Preindustrial Holocene Research Articles

Paleo±Dust: quantifying uncertainty in paleo-dust deposition across archive types

Abstract. Mineral dust aerosol concentrations in the atmosphere varied greatly on glacial–interglacial timescales. The greatest changes in global dust activity occurred in response to changes in orbital parameters (which affect dust emission intensity through glacial activity) and the lifetime of dust in the atmosphere (caused by changes in the global hydrological cycle). Long-term changes in the surface dust deposition rate are registered in geological archives such as loess, peats, lakes, marine sediments, and ice. Data provided by these archives are crucial for guiding simulations of dust and for better understanding the natural global dust cycle. However, the methods employed to derive paleo-dust deposition rates differ markedly between archives and are subject to different sources of uncertainty. Here, we present Paleo±Dust, an updated compilation of bulk and <10 µm paleo-dust deposition rates with quantitative 1σ uncertainties that are inter-comparable among archive types. Paleo±Dust incorporates a total of 285 pre-industrial Holocene (pi-HOL) and 209 Last Glacial Maximum (LGM) dust flux constraints from studies published until December 2022, including, for the first time, peat records. We also recalculate previously published dust fluxes to exclude data from the last deglaciation and thus obtain more representative constraints for the last pre-industrial interglacial and glacial end-member climate states. Based on Paleo±Dust, the global LGM:pi-HOL ratio of <10 µm dust deposition rates is 3.1 ± 0.7 (1σ). We expect Paleo±Dust to be of use for future paleoclimate dust studies and simulations using Earth system models of high to intermediate complexity. Paleo±Dust is publicly accessible at https://doi.org/10.1594/PANGAEA.962969 (Cosentino et al., 2024).

Relative importance of forcings and feedbacks in the Holocene temperature conundrum

The pre-industrial Holocene provides the backdrop for the emergence of civilisations and the starting point of anthropogenic climate change. Several reconstructions show an early Holocene warming that was followed by cooling for several thousand years before Industrialisation. In contrast climate simulations show warming throughout the Holocene. Whilst reconstructed trends over ocean can be reconciled with warming through either seasonality or uncertainties, a consistent explanation for cooling trends over some land areas is missing. We present a suite of transient Holocene climate model simulations with a coupled general circulation model and show that a widespread mid- to late-Holocene cooling emerges over some regions of the northern hemisphere with the inclusion of anthropogenic land-use. This is mostly because in regions of prescribed late Holocene deforestation, the simulated early to mid-Holocene is characterised by a lower albedo than the late Holocene. Whilst this cooling through time can quantitatively explain some regional aspects of the reconstructions, the model-data agreement remains imperfect, and differences between reconstructions also hinders the evaluation. Moreover, model-dependency in the response of several feedbacks, particularly sea-ice, but potentially also clouds, means that it is difficult to uniquely attribute Holocene temperature evolution to specific factors. Future work should aim to derive a consensus-signal including uncertainties from the available proxy data which could be used to fingerprint simulations covering a range of plausible feedback strengths.

Pre-industrial Holocene glacier variability in the tropical Andes as context for anthropogenically driven ice retreat

Pre-industrial Holocene glacier variability in the tropical Andes as context for anthropogenically driven ice retreat

Tropospheric Ozone During the Last Interglacial

Tropospheric Ozone During the Last Interglacial

Clumped‐Isotope Constraint on Upper‐Tropospheric Cooling During the Last Glacial Maximum

