area does not preclude local growth of tree species, as the local climate combined with high nutrient input may have led to periodically suitable environmental ‘hotspot’ conditions.
Alsos, I, Sjögren, P, Brown, A, Gielly L, Merkel, M, Paus, A, Lammers, Y, Edwards, M, Alm, T, Leng, M, Goslar, T, Langdon, C, Bakke, J, and van der Bilt, W. 2020. Last Glacial Maximum environmental conditions at Andøya, northern Norway; evidence for a northern ice-edge ecological “hotspot”. Quaternary Science Reviews, 239.
June 2020: Chemical Geography
Although the oxygen isotope composition (δ18O) of calcite (δ18Ocalcite) and, to a lesser extent, diatom silica (δ18Odiatom) are widely used tracers of past hydroclimates (especially temperature and surface water hydrology), the degree to which these two hosts simultaneously acquire their isotope signals in modern lacustrine environments, or how these are altered during initial sedimentation, is poorly understood. This study compares oxygen and hydrogen isotope data (δ18O, δ2H) of contemporary lake water samples at ~2-weekly intervals over a 2-year period (2010–12) with matching collections of diatoms (δ18Odiatom) and calcite (δ18Ocalcite) from sediment traps (at 10 m and 25 m) at Rostherne Mere (maximum depth 30 m), a well-monitored, eutrophic, seasonally stratified monomictic lake in the UK. The epilimnion shows a seasonal pattern of rising temperature and summer evaporative enrichment in 18O, and while there is a temperature imprint in both δ18Odiatom and δ18Ocalcite, there is significant inter-annual variability in both of these signals. The interpretation of δ18Odiatom and δ18Ocalcite values is complicated due to in-lake processes (e.g. non-equilibrium calcite precipitation, especially in spring, leading to significant 18Ocalcite depletion), and for δ18Odiatom, by post-mortem, depositional and possibly dissolution or diagenetic effects. For 2010 and 2011 respectively, there is a strong temperature dependence of δ18Ocalcite and δ18Odiatom in fresh trap material, with the fractionation slope for δ18Odiatom of ca. −0.2‰/°C, in agreement with several other studies. The δ18Odiatom data indicate the initiation of rapid post-mortem secondary alteration of fresh diatom silica (within ~6 months), with some trap material undergoing partial maturation in situ. Diatom δ18O of the trap material is also influenced by resuspension of diatom frustules from surface sediments (notably in summer 2011), with the net effect seen as an enrichment of deep-trap 18Odiatom by about +0.7‰ relative to shallow-trap values. Contact with anoxic water and anaerobic bacteria are potentially key to initiating this silica maturation process, as deep-trap samples that were removed prior to anoxia developing do not show enrichment. Dissolution (perhaps enhanced by anaerobic bacterial communities) may also be responsible for changes to δ18Odiatom that lead to increasing, but apparently predictable, error in inferred temperatures using this proxy. High resolution, multi-year monitoring can shed light on the complex dynamics affecting δ18Odiatom and δ18Ocalcite and supports the careful use of sedimentary δ18Odiatom and δ18Ocalcite as containing valuable hydroclimatic signals especially at a multi-annual resolution, although there remain substantial challenges to developing a reliable geothermometer on paired δ18Odiatom and δ18Ocalcite. In particular, δ18Odiatom needs cautious interpretation where silica post-mortem secondary alteration is incomplete and diatom preservation is not perfect, and we recommend dissolution be routinely assessed on diatom samples used for isotopic analyses.
Ryves, D B, Leng, M J, Barker, P A, Snelling, A M, Sloane, H J, Arrowsmith, C, Tyler, J J, Scott, D R, Radbourne, A D, and Anderson, N J. 2020. Understanding the transfer of contemporary temperature signals into lake sediments via paired oxygen isotope ratios in carbonates and diatom silica: Problems and potential, Chemical Geology, 119705.
May 2020: Quaternary Science Reviews
We provide evidence for a large-scale geomorphic event in Cambodia’s great lake, the Tonlé Sap, during the middle Holocene. The present-day hydrology of the basin is dominated by an annual flood pulse where water from the Mekong River raises the lake level by c. 8 m during the monsoon season. We present new subsurface geophysical data, allied to new and past core studies, which unequivocally show a period of major mid-Holocene erosion across the entire Tonlé Sap basin that is coincident with establishment of the lake’s flood pulse. We argue that this widespread erosion, which removed at least 1.2 m of sediment across the lake’s extent, was triggered by up to three, likely interacting, processes: (1) base-level lowering due to mid-Holocene sea-level fall, leading to (2) capture of the Tonlé Sap drainage by the Mekong River, and (3) a drying climate that also reduced lake level. Longer-term landscape evolution was thus punctuated by a rapid, river capture- and base-level fall- induced, lake drainage that established the ecosystem that flourishes today. The scale of change induced by this mid-Holocene river capture event demonstrates the susceptibility of the Tonlé Sap lake to ongoing changes in local base-level and hydrology induced by anthropogenic activity, such as damming and sand mining, within the Mekong River Basin.
