Effects of Shoreline Development on Composition and Physical Structure of Plants in a South Carolina High Marsh

Increased freshwater and nutrient runoff associated with coastal development is implicated in dramatically altering estuarine communities along eastern US shorelines. We examined effects of three categories of shoreline development on high-marsh environments within Murrells Inlet, South Carolina, USA by measuring sediment nutrients, porewater salinity, plant species diversity, and above- and belowground plant biomass. Effects on new plant growth also were examined in plot clearing and transplantation experiments. Greater nutrient availability in sediments along developed shorelines was reflected in greater aboveground biomass and nitrogen storage in Juncus roemerianus plant tissue. Plant species composition was not significantly different among levels of shoreline development. Zinc concentrations were greater in sediments from developed shorelines and may represent an easily measured indicator of shoreline development. Recently accelerating shoreline development in the southeastern USA may alter plant production, nitrogen storage, and sediment metal content in salt marshes.     

CO<Subscript>2</Subscript> flux variation and its contribution area in the debris-covered area of Koxkar Glacier,Mt. Tianshan in China

During the formation and development of glacial meltwater runoff, hydrochemical erosion is abundant, especially the hydrolysis of K/Na feldspar and carbonates, which can consume H⁺ in the water, promote the formation of bicarbonate by dissolving atmospheric CO₂, and affect the regional carbon cycle. From July 21, 2015, to July 18, 2017, the CO₂ concentration and flux were observed by the eddy covariance (EC) method in the relatively flat and open moraine cover area of Koxkar Glacier in western Mt. Tianshan, China. We found that: (1) atmospheric CO₂ fluxes ranged from ??408.95 to 81.58 mmol m^?2 day^?1 (average ? 58.68 mmol m^?2 day^?1), suggesting that the study area is a significant carbon sink, (2) the CO₂ flux footprint contribution areas were primarily within 150 m of the EC station, averaging total contribution rates of 93.30%, 91.39%, and 90.17% of the CO₂ flux in the snow accumulation, snow melting, and glacial melting periods, respectively. Therefore, the contribution areas with significant influences on CO₂ flux observed at EC stations were concentrated, demonstrating that grassland CO₂ flux around the glaciers had little effect at the EC stations, (3) in the predominant wind direction, under stable daytime atmospheric stratification, the measurement of CO₂ flux, as interpreted by the Agroscope Reckenholz Tanikon footprint tool, was 79.09% ± 1.84% in the contribution area. This was slightly more than seen at night, but significantly lower than the average under unstable atmospheric stratification across the three periods of interest (89%). The average distance of the farthest point of the flux footprint under steady state atmospheric conditions was 202.61?±?69.33 m, markedly greater than that under non-steady state conditions (68.55?±?10.34 m). This also indicates that the CO₂ flux observed using EC was affected primarily by hydrochemical erosion reactions in the glacier area, (4) a good negative correlation was found between net glacier exchange (NGE) of CO₂ and air temperature on precipitation-free days. Strong ice and snow ablation could promote hydrochemical reactions of soluble substances in the debris area and accelerated sinking of atmospheric CO₂. Precipitation events might reduce snow and ice melting, driven by reduced regional temperatures. However, a connection between NGE and precipitation, when less than 8.8 mm per day, was not obvious. When precipitation was greater than 8.8 mm per day, NGE decreased with increasing precipitation, (5) graphically, the slope of NGE, related to daily runoff, followed a trend: snow melting period?>?snow accumulation period?>?early glacial ablation period?>?late glacier ablation period?>?dramatic glacier ablation period. The slope was relatively large during snow melting, likely because of CO₂ sinking caused by water–rock interactions. The chemical reaction during elution in the snow layer might also promote atmospheric CO₂ drawdown. At the same time, the damping effect of snow cover and the almost-closed glacier hydrographic channel inhibited the formation of regional runoff, possibly providing sufficient time for the chemical reaction, thus promoting further CO₂ drawdown.     

