Is pedogenic carbonate an important atmospheric CO<Subscript>2</Subscript> sink?

Nearly 18 years after the proposal of the weathering-related carbon sink concept(Berner R A.Weathering,plants and the long-term carbon cycle.Geochim Cosmochim Acta,1992,56:3225-3231),it is an appropriate timing to re-evaluate its geological context with the updated dataset.Ryskov et al.(Ryskov Ya G,Demkin V A,Oleynik S A,et al.Dynamics of pedogenic carbonate for the last 5000 years and its role as a buffer reservoir for atmospheric carbon dioxide in soils of Russia.Glob Planet Change,2008,61:63-69) lately c...     

Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event

In the Jurassic period, the Early Toarcian oceanic anoxic event (about 183 million years ago) is associated with exceptionally high rates of organic-carbon burial, high palaeotemperatures and significant mass extinction. Heavy carbon-isotope compositions in rocks and fossils of this age have been linked to the global burial of organic carbon, which is isotopically light. In contrast, examples of light carbon-isotope values from marine organic matter of Early Toarcian age have been explained principally in terms of localized upwelling of bottom water enriched in 12C versus 13C (refs 1,2,5,6). Here, however, we report carbon-isotope analyses of fossil wood which demonstrate that isotopically light carbon dominated all the upper oceanic, biospheric and atmospheric carbon reservoirs, and that this occurred despite the enhanced burial of organic carbon. We propose that--as has been suggested for the Late Palaeocene thermal maximum, some 55 million years ago--the observed patterns were produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental-margin sediments.     

Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system

Continental erosion controls atmospheric carbon dioxide levels on geological timescales through silicate weathering, riverine transport and subsequent burial of organic carbon in oceanic sediments. The efficiency of organic carbon deposition in sedimentary basins is however limited by the organic carbon load capacity of the sediments and organic carbon oxidation in continental margins. At the global scale, previous studies have suggested that about 70 per cent of riverine organic carbon is returned to the atmosphere, such as in the Amazon basin. Here we present a comprehensive organic carbon budget for the Himalayan erosional system, including source rocks, river sediments and marine sediments buried in the Bengal fan. We show that organic carbon export is controlled by sediment properties, and that oxidative loss is negligible during transport and deposition to the ocean. Our results indicate that 70 to 85 per cent of the organic carbon is recent organic matter captured during transport, which serves as a net sink for atmospheric carbon dioxide. The amount of organic carbon deposited in the Bengal basin represents about 10 to 20 per cent of the total terrestrial organic carbon buried in oceanic sediments. High erosion rates in the Himalayas generate high sedimentation rates and low oxygen availability in the Bay of Bengal that sustain the observed extreme organic carbon burial efficiency. Active orogenic systems generate enhanced physical erosion and the resulting organic carbon burial buffers atmospheric carbon dioxide levels, thereby exerting a negative feedback on climate over geological timescales.     

Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals

The marine sedimentary record exhibits evidence for episodes of enhanced organic carbon burial known as 'oceanic anoxic events' (OAEs). They are characterized by carbon-isotope excursions in marine and terrestrial reservoirs and mass extinction of marine faunas. Causal mechanisms for the enhancement of organic carbon burial during OAEs are still debated, but it is thought that such events should draw down significant quantities of atmospheric carbon dioxide. In the case of the Toarcian OAE (approximately 183 million years ago), a short-lived negative carbon-isotope excursion in oceanic and terrestrial reservoirs has been interpreted to indicate raised atmospheric carbon dioxide caused by oxidation of methane catastrophically released from either marine gas hydrates or magma-intruded organic-rich rocks. Here we test these two leading hypotheses for a negative carbon isotopic excursion marking the initiation of the Toarcian OAE using a high-resolution atmospheric carbon dioxide record obtained from fossil leaf stomatal frequency. We find that coincident with the negative carbon-isotope excursion carbon dioxide is first drawn down by 350 +/- 100 p.p.m.v. and then abruptly elevated by 1,200 +/- 400 p.p.m.v, and infer a global cooling and greenhouse warming of 2.5 +/- 0.1 degrees C and 6.5 +/- 1 degrees C, respectively. The pattern and magnitude of carbon dioxide change are difficult to reconcile with catastrophic input of isotopically light methane from hydrates as the cause of the negative isotopic signal. Our carbon dioxide record better supports a magma-intrusion hypothesis, and suggests that injection of isotopically light carbon from the release of thermogenic methane occurred owing to the intrusion of Gondwana coals by Toarcian-aged Karoo-Ferrar dolerites.     