Darby, S E Langdon, P G, Best, J L, Leyland, J, Hackney, C R Marti, M, Morgan, P R, Ben, S, Aalto, R, Parsons, D R, Nicholas, A P, and Leng, M J. 2020. Drainage and erosion of Cambodia’s great lake in the middle-late Holocene: The combined role of climatic drying, base-level fall and river capture. Quaternary Science Reviews, 236, 106265.
May 2020: Scientific Drilling
The Neogene and Quaternary are characterised by enormous changes in global climate and environments, including global cooling and the establishment of northern high–latitude glaciers. These changes reshaped global ecosystems, including the emergence of tropical dry forests and savannahs that are found in Africa today, which in turn may have influenced the evolution of humans and their ancestors. However, despite decades of research we lack long, continuous, well-resolved records of tropical climate, ecosystem changes, and surface processes necessary to understand their interactions and influences on evolutionary processes. Lake Tanganyika, Africa, contains the most continuous, long continental climate record from the mid-Miocene (~10 Ma) to the present anywhere in the tropics and has long been recognised as a top priority site for scientific drilling. The lake is surrounded by the Miombo woodlands, part of the largest dry tropical biome on Earth. Lake Tanganyika also harbors incredibly diverse endemic biota and an entirely unexplored deep microbial biosphere, and it provides textbook examples of rift segmentation, fault behaviour, and associated surface processes. To evaluate the interdisciplinary scientific opportunities that an ICDP drilling program at Lake Tanganyika could offer, more than 70 scientists representing 12 countries and a variety of scientific disciplines met in Dar es Salaam, Tanzania, in June 2019. The team developed key research objectives in basin evolution, source-to-sink sedimentology, organismal evolution, geomicrobiology, palaeoclimatology, palaeolimnology, terrestrial palaeoecology, palaeoanthropology, and geochronology to be addressed through scientific drilling on Lake Tanganyika. They also identified drilling targets and strategies, logistical challenges, and education and capacity building programs to be carried out through the project. Participants concluded that a drilling program at Lake Tanganyika would produce the first continuous Miocene–present record from the tropics, transforming our understanding of global environmental change, the environmental context of human origins in Africa, and providing a detailed window into the dynamics, tempo and mode of biological diversification and adaptive radiations.
Russell, J M, Barker, P, Cohen, A, Ivory, S, Kimirei, I, Lane, C, Leng, M, Maganza, N, McGlue, M, Msaky, E, Noren, A, Park Boush, L, Salzburger, W, Scholz, C, Tiedemann, R, Nuru, S, and the Lake Tanganyika Scientific Drilling Project (TSDP) Consortium. 2020. ICDP workshop on the Lake Tanganyika Scientific Drilling Project: a late Miocene–present record of climate, rifting, and ecosystem evolution from the world’s oldest tropical lake, Sci. Dril., 27, 53–60.
May 2020: Geography and the Environment
Rapid development and climate change in southeast Asia is placing unprecedented pressures on freshwater ecosystems, but long term records of the ecological consequences are rare. Here we examine one basin of Tasik Chini (Malaysia), a UNESCO‐designated flood pulse wetland, where human disturbances (dam installation, iron ore mining, oil palm and rubber cultivation) have escalated since the 1980s. Diatom analysis and organic matter geochemistry (δ13Corg and C/N ratios) were applied to a sediment sequence to infer ecological changes in the basin since c. 1900 CE. As the Tasik Chini wetland is a rare ecosystem with an unknown diatom ecology, contemporary diatom habitats (plant surfaces, mud surfaces, rocks, plankton) were sampled from across the lake to help interpret the sedimentary record. Habitat specificity of diatoms was not strongly defined and, although planktonic and benthic groupings were distinctive, there was no difference in assemblages among the benthic habitat surfaces. An increase in the proportion of benthic diatom taxa suggests that a substantial decrease in water level occurred between c. 1938 and 1995 CE, initiated by a decline in rainfall (supported by regional meteorological data), which increased the hydrological isolation of the sub‐basin. Changes in the diatom assemblages were most marked after 1995 CE when the Chini dam was installed. After this time both δ13Corg and C/N decreased, suggesting an increase in autochthonous production relative to allochthonous river flood pulse inputs. Oil palm plantations and mining continued to expand after c. 1995 CE and we speculate that inputs of pollutants from these activities may have contributed to the marked ecological change. Together, our work shows that this sub‐basin of Tasik Chini has been particularly sensitive to, and impacted by, a combination of human and climatically induced changes due to its hydrologically isolated position.