Late Pleistocene/Holocene Environments in the middle Yangtze River Valley,China and rice (oryza sativa L.) domestication: The phytolith evidence

Phytolith data from Poyang Lake, southern China, indicate that significant natural and human‐induced vegetational changes have occurred in the middle Yangtze River valley, the likely hearth of rice (Oryza sativa L.) domestication, during the Late Pleistocene and Holocene periods. During the Late Pleistocene (from >13,500 to ca. 10,500 yr B.P.) the climate was cooler and drier than today's. Oryza appears to have been a natural component of the vegetation at that time, but may not have been well adapted to the glacial climatic conditions. The early Holocene climate may have been wetter and more markedly seasonal that at present, and wild Oryza species may have been distributed further north than seen today. By 4000 yr B.P., rice agriculture appears to have been well developed in the middle Yangtze River Valley. Environmental factors such as atmospheric CO₂ concentrations and the seasonality of precipitation and temperature in addition to overall cooler and drier Pleistocene climates may have significantly influenced human exploitation of Oryza during the Late Pleistocene and early Holocene in southern China. © 2000 John Wiley & Sons, Inc.     

Milestones in the evolution of the atmosphere with reference to climate change

The history of life on Earth is critically dependent on the carbon, sulfur and oxygen cycles of the lithosphere – hydrosphere – atmosphere – biosphere system. An Archean oxygen-poor greenhouse atmosphere developed through: (i) accumulation of CO₂ and CH₄ from episodic injections of CO₂ from volcanic activity, volatilised crust impacted by asteroids and comets, metamorphic devolatilisation processes and release of methane from sediments; and (ii) little CO₂ weathering-capture due to both high temperatures of the hydrosphere (low CO₂ solubility) and a low ratio of exposed continents to oceans. In the wake of the Sturtian glaciation, enrichment in oxygen and appearance of multicellular eukaryotes heralded the onset of the Phanerozoic where greenhouse conditions were interrupted by periods of strong CO₂-sequestration through intensified capture of CO₂ by marine plants, onset of land plants and burial of carbonaceous shale and coal (Late Ordovician; Carboniferous – Permian; Late Jurassic; Late Tertiary – Quaternary). The progression from Late Mesozoic and Early Tertiary greenhouse conditions to Late Tertiary – Quaternary ice ages was related to the sequestration of CO₂ by rapid weathering of the emerging Alpine and Himalayan mountain chains. A number of peak warming and sea-level-rise events include the Late Oligocene, mid-Miocene, mid-Pliocene and Pleistocene glacial terminations. The Late Tertiary – Quaternary ice ages were dominated by cyclic orbital-forcing-triggered terminations which involved CO₂-feedback effects from warming seas and the biosphere and albedo flips due to ice-sheet melting. Since ca AD 1750 human emissions were ～305 Gt of carbon, as compared with ～750 Gt C in the atmosphere. The emissions constitute ～12% of the terrestrial biosphere and ～10% of the known global fossil fuel reserve of ～4000 Gt C, whose combustion would compare to the ～ 4600 Gt C released to the atmosphere during the K – T impact event 65 million years ago, with associated ～65% mass extinction of species. The current growth rate of atmospheric greenhouse gases and global mean temperatures exceed those of Pleistocene glacial terminations by one to two orders of magnitude. The relationship between temperatures and sea-levels for the last few million years project future sea-level rises toward time-averaged values of at least 5 m per 1°C. The instability of ice sheets suggested by the Dansgaard – Oeschinger glacial cycles during 50 – 20 ka, observed ice melt lag effects of glacial terminations, spring ice collapse dynamics and the doubling per-decade of Greenland and west Antarctic ice melt suggest that the Intergovernmental Panel on Climate Change's projected sea-level rises (<59 cm) for the 21st century may be exceeded. The biological and philosophical rationale underlying climate change and mass extinction perpetrated by an intelligent carbon-emitting mammal species may never be known.     

New U/Th ages for Pleistocene megafauna deposits of southeastern Queensland,Australia

Arguments over the extinction of Pleistocene megafauna have become particularly polarised in recent years. Causes for the extinctions are widely debated with climate change, human hunting and/or habitat modification, or a combination of those factors, being the dominant hypotheses. However, a lack of a spatially constrained chronology for many megafauna renders most hypotheses difficult to test. Here, we present several new U/Th dates for a series of previously undated, megafauna-bearing localities from southeastern Queensland, Australia. The sites were previously used to argue for or against various megafauna extinction hypotheses, and are the type localities for two now-extinct Pleistocene marsupials (including the giant koala, Phascolarctos stirtoni). The new dating allows the deposits to be placed in a spatially- and temporally constrained context relevant to the understanding of Australian megafaunal extinctions. The results indicate that The Joint (Texas Caves) megafaunal assemblage is middle Pleistocene or older (>292 ky); the Cement Mills (Gore) megafaunal assemblage is late Pleistocene or older (>53 ky); and the Russenden Cave Bone Chamber (Texas Caves) megafaunal assemblage is late Pleistocene (～55 ky). Importantly, the new results broadly show that the sites date prior to the hypothesised megafaunal extinction ‘window’ (i.e., ～30–50 ky), and therefore, cannot be used to argue exclusively for or against human/climate change extinction models, without first exploring their palaeoecological significance on wider temporal and spatial scales.     