AbstractIce cores and other paleotemperature proxies, together with general circulation models, have provided information on past surface temperatures and the atmosphere's composition in different climates. Little is known, however, about past temperatures at high altitudes, which play a crucial role in Earth's radiative energy budget. Paleoclimate records at high‐altitude sites are sparse, and the few that are available show poor agreement with climate model predictions. These disagreements could be due to insufficient spatial coverage, spatiotemporal biases, or model physics; new records that can mitigate or avoid these uncertainties are needed. Here, we constrain the change in upper‐tropospheric temperature at the global scale during the Last Glacial Maximum (LGM) using the clumped‐isotope composition of molecular oxygen trapped in polar ice cores. Aided by global three‐dimensional chemical transport modeling, we exploit the intrinsic temperature sensitivity of the clumped‐isotope composition of atmospheric oxygen to infer that the upper troposphere (effective mean altitude 10–11 km) was 6–9°C cooler during the LGM than during the late preindustrial Holocene. A complementary energy balance approach supports a minor or negligible steepening of atmospheric lapse rates during the LGM, which is consistent with a range of climate model simulations. Proxy‐model disagreements with other high‐altitude records may stem from inaccuracies in regional hydroclimate simulation, possibly related to land‐atmosphere feedbacks.

Stability of the Antarctic Ice Sheet during the pre-industrial Holocene

The rate and magnitude of the Antarctic Ice Sheet (AIS) contribution to global sea-level rise beyond 2100 ce remains highly uncertain. Past changes of the AIS, however, offer opportunities to understand contemporary and future ice sheet behaviour. In this Review, we outline how the AIS evolved through the pre-industrial Holocene, 11,700 years ago to 1850 ce. Three main phases of ice sheet behaviour are identified: a period of rapid ice volume loss across all sectors in the Early and Mid Holocene; a retreat inland of the present-day ice sheet margin in some sectors, followed by readvance; and continued ice volume loss in several sectors during the past few millennia, and in some areas up to and into the industrial era. Global sea levels rose by 2.4–12 m owing to the period of rapid Antarctic ice loss and possibly fell by 0.35–1.2 m owing to subsequent readvance. Changes in the AIS during the Holocene were likely driven by similar processes to those acting today and predicted for the future, which are associated with oceanic and atmospheric conditions as well as bed topography. Further work is required to better understand these processes and to quantify Antarctica’s contribution to past sea-level change. The Antarctic Ice Sheet (AIS) underwent dramatic changes over the Holocene, impacting global sea levels. In this Review, Jones et al. discuss changes in this ice sheet during the pre-industrial Holocene. The drivers behind these past changes are explored, as well as their relevance for current and future changes in the AIS.

ПРИРОДНЫЕ ПОЖАРЫ: ДАННЫЕ НАБЛЮДЕНИЙ И МОДЕЛИРОВАНИЕ

A review of the contemporary knowledge on natural fires (NF) and on contemporary approaches for their modelling is presented. The review is targeted to the implementation of NF models into the global Earth system models. In the last years, satellite retrievals are able to detect even small (with the size < 500 m) fires. Nonetheless, marked uncertainties of the estimates for the burned are and for the NF-associated release of different substances into the atmosphere still persist. For the most detailed satellite dataset GFED-4.1, the annual burned area is close to 5 mln km2 , including at least 2.9 Mkm2 due to NF themselves. This leads to the carbon release of ~2 PgC year into the atmosphere, mostly in the form of CO2.In the Russian territory, the annual burned area is close to 135 thnd km2 , and the annual carbon release into the atmosphere is equal to 0.12 PgC year. These estimates appear much larger the respective estimates reported in the Russian National Report for 1900-2015. NF Characteristics follow the interannual climate variability, including that in the El Niño years. Nevertheless, no significant trends are found at the global and subcontinental spatial scales based on the satellite data. Proxy data and modelling results exhibit a general increase of the NF activity during the preindustrial Holocene (including cold epochs) as well as an overall enhancement of this activity from cold stadials to warm interstadials in the Pleistocene. Currently the NF simulation modules are started to be implemented into the Earth system models. These modules take into account fire-affecting weather and/ or climate characteristics, the number of ignitions depending on lightning activity and population density (sometimes, depending on landuse type as well), fire suppression, amount and type of fuel for fires, and combustion completeness. A number of models is able to simulate even the simplified dynamics of individual fires. However, the limited contemporary knowledge on NF leads to the marked intermodel scatter even in the simulated climatology of the NF characteristics and in the simulated trend slopes of such characteristics. In particular, the intermodel differences in the 20th century variations in the simulated NF characteristics are mostly caused by the implemented assumptions on the landuse impact on NF dynamics. As a result, further development of the NF modules is needed.