Briddon, C L, McGowan, S, Metcalfe, S E, Panizzo, V, Lacey, J, Engels, S, Leng, M, Mills, K, Shafiq, M, and Idris, M. 2020. Diatoms in a sediment core from a flood pulse wetland in Malaysia record strong responses to human impacts and hydro–climate over the past 150 years. Geo: Geography and Environment.
April 2020: PNAS
The timing of human colonization of East Polynesia, a vast area lying between Hawai‘i, Rapa Nui, and New Zealand, is much debated and the underlying causes of this great migration have been enigmatic. Our study generates evidence for human dispersal into eastern Polynesia from islands to the west from around AD 900 and contemporaneous paleoclimate data from the likely source region. Lake cores from Atiu, Southern Cook Islands (SCIs) register evidence of pig and/or human occupation on a virgin landscape at this time, followed by changes in lake carbon around AD 1000 and significant anthropogenic disturbance from c. AD 1100. The broader paleoclimate context of these early voyages of exploration are derived from the Atiu lake core and complemented by additional lake cores from Samoa (directly west) and Vanuatu (southwest) and published hydroclimate proxies from the Society Islands (northeast) and Kiribati (north). Algal lipid and leaf wax biomarkers allow for comparisons of changing hydroclimate conditions across the region before, during, and after human arrival in the SCIs. The evidence indicates a prolonged drought in the likely western source region for these colonists, lasting c. 200 to 400 y, contemporaneous with the phasing of human dispersal into the Pacific. We propose that drying climate, coupled with documented social pressures and societal developments, instigated initial eastward exploration, resulting in SCI landfall(s) and return voyaging, with colonization a century or two later. This incremental settlement process likely involved the accumulation of critical maritime knowledge over several generations.
Sear, D A, Allen, M S, Hassall, J D, Maloney, A E, Langdon, P G, Morrison, A E, Henderson, A C G, Mackay, H, Croudace, I W, Clarke, C, Sachs, J, Macdonald, G, Chiverrell, R, Leng, M, Cisneros-Dozal, L, and Fonville, T. 2020. Human settlement of East Polynesia earlier, incremental, and coincident with prolonged South Pacific drought. Proceedings of the National Academy of Sciences of the United States of America, 117, 8813–8819.
March 2020: Palaeogeography, Palaeoclimatology, Palaeoecology
Wetland sediments archive information about past terrestrial ecosystem change including variations in fire occurrence and terrestrial carbon fluxes. The charcoal content of sediments is important for understanding past fire regimes, as well as the role this recalcitrant carbon plays in the global carbon cycle. Infrared (IR) spectroscopy provides a rapid, non-destructive and cost effective method for simultaneously analysing numerous organic and inorganic sediment properties. The use of IR spectroscopy is well developed for determining concentrations of total organic carbon (TOC), total nitrogen (TN), biogenic silica and carbonate in lacustrine sediments. In soil science IR spectroscopy is also routinely used to determine charcoal content, however the potential for analysing charcoal content from lacustrine sediments has yet to be investigated. Here we develop IR spectroscopy and partial least squares regressions (PLSR) to predict the charcoal and TOC content of an organic, 130,000 year old sediment sequence from North Stradbroke Island (Minjerribah), Australia. Charcoal concentrations used for model development were derived using both traditional palaeoecological area measures (cm2 g−1) and solid state 13C nuclear magnetic resonance (13C NMR) of poly-aryl structures. The IR PLSR models yielded significant correlations for the two charcoal methodologies (area measurements, R2 = 0.57, p < .05; 13C NMR, R2 = 0.70, p < .05). We additionally find a very strong, significant, correlation for TOC (R2 = 0.92, p < .05), consistent with previous studies. Hence, IR is a promising tool for determining the charcoal content of lacustrine sediments, particularly for first-order sample screening, as part of a multi-proxy framework. IR spectroscopy can therefore provide a reliable and rapid technique for the initial investigation of fire histories and organic constituents of sedimentary sequences.
Cadd, H R, Tyler, J, Tibby, J, Baldock, J, Hawke, B, Barr, C, and Leng, M.J. 2020. The potential for rapid determination of charcoal from wetland sediments using infrared spectroscopy. Palaeogeography, Palaeoclimatology, Palaeoecology, 542,109562.