Nutrient Inputs, Phytoplankton Response, and CO2 Variations in a Semi-Enclosed Subtropical Embayment, Kaneohe Bay, Hawaii

The marine shelf areas in subtropical and tropical regions represent only 35% of the total shelf areas globally, but receive a disproportionately large amount of water (65%) and sediment (58%) discharges that enter such environments. Small rivers and/or streams that drain the mountainous areas in these climatic zones deliver the majority of the sediment and nutrient inputs to these narrow shelf environments; such inputs often occur as discrete, episodic introductions associated with storm events. To gain insight into the linked biogeochemical behavior of subtropical/tropical mountainous watershed-coastal ocean ecosystems, this work describes the use of a buoy system to monitor autonomously water quality responses to land-derived nutrient inputs and physical forcing associated with local storm events in the coastal ocean of southern Kaneohe Bay, Oahu, Hawaii, USA. The data represent 2.5 years of near-real time observations at a fixed station, collected concurrently with spatially distributed synoptic sampling over larger sections of Kaneohe Bay. Storm events cause most of the fluvial nutrient, particulate, and dissolved organic carbon inputs to Kaneohe Bay. Nutrient loadings from direct rainfall and/or terrestrial runoff produce an immediate increase in the N:P ratio of bay waters up to values of 48 and drive phytoplankton biomass growth. Rapid uptake of such nutrient subsidies by phytoplankton causes rapid declines of N levels, return to N-limited conditions, and subsequent decline of phytoplankton biomass over timescales ranging from a few days to several weeks, depending on conditions and proximity to the sources of runoff. The enhanced productivity may promote the drawing down of pCO₂ and lowering of surface water column carbonate saturation states, and in some events, a temporary shift from N to P limitation. The productivity-driven CO₂ drawdown may temporarily lead to air-to-sea transfer of atmospheric CO₂ in a system that is on an annual basis a source of CO₂ to the atmosphere due to calcification and perhaps heterotrophy. Storms may also strongly affect proximal coastal zone pCO₂ and hence carbonate saturation state due to river runoff flushing out high pCO₂ soil and ground waters. Mixing of the CO₂-charged water with seawater causes a salting out effect that releases CO₂ to the atmosphere. Many subtropical and tropical systems throughout the Pacific region are similar to Kaneohe Bay, and our work provides an important indication of the variability and range of CO₂ dynamics that are likely to exist elsewhere. Such variability must be taken into account in any analysis of the direction and magnitude of the air?Csea CO₂ exchange for the integrated coastal ocean, proximal and distal. It cannot be overemphasized that this research illustrates several examples of how high frequency sampling by a moored autonomous system can provide details about ecosystem responses to stochastic atmospheric forcing that are commonly missed by traditional synoptic observational approaches. Finally, the work exemplifies the utility of combining synoptic sampling and real-time autonomous observations to elucidate the biogeochemical and physical responses of coastal subtropical/tropical coral reef ecosystems to climatic perturbations.     

Trends in intrinsic water-use efficiency of natural trees for the past 100–200 years: a response to atmospheric CO2 concentration

To evaluate how the land carbon reservoir has been acting as a sink to the anthropogenic CO₂ input to the atmosphere, it is important to study how plants in natural forests physiologically adjust to the changing atmospheric conditions. This has been studied intensively using controlled experiments, but it has been difficult to scale short-term observations to long-term ecosystem-level response. This paper derives variations of plant intrinsic water-use efficiency from natural trees for the past 100–200 years using carbon isotope chronologies. This parameter may potentially cause an increase in plant growth rate by improving the efficiency of plant water use, especially in arid environments. Attempts were made to isolate the variations of intrinsic water-use efficiency as a function of only the CO₂ concentration of the atmosphere. The intrinsic water-use efficiency of almost all trees increased with increasing atmospheric CO₂ concentration. This is caused by an increase in the carbon assimilation rate (A) and/or a decrease in the stomatal conductance (g). The increase in plant intrinsic water-use efficiency may imply an increase in plant transpiration efficiency which may have a direct connection with changes in plant biomass.     