Simulating the climate response to atmospheric oxygen variability in the Phanerozoic: a focus on the Holocene, Cretaceous and Permian

Abstract. The amount of dioxygen (O2) in the atmosphere may have varied from as little as 5 % to as much as 35 % during the Phanerozoic eon (54 Ma–present). These changes in the amount of O2 are large enough to have led to changes in atmospheric mass, which may alter the radiative budget of the atmosphere, leading to this mechanism being invoked to explain discrepancies between climate model simulations and proxy reconstructions of past climates. Here, we present the first fully 3-D numerical model simulations to investigate the climate impacts of changes in O2 under different climate states using the coupled atmosphere–ocean Hadley Centre Global Environmental Model version 3 (HadGEM3-AO) and Hadley Centre Coupled Model version 3 (HadCM3-BL) models. We show that simulations with an increase in O2 content result in increased global-mean surface air temperature under conditions of a pre-industrial Holocene climate state, in agreement with idealised 1-D and 2-D modelling studies. We demonstrate the mechanism behind the warming is complex and involves a trade-off between a number of factors. Increasing atmospheric O2 leads to a reduction in incident shortwave radiation at the Earth's surface due to Rayleigh scattering, a cooling effect. However, there is a competing warming effect due to an increase in the pressure broadening of greenhouse gas absorption lines and dynamical feedbacks, which alter the meridional heat transport of the ocean, warming polar regions and cooling tropical regions. Case studies from past climates are investigated using HadCM3-BL and show that, in the warmest climate states in the Maastrichtian (72.1–66.0 Ma), increasing oxygen may lead to a temperature decrease, as the equilibrium climate sensitivity is lower. For the Asselian (298.9–295.0 Ma), increasing oxygen content leads to a warmer global-mean surface temperature and reduced carbon storage on land, suggesting that high oxygen content may have been a contributing factor in preventing a “Snowball Earth” during this period of the early Permian. These climate model simulations reconcile the surface temperature response to oxygen content changes across the hierarchy of model complexity and highlight the broad range of Earth system feedbacks that need to be accounted for when considering the climate response to changes in atmospheric oxygen content.

Palaeoclimate constraints on the impact of 2 °C anthropogenic warming and beyond

Over the past 3.5 million years, there have been several intervals when climate conditions were warmer than during the pre-industrial Holocene. Although past intervals of warming were forced differently than future anthropogenic change, such periods can provide insights into potential future climate impacts and ecosystem feedbacks, especially over centennial-to-millennial timescales that are often not covered by climate model simulations. Our observation-based synthesis of the understanding of past intervals with temperatures within the range of projected future warming suggests that there is a low risk of runaway greenhouse gas feedbacks for global warming of no more than 2 °C. However, substantial regional environmental impacts can occur. A global average warming of 1–2 °C with strong polar amplification has, in the past, been accompanied by significant shifts in climate zones and the spatial distribution of land and ocean ecosystems. Sustained warming at this level has also led to substantial reductions of the Greenland and Antarctic ice sheets, with sea-level increases of at least several metres on millennial timescales. Comparison of palaeo observations with climate model results suggests that, due to the lack of certain feedback processes, model-based climate projections may underestimate long-term warming in response to future radiative forcing by as much as a factor of two, and thus may also underestimate centennial-to-millennial-scale sea-level rise.

Constraining the Deforestation History of Europe: Evaluation of Historical Land Use Scenarios with Pollen-Based Land Cover Reconstructions