Influence of the carbon cycle on the attribution of responsibility for climate change

The carbon cycle is one of the fundamental climate change issues.Its long-term evolution largely affects the amplitude and trend of human-induced climate change,as well as the formulation and implementation of emission reduction policy and technology for stabilizing the atmospheric CO2concentration.Two earth system models incorporating the global carbon cycle,the Community Earth System Model and the Beijing Normal University-Earth System Model,were used to investigate the effect of the carbon cycle on the attribution of the historical responsibility for climate change.The simulations show that when compared with the criterion based on cumulative emissions,the developed(developing)countries’responsibility is reduced(increased)by 6%–10%using atmospheric CO2concentration as the criterion.This discrepancy is attributed to the fact that the developed world contributed approximately61%–68%(61%–64%)to the change in global oceanic(terrestrial)carbon sequestration for the period from 1850 to2005,whereas the developing world contributed approximately 32%–49%(36%–39%).Under a developed world emissions scenario,the relatively larger uptake of global carbon sinks reduced the developed countries’responsibility for carbon emissions but increased their responsibility for global ocean acidification(68%).In addition,the large emissions from the developed world reduced the efficiency of the global carbon sinks,which may affect the long-term carbon sequestration and exacerbate global warming in the future.Therefore,it is necessary to further consider the interaction between carbon emissions and the carbon cycle when formulating emission reduction policy.     

Effect of evaporite deposition on Early Cretaceous carbon and sulphur cycling

The global carbon and sulphur cycles are central to our understanding of the Earth's history, because changes in the partitioning between the reduced and oxidized reservoirs of these elements are the primary control on atmospheric oxygen concentrations. In modern marine sediments, the burial rates of reduced carbon and sulphur are positively coupled, but high-resolution isotope records indicate that these rates were inversely related during the Early Cretaceous period. This inverse relationship is difficult to reconcile with our understanding of the processes that control organic matter remineralization and pyrite burial. Here we show that the inverse correlation can be explained by the deposition of evaporites during the opening of the South Atlantic Ocean basin. Evaporite deposition can alter the chemical composition of sea water, which can in turn affect the ability of sulphate-reducing bacteria to remineralize organic matter and mediate pyrite burial. We use a reaction-transport model to quantify these effects, and the resulting changes in the burial rates of carbon and sulphur, during the Early Cretaceous period. Our results indicate that deposition of the South Atlantic evaporites removed enough sulphate from the ocean temporarily to reduce biologically mediated pyrite burial and organic matter remineralization by up to fifty per cent, thus explaining the inverse relationship between the burial rates of reduced carbon and sulphur during this interval. Furthermore, our findings suggest that the effect of changing seawater sulphate concentrations on the marine subsurface biosphere may be the key to understanding other large-scale perturbations of the global carbon and sulphur cycles.     

Atmospheric carbon dioxide concentrations over the past 60 million years

Knowledge of the evolution of atmospheric carbon dioxide concentrations throughout the Earth's history is important for a reconstruction of the links between climate and radiative forcing of the Earth's surface temperatures. Although atmospheric carbon dioxide concentrations in the early Cenozoic era (about 60 Myr ago) are widely believed to have been higher than at present, there is disagreement regarding the exact carbon dioxide levels, the timing of the decline and the mechanisms that are most important for the control of CO2 concentrations over geological timescales. Here we use the boron-isotope ratios of ancient planktonic foraminifer shells to estimate the pH of surface-layer sea water throughout the past 60 million years, which can be used to reconstruct atmospheric CO2 concentrations. We estimate CO2 concentrations of more than 2,000 p.p.m. for the late Palaeocene and earliest Eocene periods (from about 60 to 52 Myr ago), and find an erratic decline between 55 and 40 Myr ago that may have been caused by reduced CO2 outgassing from ocean ridges, volcanoes and metamorphic belts and increased carbon burial. Since the early Miocene (about 24 Myr ago), atmospheric CO2 concentrations appear to have remained below 500 p.p.m. and were more stable than before, although transient intervals of CO2 reduction may have occurred during periods of rapid cooling approximately 15 and 3 Myr ago.     