Responses of <Emphasis Type="Italic">Spartina alterniflora</Emphasis> to Multiple Stressors: Changing Precipitation Patterns,Accelerated Sea Level Rise,and Nutrient Enrichment

Coastal wetlands, well recognized for their ecosystem services, have faced many threats throughout the USA and elsewhere. While managers require good information on the net impact of these combined stressors on wetlands, little such information exists. We conducted a 4-month mesocosm study to analyze the multiple stressor effects of precipitation changes, sea level rise, and eutrophication on the salt marsh plant Spartina alterniflora. Pots containing plants in an organic soil matrix were positioned in tanks and received Narragansett Bay (RI, USA) water. The study simulated three precipitation levels (ambient daily rain, biweekly storm, and drought), three levels of tidal inundations (high (15 cm below mean high water (MHW)), mean (MHW), and low (15 cm above MHW)), and two nutrient enrichment levels (unenriched and nutrient-enriched bay water). Our results demonstrate that storm and drought stressors led to significantly less above- and belowground biomass than those in ambient rain conditions. Plants that were flooded at high inundation had less belowground biomass, fine roots, and shoots. Nutrients had no detectable effect on aboveground biomass, but the enriched pots had higher stem counts and more fine roots than unenriched pots, in addition to greater CO₂ emission rates; however, the unenriched pots had significantly more coarse roots and rhizomes, which help to build peat in organogenic marshes. These results suggest that multiple stressors of altered precipitation, sea level rise, and nutrient enrichment would lead to reduced marsh sustainability.     

Human perturbations on the global biogeochemical cycles of coupled Si–C and responses of terrestrial processes and the coastal ocean

We present a model of the global biogeochemical cycle of silicon (Si) that emphasizes its linkages to the carbon cycle and temperature. The Si cycle is a crucial part of global nutrient biogeochemistry regulating long-term atmospheric CO₂ concentrations due to silicate mineral weathering reactions involving the uptake of atmospheric CO₂ and production of riverine dissolved silica, cations and bicarbonate. In addition and importantly, the Si cycle is strongly coupled to the other nutrient cycles of N, P, and Fe; hence siliceous organisms represent a significant fraction of global primary productivity and biomass. Human perturbations involving land-use changes, burning of fossil fuel, and inorganic N and P fertilization have greatly altered the terrestrial Si cycle, changing the river discharge of Si and consequently impacting marine primary productivity primarily in coastal ocean waters.     

Ocean chemistry during glacial time

Measurements of CO₂ to air ratios in the gas trapped in bubbles in ice of glacial age suggest that the CO₂ content of the atmosphere was considerably lower during peak glacial time than during Holocene time. The purpose of this paper is to show that such a change must in all likelihood be the result of alterations in the nutrient element chemistry of sea water. Two possible scenarios are presented. One involves alternate storage and erosion of phosphorous leaving residues from shelf sediments. The other involves changes in the $\text{C}\text{P}$ ratio in the organic debris falling to the deep sea. Means of verifying the nutrient cycle hypothesis are also given. It is shown that the ¹³C record as we know it in planktonic and benthic foraminifera, the oxygen record as inferred from benthic foraminifera species distributions, and the early post glacial CaCo₃ preservation event as recorded by aragonitic pteropods are consistent with both of the hypotheses presented. Only if an early post glacial spike in the ¹³C record for planktonic shells could be found would it be possible to eliminate one of these hypotheses (i.e., that involving shelf storage). The implications of these nutrient hypotheses to climate theory are as follows. If shelf storage is responsible for the glacial to interglacial CO₂ increase, then the CO₂ change must be considered an amplifier of some primary cause. The reason is that sea level changes are needed to drive deposition on (and erosion from) the shelves. On the other hand, if changes in the $\text{C}\text{P}$ ratio for falling debris are responsible, then the CO₂ change could either be an amplifier or a primary cause for the major glacial to interglacial climatic cycle. The latter is possible as self-sustained oscillations in ocean chemistry might be driven by interactions between ocean ecology and ocean nutrient chemistry.     