Anthropogenic land cover change (ALCC) is the most important transformation of the Earth system that occurred in the preindustrial Holocene, with implications for carbon, water and sediment cycles, biodiversity and the provision of ecosystem services and regional and global climate. For example, anthropogenic deforestation in preindustrial Eurasia may have led to feedbacks to the climate system: both biogeophysical, regionally amplifying winter cold and summer warm temperatures, and biogeochemical, stabilizing atmospheric CO 2 concentrations and thus influencing global climate. Quantification of these effects is difficult, however, because scenarios of anthropogenic land cover change over the Holocene vary widely, with increasing disagreement back in time. Because land cover change had such widespread ramifications for the Earth system, it is essential to assess current ALCC scenarios in light of observations and provide guidance on which models are most realistic. Here, we perform a systematic evaluation of two widely-used ALCC scenarios (KK10 and HYDE3.1) in northern and part of central Europe using an independent, pollen-based reconstruction of Holocene land cover (REVEALS). Considering that ALCC in Europe primarily resulted in deforestation, we compare modeled land use with the cover of non-forest vegetation inferred from the pollen data. Though neither land cover change scenario matches the pollen-based reconstructions precisely, KK10 correlates well with REVEALS at the country scale, while HYDE systematically underestimates land use with increasing magnitude with time in the past. Discrepancies between modeled and reconstructed land use are caused by a number of factors, including assumptions of per-capita land use and socio-cultural factors that cannot be predicted on the basis of the characteristics of the physical environment, including dietary preferences, long-distance trade, the location of urban areas and social organization.

Young people's burden: requirement of negative CO<sub>2</sub> emissions

Abstract. Global temperature is a fundamental climate metric highly correlated with sea level, which implies that keeping shorelines near their present location requires keeping global temperature within or close to its preindustrial Holocene range. However, global temperature excluding short-term variability now exceeds +1 °C relative to the 1880–1920 mean and annual 2016 global temperature was almost +1.3 °C. We show that global temperature has risen well out of the Holocene range and Earth is now as warm as it was during the prior (Eemian) interglacial period, when sea level reached 6–9 m higher than today. Further, Earth is out of energy balance with present atmospheric composition, implying that more warming is in the pipeline, and we show that the growth rate of greenhouse gas climate forcing has accelerated markedly in the past decade. The rapidity of ice sheet and sea level response to global temperature is difficult to predict, but is dependent on the magnitude of warming. Targets for limiting global warming thus, at minimum, should aim to avoid leaving global temperature at Eemian or higher levels for centuries. Such targets now require negative emissions, i.e., extraction of CO2 from the air. If phasedown of fossil fuel emissions begins soon, improved agricultural and forestry practices, including reforestation and steps to improve soil fertility and increase its carbon content, may provide much of the necessary CO2 extraction. In that case, the magnitude and duration of global temperature excursion above the natural range of the current interglacial (Holocene) could be limited and irreversible climate impacts could be minimized. In contrast, continued high fossil fuel emissions today place a burden on young people to undertake massive technological CO2 extraction if they are to limit climate change and its consequences. Proposed methods of extraction such as bioenergy with carbon capture and storage (BECCS) or air capture of CO2 have minimal estimated costs of USD 89–535 trillion this century and also have large risks and uncertain feasibility. Continued high fossil fuel emissions unarguably sentences young people to either a massive, implausible cleanup or growing deleterious climate impacts or both.

The evolution of sub-monsoon systems in the Afro-Asian monsoon region during the Holocene– comparison of different transient climate model simulations

Abstract. The recently proposed global monsoon hypothesis interprets monsoon systems as part of one global-scale atmospheric overturning circulation, implying a connection between the regional monsoon systems and an in-phase behaviour of all northern hemispheric monsoons on annual timescales (Trenberth et al., 2000). Whether this concept can be applied to past climates and variability on longer timescales is still under debate, because the monsoon systems exhibit different regional characteristics such as different seasonality (i.e. onset, peak and withdrawal). To investigate the interconnection of different monsoon systems during the pre-industrial Holocene, five transient global climate model simulations have been analysed with respect to the rainfall trend and variability in different sub-domains of the Afro-Asian monsoon region. Our analysis suggests that on millennial timescales with varying orbital forcing, the monsoons do not behave as a tightly connected global system. According to the models, the Indian and North African monsoons are coupled, showing similar rainfall trend and moderate correlation in centennial rainfall variability in all models. The East Asian monsoon changes independently during the Holocene. The dissimilarities in the seasonality of the monsoon sub-systems lead to a stronger response of the North African and Indian monsoon systems to the Holocene insolation forcing than of the East Asian monsoon and affect the seasonal distribution of Holocene rainfall variations. Within the Indian and North African monsoon domain, precipitation solely changes during the summer months, showing a decreasing Holocene precipitation trend. In the East Asian monsoon region, the precipitation signal is determined by an increasing precipitation trend during spring and a decreasing precipitation change during summer, partly balancing each other. A synthesis of reconstructions and the model results do not reveal an impact of the different seasonality on the timing of the Holocene rainfall optimum in the different sub-monsoon systems. Rather they indicate locally inhomogeneous rainfall changes and show that single palaeo-records should not be used to characterise the rainfall change and monsoon evolution for entire monsoon sub-systems.