February 2020: PNAS
Global perturbations to the Early Jurassic environment (∼201 to ∼174 Ma), notably during the Triassic–Jurassic transition and Toarcian Oceanic Anoxic Event, are well studied and largely associated with volcanogenic greenhouse gas emissions released by large igneous provinces. The long-term secular evolution, timing, and pacing of changes in the Early Jurassic carbon cycle that provide context for these events are thus far poorly understood due to a lack of continuous high-resolution δ13C data. Here we present a δ13CTOC record for the uppermost Rhaetian (Triassic) to Pliensbachian (Lower Jurassic), derived from a calcareous mudstone succession of the exceptionally expanded Llanbedr (Mochras Farm) borehole, Cardigan Bay Basin, Wales, United Kingdom. Combined with existing δ13CTOC data from the Toarcian, the compilation covers the entire Lower Jurassic. The dataset reproduces large-amplitude δ13CTOC excursions (>3‰) recognized elsewhere, at the Sinemurian–Pliensbachian transition and in the lower Toarcian serpentinum zone, as well as several previously identified medium-amplitude (∼0.5 to 2‰) shifts in the Hettangian to Pliensbachian interval. In addition, multiple hitherto undiscovered isotope shifts of comparable amplitude and stratigraphic extent are recorded, demonstrating that those similar features described earlier from stratigraphically more limited sections are nonunique in a long-term context. These shifts are identified as long-eccentricity (∼405-ky) orbital cycles. Orbital tuning of the δ13CTOC record provides the basis for an astrochronological duration estimate for the Pliensbachian and Sinemurian, giving implications for the duration of the Hettangian Stage. Overall the chemostratigraphy illustrates particular sensitivity of the marine carbon cycle to long-eccentricity orbital forcing.
Storm, M S, Hesselbo, S P Jenkyns, H C, Ruhl, M, Ullmann, C V, Xu, W, Leng, M J, Riding, J B, and Gorbanenko, O. 2020. Orbital pacing and secular evolution of the Early Jurassic carbon cycle. Proceedings of the National Academy of Sciences of the United States of America, 117, 3974–3982.
February 2020: Scientific Reports
Field data about the effect of soil pH on phosphorus (P) cycling is limited. A promising tool to study P cycling under field conditions is the 18O/16O ratio of phosphate (δ18OP). In this study we investigate whether the δ18OP can be used to elucidate the effect of soil pH on P cycling in grasslands. Soils and plants were sampled from different fertilisation and lime treatments of the Park Grass long term experiment at Rothamsted Research, UK. The soils were sequentially extracted to isolate different soil P pools, including available P and corresponding δ18OP values were determined. We did not observe changes in plant δ18OP value, but soil P δ18OP values changed, and lower δ18OP values were associated with higher soil pH values. At sites where P was not limiting, available P δ18OP increased by up to 3‰ when lime was applied. We show that the δ18OP method is a useful tool to investigate the effect of pH on soil P cycling under field conditions as it highlights that different soil processes must govern P availability as pH shifts. The next challenge is now to identify these underlying processes, enabling better management of soil P at different pH.
Pfahler, V, Macdonald, A, Mead, A, Smith, A C, and Tamburini, F. 2020. Changes of oxygen isotope values of soil P pools associated with changes in soil pH. Scientific Reports 10.
January 2020: Quaternary Science Reviews
Mediterranean mid-altitude sites are critical for the survival of plant species allowing for elevational vegetation shifts in response to high-amplitude climate variability. Pollen records from the southern Balkans have underlined the importance of the region in preserving plant diversity over at least the last half a million years. So far, there are no records of vegetation and climate dynamics from Balkan refugia with an Early Pleistocene age. Here we present a unique palynological archive from such a refugium, the Lake Ohrid basin, recording continuously floristic diversity and vegetation succession under obliquity-paced climate oscillations. Palynological data are complemented by biomarker, diatom, carbonate isotope and sedimentological data to identify the mechanisms controlling shifts in the aquatic and terrestrial ecosystems within the lake and its catchment. The study interval encompasses four complete glacial-interglacial cycles (1365–1165 ka; MIS 43–35). Within the first 100 kyr of lake ontogeny, lake size and depth increase before the lake system enters a new equilibrium state as observed in a distinct shift in biotic communities and sediment composition. Several relict tree genera such as Cedrus, Tsuga, Carya, and Pterocarya played an important role in ecological succession cycles, while total relict abundance accounts for up to half of the total arboreal vegetation. The most prominent biome during interglacials is cool mixed evergreen needleleaf and deciduous broadleaf forests, while cool evergreen needleleaf forests dominate within glacials. A rather forested landscape with a remarkable plant diversity provide unique insights into Early Pleistocene ecosystem resilience and vegetation dynamics.