Carbonate factories: A conundrum in sedimentary geology

Describing, characterizing and interpreting the nearly infinite variety of carbonate rocks are conundrums – intricate and difficult problems having only conjectural answers – that have occupied geologists for more than two centuries. Depositional features including components, rock textures, lithofacies, platform types and architecture, all vary in space and time, as do the results of diagenetic processes on those primary features. Approaches to the study of carbonate rocks have become progressively more analytical. One focus has evolved from efforts to build reference models for specific Phanerozoic windows to scrutinize the effect of climate and long-term oscillations of the ocean–atmosphere system in influencing the mineralogy of carbonate components.This paper adds to the ongoing lively debates by attempting to understand changes in the predominant types of carbonate-producing organisms during the Mesozoic–Cenozoic, while striving to minimize the uniformitarian bias. Our approach integrates estimates of changes in Ca²⁺ concentration in seawater and atmospheric CO₂, with biological evolution and ecological requirements of characteristic carbonate-producing marine communities. The underlying rationale for our approach is the fact that CO₂ is basic to both carbonates and organic matter, and that photosynthesis is a fundamental biological process responsible for both primary production of organic matter and providing chemical environments that promote calcification. Gross photosynthesis and hypercalcification are dependent largely upon sunlight, while net primary production and, e.g., subsequent burial of organic matter typically requires sources of new nutrients (N, P and trace elements). Our approach plausibly explains the changing character of carbonate production as an evolving response to changing environmental conditions driven by the geotectonic cycle, while identifying uncertainties that deserve further research.With metazoan consumer diversity reduced by the end-Permian extinctions, excess photosynthesis by phytoplankton and microbial assemblages in surface waters, induced by moderately high CO₂ and temperature during the Early Mesozoic, supported proliferation of non-tissular metazoans (e.g., sponges) and heterotrophic bacteria at the sea floor. Metabolic activity by those microbes, especially sulfate reduction, resulted in abundant biologically-induced geochemical carbonate precipitation on and within the sea floor. For example, with the opening of Tethyan seaways during the Triassic, massive sponge/microbe boundstones (the benthic automicrite factory) formed steep, massive and thick progradational slopes and, locally, mud-mounds. As tectonic processes created shallow epicontinental seas, photosynthesis drove lime-mud precipitation in the illuminated zone of the water column. The resulting neritic lime-mud component of the shallow-water carbonate factory became predominant during the Jurassic, paralleling the increase in atmospheric pCO₂, while the decreasing importance of the benthic automicrite factory parallels the diversification of calcifying metazoans, phytoplankton and zooplankton.With atmospheric pCO₂ declining through the Cretaceous, the potential habitats for neritic lime-mud precipitation declined. At the same time, peak oceanic Ca²⁺ concentrations promoted biotically-controlled calcification by the skeletal factory. With changes produced by extinctions and turnovers at the Cretaceous–Tertiary boundary, adaptations to decreasing Ca²⁺ and pCO₂, coupled with increasing global temperature gradients (i.e., high-latitude and deep-water cooling), and strategies that efficiently linked photosynthesis and calcification, promoted successive changes of the dominant skeletal factory through the Cenozoic: larger benthic foraminifers (protist–protist symbiosis) during the Paleogene, red algae during the Miocene and modern coral reefs (metazoan–protist symbiosis) since Late Miocene.     

Flooding of the continental shelves as a contributor to deglacial CH4 rise

The transition from the last glacial and beginning of Bølling–Allerød and Pre‐Boreal periods in particular is marked by rapid increases in atmospheric methane (CH₄) concentrations. The CH₄ concentrations reached during these intervals, ～650–750 ppb, is twice that at the last glacial maximum and is not exceeded until the onset of industrialization at the end of the Holocene. Periods of rapid sea‐level rise as the Last Glacial Maximum ice sheets retreated and associated with ‘melt‐water pulses’ appear to coincide with the onset of elevated concentrations of CH₄, suggestive of a potential causative link. Here we identify and outline a mechanism involving the flooding of the continental shelves that were exposed and vegetated during the glacial sea‐level low stand and that can help account for some of these observations. Specifically, we hypothesize that waterlogging (and later, flooding) of large tracts of forest and savanna in the Tropics and Subtropics during the deglacial transition and early Holocene would have resulted in rapid anaerobic decomposition of standing biomass and emission of methane to the atmosphere. This novel mechanism, akin to the consequences of filling new hydroelectric reservoirs, provides a mechanistic explanation for the apparent synchronicity between rate of sea‐level rise and occurrence of elevated concentrations of ice core CH₄. However, shelf flooding and the creation of transient wetlands are unlikely to explain more than ～60 ppb of the increase in atmospheric CH₄ during the deglacial transition, requiring additional mechanisms to explain the bulk of the glacial to interglacial increase. Similarly, this mechanism has the potential also to play some role in the rapid changes in atmospheric methane associated with the Dansgaard–Oeschger cycles. Copyright © 2012 John Wiley & Sons, Ltd.     