Relative importance of climatic and autogenic controls on Holocene carbon accumulation in a temperate bog in southern Ontario, Canada

Peat cores from Wylde Lake Bog in temperate southern Ontario, Canada, were examined to determine factors affecting Holocene ecological change and long-term rate of carbon accumulation (LORCA). By 10,250 cal. BP, a marsh was established, characterized by sediments with higher bulk density, highly decomposed material in the macrofossil record, and lower LORCA. By 8100 cal. BP, the marsh was replaced by a peat-accumulating fen dominated by herbaceous taxa and non- Sphagnum mosses. Around 4000 cal. BP, transition to a Sphagnum-dominated bog took place. These directional changes suggest a strong role for autogenesis, although periodic fluctuations in macrofossil assemblages and in LORCA suggest an important secondary role for climatic change and disturbance in explaining Holocene changes. LORCA remained fairly stable through the fen and bog zones, ~18 g C/m2/yr. In the high-resolution record spanning the most recent 1800 years, relatively higher values for LORCA and macrofossils associated with wetter conditions were recorded around 1400 cal. BP, partially coincident with the ‘Medieval Climate Anomaly’. LORCA decreases to minimum values for the record during part of the ‘Little Ice Age’ as woody macrofossils increase, suggesting a drier peat surface. The most recent portion of the record shows significant changes in LORCA and in bog vegetation associated with anthropogenic land clearance and damming. The charcoal record suggests that fire did not play an important role in peatland dynamics in the pre-industrial Holocene; however, a major anthropogenic fire in ad 1870 significantly altered the peatland, affecting surface vegetation heterogeneously.

Changes in calcification of coccoliths under stable atmospheric CO<sub>2</sub>

Abstract. The response of coccolithophore calcification to ocean acidification has been studied in culture experiments as well as in present and past oceans. The response, however, is different between species and strains, and for the relatively small carbonate chemistry changes observed in natural environments, a uniform response of the entire coccolithophore community has not been documented so far. Moreover, previous palaeo-studies basically focus on changes in coccolith weight due to increasing CO2 and the resulting changes in the carbonate system, and only few studies focus on the influence of other environmental factors. In order to untangle changes in coccolithophore calcification due to environmental factors such as temperature and/or productivity from changes caused by increasing pCO2 and decreasing carbonate ion concentration, we here present a study on coccolith calcification from the Holocene North Atlantic Ocean. The pre-industrial Holocene, with its predominantly stable atmospheric CO2, provides the conditions for such a comprehensive analysis. For an analysis on changes in major components of Holocene coccolithophores under natural conditions, the family Noelaerhabdaceae was selected, which constitutes the main part of the assemblage in the North Atlantic. Records of average coccolith weights from three Holocene sediment cores along a north–south transect in the North Atlantic were analysed. During the Holocene, mean weight (and therefore calcification) of Noelaerhabdaceae (Emiliania huxleyi and Gephyrocapsa) coccoliths decreased at the Azores (Geofar KF 16) from around 7 to 6 pg, but increased at the Rockall Plateau (ODP site 980) from around 6 to 8 pg, and at the Vøring Plateau (MD08-3192) from 7 to 10 pg. The amplitude of average weight variability is within the range of glacial–interglacial changes that were interpreted to be an effect of decreasing carbonate ion concentration. By comparison with SEM assemblage counts, we show that weight changes are not only partly due to variations in the coccolithophore assemblage but also an effect of a change in calcification and/or morphotype variability within single species. Our results indicate that there is no single key factor responsible for the observed changes in coccolith weight. A major increase in coccolith weight occurs during a slight decrease in carbonate ion concentration in the late Holocene at the Rockall Plateau and Vøring Plateau. Here, more favourable productivity conditions apparently lead to an increase in coccolith weight, either due to the capability of coccolithophore species, especially E. huxleyi, to adapt to decreasing carbonate ion concentration or due to a shift towards heavier calcifying morphotypes.