Panagiotopoulos, K, Holtvoeth, J, Kouli, K, Marinova, E, Francke, A, Cvetkoska, A, Jovanovska, E, Lacey, J, Lyons, E, Buckel, C, Bertini, A, Donders, T, Just, J, Leicher, N, Leng, M, Melles, M, Pancost, R, Sadori, L, Tauber, P, Vogel, H, Wagner, B, and Wilke, T 2020. Insights into the evolution of the young Lake Ohrid ecosystem and vegetation succession from a southern European refugium during the Early Pleistocene. Quaternary Science Reviews, 227.
January 2020: Marine Geology
Remotely operated vehicle (ROV) explorations of the Kahouanne Seamounts, located 25 km SE of Montserrat in the northern Lesser Antilles island arc, have discovered the occurrence of honeycomb-scalloped erosional features on volcanic and limestone outcrops at depths of up to 600 m below sea level (mbsl). These features, combined with the flat-topped morphology of the seamounts, the occurrence of shallow-water carbonates (rhodoliths, benthic foraminifera), and the presence of oxidized, highly-vesicular volcanic fragments, suggest that the seamounts were once subaerial islands and have subsequently subsided to their present depth. The subsidence is likely to have been in response to a combination of 1) graben development southeast of the island of Montserrat where faulting and extension are driven by the accommodation of slip convergence associated with oblique subduction along the Lesser Antilles volcanic arc and 2) regional subsidence inferred from studies of nearby carbonate platforms. 40Ar/39Ar dating of basalt/andesite lavas from the central seamount indicates active volcanism around 5.4 Ma. Shallow-water carbonates recovered from an apparent wave-cut terrace at ~600 mbsl in the same area, were deposited approximately 3 Ma, based on Sr-isotope stratigraphy, indicating significant subsidence of the complex since that time. The total subsidence is in line with displacements in the Kahouanne valley graben and regional subsidence rates inferred from carbonate platform depths around the islands of Guadeloupe and Martinique, implying that intra-arc subsidence has been a significant process shaping the present bathymetry and topography of this island arc.
Carey, S, Sparks, R S J, Tucker, M E, Li, T, Robinson, L, Watt, S F L, Gee, M, Hastie, A, Barfod, D N, Stinton, A, Leng, M, Raineault, N, and Ballard, R.D. 2020. The polygenetic Kahouanne Seamounts in the northern Lesser Antilles island arc: Evidence for large-scale volcanic island subsidence. Marine Geology, 419, 106046.
January 2020: Journal of Biogeography
A sedimentary record covering the last c. 10 500 years was recovered from the volcanic crater that contains Lake Lanoto’o near the centre of Upolu Island. Information on past ecological change was obtained from microscopic and macroscopic remains extracted from the sediments: charcoal (fire history), pollen/spores and plant remains (vegetation history), and lake status (algae/cyanobacteria). Information on the depositional environment and climate was obtained from geochemical and sedimentary analysis: loss-on-ignition (sediment composition), cryptotephras (volcanic eruptions) and precipitation regime (Ti/inc). The environmental history developed was compared with the archaeological record from the region. Charcoal material was found in the Lake Lanoto’o sediments at higher abundances and more frequently in samples from the period after the first archaeological evidence of people on Upolu (c. 2900–2700 years ago). No abrupt shift is recognized in the vegetation or aquatic ecosystem assemblages coincident with the arrival of people on the island. Macrocharcoal is demonstrated to be an effective proxy for detecting human occupation of Upolu around 2800 years ago. The immediate impact of these settlers on the vegetation seems to have been minimal; however, a subsequent opening up of the landscape is suggested through the gradual increase in ferns. The absence of any significant change in the aquatic community associated with, or after, the arrival of people on the islands suggests that humans rarely visited the lake. We suggest that on Upolu a simple model of decreasing human impact away from coastal areas is applicable.
Gosling, W D, Sear, D A, Hassall, J D, Langdon, P G, Bönnen, M N T, Driessen, T D, van Kemenade, Z R, Noort, K, Leng, M J, Croudace, I W, Bourne, A J, and McMichael, C N H. 2020. Human occupation and ecosystem change on Upolu (Samoa) during the Holocene. Journal of Biogeography.
Contact Béatrice Bullock-von Moos for further information about the Stable isotope facility.