Land–sea carbon and nutrient fluxes and coastal ocean CO2 exchange and acidification: Past,present, and future

Epochs of changing atmospheric CO₂ and seawater CO₂–carbonic acid system chemistry and acidification have occurred during the Phanerozoic at various time scales. On the longer geologic time scale, as sea level rose and fell and continental free board decreased and increased, respectively, the riverine fluxes of Ca, Mg, DIC, and total alkalinity to the coastal ocean varied and helped regulate the C chemistry of seawater, but nevertheless there were major epochs of ocean acidification (OA). On the shorter glacial–interglacial time scale from the Last Glacial Maximum (LGM) to late preindustrial time, riverine fluxes of DIC, total alkalinity, and N and P nutrients increased and along with rising sea level, atmospheric PCO₂ and temperature led, among other changes, to a slightly deceasing pH of coastal and open ocean waters, and to increasing net ecosystem calcification and decreasing net heterotrophy in coastal ocean waters. From late preindustrial time to the present and projected into the 21st century, human activities, such as fossil fuel and land-use emissions of CO₂ to the atmosphere, increasing application of N and P nutrient subsidies and combustion N to the landscape, and sewage discharges of C, N, P have led, and will continue to lead, to significant modifications of coastal ocean waters. The changes include a rapid decline in pH and carbonate saturation state (modern problem of ocean acidification), a shift toward dissolution of carbonate substrates exceeding production, potentially leading to the “demise” of the coral reefs, reversal of the direction of the sea-to-air flux of CO₂ and enhanced biological production and burial of organic C, a small sink of anthropogenic CO₂, accompanied by a continuous trend toward increasing autotrophy in coastal waters.     

Mid Holocene origin of the sea-surface salinity low in the subarctic North Pacific

IMAGES core MD01-2416 (51°N, 168°E) provides the first centennial-scale multiproxy record of Holocene variation in North Pacific sea-surface temperature (SST), salinity, and biogenic productivity. Our results reveal a gradual decrease in subarctic SST by 3–5 °C from 11.1 to 4.2 ka and a stepwise long-term decrease in sea surface salinity (SSS) by 2–3 p.s.u. Early Holocene SSS were as high as in the modern subtropical Pacific. The steep halocline and stratification that is characteristic of the present-day subarctic North Pacific surface ocean is a fairly recent feature, developed as a product of mid-Holocene environmental change. High SSS matched a salient productivity maximum of biogenic opal during Bølling-to-Early Holocene times, reaching levels similar to those observed during preglacial times in the warm mid-Pliocene prior to 2.73 Ma. Similar productivity spikes marked every preceding glacial termination of the last 800 ka, indicating recurrent short-term events of mid-Pliocene-style intense upwelling of nutrient-rich Pacific Deepwater in the Pleistocene. Such events led to a repeated exposure of CO₂-rich deepwater at the ocean surface facilitating a transient CO₂ release to the atmosphere, but the timing and duration of these events repudiate a long-term influence of the subarctic North Pacific on global atmospheric CO₂ concentration.     

Dating megafaunal extinction on the Pleistocene Darling Downs,eastern Australia: the promise and pitfalls of dating as a test of extinction hypotheses