A new Himalayan ice core CH<sub>4</sub> record: possible hints at the preindustrial latitudinal gradient

Abstract. Two ice cores recovered from the Himalayan East Rongbuk (ER) Glacier on the northeast saddle of Mt. Qomolangma (Everest) (28°01' N, 86°58' E, 6518 m above sea level) give access to a tentative record of past Himalayan atmospheric mixing ratio of CH4 spanning the past 1200 yr. The major part of the record is affected by artifacts probably due to in situ production. After selecting what may represent the true atmospheric mixing ratio, an average of 749 ± 25 ppbv of CH4 is estimated for the late preindustrial Holocene, which is ~ 36 ± 17 (~ 73 ± 18) ppbv higher than the atmospheric levels recorded in the Greenland (Antarctic) ice cores. A comparison of these new data with model simulations of the CH4 latitudinal gradient suggests either that the models do not get a correct balance between high and low latitude CH4 sources, or that the filtered CH4 profile from the ER cores remains biased by small artifacts.

How warm was the last interglacial? New model–data comparisons

A Community Climate System Model, Version 3 (CCSM3) simulation for 125 ka during the Last Interglacial (LIG) is compared to two recent proxy reconstructions to evaluate surface temperature changes from modern times. The dominant forcing change from modern, the orbital forcing, modified the incoming solar insolation at the top of the atmosphere, resulting in large positive anomalies in boreal summer. Greenhouse gas concentrations are similar to those of the pre-industrial (PI) Holocene. CCSM3 simulates an enhanced seasonal cycle over the Northern Hemisphere continents with warming most developed during boreal summer. In addition, year-round warming over the North Atlantic is associated with a seasonal memory of sea ice retreat in CCSM3, which extends the effects of positive summer insolation anomalies on the high-latitude oceans to winter months. The simulated Arctic terrestrial annual warming, though, is much less than the observational evidence, suggesting either missing feedbacks in the simulation and/or interpretation of the proxies. Over Antarctica, CCSM3 cannot reproduce the large LIG warming recorded by the Antarctic ice cores, even with simulations designed to consider observed evidence of early LIG warmth in Southern Ocean and Antarctica records and the possible disintegration of the West Antarctic Ice Sheet. Comparisons with a HadCM3 simulation indicate that sea ice is important for understanding model polar responses. Overall, the models simulate little global annual surface temperature change, while the proxy reconstructions suggest a global annual warming at LIG (as compared to the PI Holocene) of approximately 1(°)C, though with possible spatial sampling biases. The CCSM3 SRES B1 (low scenario) future projections suggest high-latitude warmth similar to that reconstructed for the LIG may be exceeded before the end of this century.

High epibenthic foraminiferal δ13C in the Recent deep Arctic Ocean: Implications for ventilation and brine release during stadials