A key to understanding Late Pleistocene megafaunal extinction dynamics is knowledge of megafaunal ecological response(s) to long-term environmental perturbations. Strategically, that requires targeting fossil deposits that accumulated during glacial and interglacial intervals both before and after human arrival, with subsequent palaeoecological models underpinned by robust and reliable chronologies. Late Pleistocene vertebrate fossil localities from the Darling Downs, eastern Australia, provide stratigraphically-intact, abundant megafaunal sequences, which allows for testing of anthropogenic versus climate change megafauna extinction hypotheses. Each stratigraphic unit at site QML796, Kings Creek Catchment, was previously shown to have had similar sampling potential, and the basal units contain both small-sized taxa (e.g., land snails, frogs, bandicoots, rodents) and megafauna. Importantly, sequential faunal horizons show stepwise decrease in taxonomic diversity with the loss of some, but not all, megafauna in the geographically-small palaeocatchment. The purpose of this paper is to present the results of our intensive, multidisciplinary dating study of the deposits (>40 dates). Dating by means of accelerator mass spectrometry (AMS) ¹⁴C (targeting bone, freshwater molluscs, and charcoal) and thermal ionisation mass spectrometry U/Th (targeting teeth and freshwater molluscs) do not agree with each other and, in the case of AMS ¹⁴C dating, lack internal consistency. Scanning electron microscopy and rare earth element analyses demonstrate that the dated molluscs are diagenetically altered and contain aragonite cements that incorporated secondary young C, suggesting that such dates should be regarded as minimum ages. AMS ¹⁴C dated charcoals provide ages that occur out of stratigraphic order, and cluster in the upper chronological limits of the technique (～40–48 ka). Again, we suggest that such results should be regarded as suspicious and only minimum ages. Subsequent OSL and U/Th (teeth) dating provide complimentary results and demonstrate that the faunal sequences actually span ～120–83 ka, thus occurring beyond the AMS ¹⁴C dating window. Importantly, the dates suggest that the local decline in biological diversity was initiated ～75,000 years before the colonisation of humans on the continent. Collectively, the data are most parsimoniously consistent with a pre-human climate change model for local habitat change and megafauna extinction, but not with a nearly simultaneous extinction of megafauna as required by the human-induced blitzkrieg extinction hypothesis. This study demonstrates the problems inherent in dating deposits that lie near the chronological limits of the radiocarbon dating technique, and highlights the need to cross-check previously-dated archaeological and megafauna deposits within the timeframe of earliest human colonisation and latest megafaunal survival.     

Isotopic evidence for temporal variation in proportion of seasonal precipitation since the last glacial time in the inland Pacific Northwest of the USA

Large-scale atmospheric circulation patterns determine the quantity and seasonality of precipitation, the major source of water in most terrestrial ecosystems. Oxygen isotope (δ¹⁸O) dynamics of the present-day hydrologic system in the Palouse region of the northwestern U.S.A. indicate a seasonal correlation between the δ¹⁸O values of precipitation and temperature, but no seasonal trends of δ¹⁸O records in soil water and shallow groundwater. Their isotope values are close to those of winter precipitation because the Palouse receives  75% of its precipitation during winter. Palouse Loess deposits contain late Pleistocene pedogenic carbonate having ca. 2 to 3‰ higher δ¹⁸O values and up to 5‰ higher carbon isotope (δ¹³C) values than Holocene and modern carbonates. The late Pleistocene δ¹⁸O values are best explained by a decrease in isotopically light winter precipitation relative to the modern winter-dominated infiltration. The δ¹³C values are attributed to a proportional increase of atmospheric CO₂ in soil CO₂ due to a decrease in soil respiration rate and ¹³C discrimination in plants under much drier paleoclimate conditions than today. The regional climate difference was likely related to anticyclonic circulation over the Pleistocene Laurentide and Ice Sheet.     

C3 and C4 Biomass Allocation Responses to Elevated CO2 and Nitrogen: Contrasting Resource Capture Strategies

Plants alter biomass allocation to optimize resource capture. Plant strategy for resource capture may have important implications in intertidal marshes, where soil nitrogen (N) levels and atmospheric carbon dioxide (CO₂) are changing. We conducted a factorial manipulation of atmospheric CO₂ (ambient and ambient?+?340?ppm) and soil N (ambient and ambient?+?25?g?m^?2?year^?1) in an intertidal marsh composed of common North Atlantic C₃ and C₄ species. Estimation of C₃ stem turnover was used to adjust aboveground C₃ productivity, and fine root productivity was partitioned into C₃?CC₄ functional groups by isotopic analysis. The results suggest that the plants follow resource capture theory. The C₃ species increased aboveground productivity under the added N and elevated CO₂ treatment (P?<?0.0001), but did not under either added N or elevated CO₂ alone. C₃ fine root production decreased with added N (P?<?0.0001), but fine roots increased under elevated CO₂ (P?=?0.0481). The C₄ species increased growth under high N availability both above- and belowground, but that stimulation was diminished under elevated CO₂. The results suggest that the marsh vegetation allocates biomass according to resource capture at the individual plant level rather than for optimal ecosystem viability in regards to biomass influence over the processes that maintain soil surface elevation in equilibrium with sea level.     

Atmospheric CO2 sink: Silicate weathering or carbonate weathering?

It is widely accepted that chemical weathering of Ca–silicate rocks could potentially control long-term climate change by providing feedback interaction with atmospheric CO₂ drawdown by means of precipitation of carbonate, and that in contrast weathering of carbonate rocks has not an equivalent impact because all of the CO₂ consumed in the weathering process is returned to the atmosphere by the comparatively rapid precipitation of carbonates in the oceans. Here, it is shown that the rapid kinetics of carbonate dissolution and the importance of small amounts of carbonate minerals in controlling the dissolved inorganic C (DIC) of silicate watersheds, coupled with aquatic photosynthetic uptake of the weathering-related DIC and burial of some of the resulting organic C, suggest that the atmospheric CO₂ sink from carbonate weathering may previously have been underestimated by a factor of about 3, amounting to 0.477 Pg C/a. This indicates that the contribution of silicate weathering to the atmospheric CO₂ sink may be only 6%, while the other 94% is by carbonate weathering. Therefore, the atmospheric CO₂ sink by carbonate weathering might be significant in controlling both the short-term and long-term climate changes. This questions the traditional point of view that only chemical weathering of Ca–silicate rocks potentially controls long-term climate change.     

Stomatal evidence for a decline in atmospheric CO2 concentration during the Younger Dryas stadial: a comparison with Antarctic ice core records

A recent high‐resolution record of Late‐glacial CO₂ change from Dome Concordia in Antarctica reveals a trend of increasing CO₂ across the Younger Dryas stadial (GS‐1). These results are in good agreement with previous Antarctic ice‐core records. However, they contrast markedly with a proxy CO₂ record based on the stomatal approach to CO₂ reconstruction, which records a ca. 70 ppm mean CO₂ decline at the onset of GS‐1. To address these apparent discrepancies we tested the validity of the stomatal‐based CO₂ reconstructions from Kråkenes by obtaining further proxy CO₂ records based on a similar approach using fossil leaves from two independent lakes in Atlantic Canada. Our Late‐glacial CO₂ reconstructions reveal an abrupt ca. 77 ppm decrease in atmospheric CO₂ at the onset of the Younger Dryas stadial, which lagged climatic cooling by ca. 130 yr. Furthermore, the trends recorded in the most accurate high‐resolution ice‐core record of CO₂, from Dome Concordia, can be reproduced from our stomatal‐based CO₂ records, when time‐averaged by the mean age distribution of air contained within Dome Concordia ice (200 to 550 yr). If correct, our results indicate an abrupt drawdown of atmospheric CO₂ within two centuries at the onset of GS‐1, suggesting that some re‐evaluation of the behaviour of atmospheric CO₂ sinks and sources during times of rapid climatic change, such as the Late‐glacial, may be required. Copyright © 2002 John Wiley & Sons, Ltd.     

Global and regional factors responsible for the drowning of the Central Apennine Chattian carbonate platforms

This work discusses and interprets the factors responsible for the Oligocene–Miocene drowning of the Central Apennine platform deposits, based on facies and stable‐isotope analyses of two representative stratigraphic sections. The Mediterranean carbonate platforms were affected during the Oligocene–Miocene boundary by a carbonate production crisis that was induced by global factors and amplified by regional events, such as volcanic activity. The positive δ¹³C shift observed in the studied sections corresponds to vertical facies changes reflecting the evolution from middle carbonate ramp to outer ramp‐hemipelagic depositional environments. This drowning event is recorded not only in the Apennine platforms, but also in other Mediterranean platforms such as in southern Apulia, Sicily and Malta, and outside the Mediterranean Basin. The ~24–23.5 Ma Mi‐1 glacial maximum may have had a significant influence on this drowning event because it was associated with high rates of accumulation of continent‐derived sediments. The increased continental weathering and runoff sustained high trophic conditions. These probably were a consequence of the Aquitanian–Burdigalian volcanic activity in the Central‐Western Mediterranean, that may have led to an increase in nutrient content in seawater and an increase in atmospheric and marine CO₂ concentrations. Copyright © 2014 John Wiley & Sons, Ltd.