Low planktic and benthic δ18O and δ13C values in sediments from the Nordic seas of cold stadials of the last glaciation have been attributed to brines, formed similar to modern ones in the Arctic Ocean. To expand on the carbon isotopes of this hypothesis, I investigated benthic δ13C from the modern Arctic Ocean. I show that mean δ13C values of live epibenthic foraminifera from the deep Arctic basins are higher than mean δ13C values of upper slope epibenthic foraminifera. This agrees with mean high δ13C values of dissolved inorganic carbon (DIC) in Arctic Bottom Water (ABW), which are higher than mean δ13CDIC values from shallower water masses of mainly Atlantic origin. However, adjustments for oceanic 13C Suess depletion raise subsurface and intermediate water δ13CDIC values over ABW δ13CDIC ones. Accordingly, during preindustrial Holocene times, the δ13CDIC of ABW was as high or even higher than today but lower than the δ13CDIC of younger subsurface and intermediate water. If brine‐enriched water significantly ventilated ABW, brines should have had high δ13CDIC values. Analogously, high‐δ13CDIC brines may have been formed in the Nordic seas during warm interstadials. During cold stadials, when most of the Arctic Ocean was perennially sea ice covered, a cessation of high‐δ13CDIC brine rejection may have lowered δ13CDIC values of ABW, and ultimately the δ13CDIC in Nordic seas intermediate and deep water. So in contrast to the idea of enhanced brine formation during cold stadials, the results of this investigation imply that a cessation of brine rejection would be more likely.

Mercury in Baltic Sea sediments—Natural background and anthropogenic impact

In total 27 short and one long sediment core, and 278 surface sediment samples from the Baltic Sea were analyzed for mercury (Hg), and organic carbon contents. Thirteen short cores and the long core were dated by radionuclide methods (210Pb, 137Cs, AMS14C). The dataset allows discriminating between natural and human induced changes on the Hg levels in Baltic Sea sediments. Preindustrial Holocene background concentrations vary between 20 and 50μg Hg per kg dry sediment and are positively correlated with organic carbon changes. Strong human induced pollution is recorded for the second half of the past century and caused high Hg concentrations of up to several hundred μg Hg per kg dry sediment even in Baltic Sea basins. Maximum concentrations are found at industrial and war waste dumping sites (local hot spots). An Hg concentration decreasing trend toward the present day is observed at most coring sites, a result of environmental measures undertaken during the last two decades. At sites where it is possible to calculate Hg fluxes, the natural accumulation rates vary between 2.1 and 5.4μg Hg per m2 per year. Anthropogenically sourced Hg accumulation rates vary in a wide range of 30 and 300μg Hg per m2 per year for the time span of maximum pollution. In areas characterized by discontinuous sedimentation only “inventories” of human sourced Hg expressed as the total amount of deposited Hg (above the natural background) per m2 can be calculated. The inventories of the investigated cores vary in the range of 1 and 8mg Hg per m2. Additionally, influences of sediment dynamics on spatial distribution pattern of Hg concentrations in surface and subsurface sediments are discussed.

Regional climate control of glaciers in New Zealand and Europe during the pre-industrial Holocene

Glacier snowlines in both the European Alps and the Southern Alps of New Zealand have retreated over the past century. An analysis of glacier fluctuations in New Zealand over the past 11,000 years suggests that this synchronous behaviour is unique to the past few hundred years.

Large inert carbon pool in the terrestrial biosphere during the Last Glacial Maximum

During each of the late Pleistocene glacial‐interglacial transitions, atmospheric carbon dioxide concentrations rose by almost 100 ppm. The sources of this carbon are unclear, and efforts to identify them are hampered by uncertainties in the magnitude of carbon reservoirs and fluxes under glacial conditions. Here we use oxygen isotope measurements from air trapped in ice cores and ocean carbon-cycle modelling to estimate terrestrial and oceanic gross primary productivity during the Last Glacial Maximum. We find that the rate of gross terrestrial primary production during the Last Glacial Maximum was about 40 10 Pg C yr 1 , half that of the pre-industrial Holocene. Despite the low levels of photosynthesis, we estimate that the late glacial terrestrial biosphere contained only 330 Pg less carbon than pre-industrial levels. We infer that the area covered by carbon-rich but unproductive biomes such as tundra and cold steppes was significantly larger during the Last Glacial Maximum, consistent with palaeoecological data. Our data also indicate the presence of an inert carbon pool of 2,300 Pg C, about 700 Pg larger than the inert carbon locked in permafrost today. We suggest that the disappearance of this carbon pool at the end of the Last Glacial Maximum may have contributed to the deglacial rise in atmospheric carbon dioxide concentrations.