Ocean productivity before about 1.9 Gyr ago limited by phosphorus adsorption onto iron oxides

After the evolution of oxygen-producing cyanobacteria at some time before 2.7 billion years ago, oxygen production on Earth is thought to have depended on the availability of nutrients in the oceans, such as phosphorus (in the form of orthophosphate). In the modern oceans, a significant removal pathway for phosphorus occurs by way of its adsorption onto iron oxide deposits. Such deposits were thought to be more abundant in the past when, under low sulphate conditions, the formation of large amounts of iron oxides resulted in the deposition of banded iron formations. Under these circumstances, phosphorus removal by iron oxide adsorption could have been enhanced. Here we analyse the phosphorus and iron content of banded iron formations to show that ocean orthophosphate concentrations from 3.2 to 1.9 billion years ago (during the Archaean and early Proterozoic eras) were probably only approximately 10-25% of present-day concentrations. We suggest therefore that low phosphorus availability should have significantly reduced rates of photosynthesis and carbon burial, thereby reducing the long-term oxygen production on the early Earth--as previously speculated--and contributing to the low concentrations of atmospheric oxygen during the late Archaean and early Proterozoic.     

Reburial of fossil organic carbon in marine sediments

Marine sediments act as the ultimate sink for organic carbon, sequestering otherwise rapidly cycling carbon for geologic timescales. Sedimentary organic carbon burial appears to be controlled by oxygen exposure time in situ, and much research has focused on understanding the mechanisms of preservation of organic carbon. In this context, combustion-derived black carbon has received attention as a form of refractory organic carbon that may be preferentially preserved in soils and sediments. However, little is understood about the environmental roles, transport and distribution of black carbon. Here we apply isotopic analyses to graphitic black carbon samples isolated from pre-industrial marine and terrestrial sediments. We find that this material is terrestrially derived and almost entirely depleted of radiocarbon, suggesting that it is graphite weathered from rocks, rather than a combustion product. The widespread presence of fossil graphitic black carbon in sediments has therefore probably led to significant overestimates of burial of combustion-derived black carbon in marine sediments. It could be responsible for biasing radiocarbon dating of sedimentary organic carbon, and also reveals a closed loop in the carbon cycle. Depending on its susceptibility to oxidation, this recycled carbon may be locked away from the biologically mediated carbon cycle for many geologic cycles.     

大洋缺氧事件的碳稳定同位素响应

从碳稳定同位素组成及其分馏机理出发 ,系统探讨了大洋缺氧事件与海相碳酸盐和有机碳稳定同位素分馏之间的关系。缺氧事件期间 ,由于生物大批死亡和快速埋藏 ,其分解消耗海水中大量的溶解氧 ,引起大洋水体缺氧 ,富含 1 2 C的有机质从而得以大量保存 ;相应地大气和海水中富 1 3 C,同期海相碳酸盐岩碳同位素 δ值 (δ1 3C)正偏。在世界各地缺氧事件层内 ,无一例外地碳酸盐岩碳稳定同位素出现了不同程度的正偏 ,Cenomanian- Turonian 界线偏幅达～2‰。海相碳酸盐与有机质碳稳定同位素变化不仅可以提供地质历史中有机碳埋藏量的记录。研究全球碳循环变化 ,还可能追溯有机碳风化和埋藏速率的变化 ,定性地恢复大气 p CO2 变化。     

Climate sensitivity constrained by CO2 concentrations over the past 420 million years

A firm understanding of the relationship between atmospheric carbon dioxide concentration and temperature is critical for interpreting past climate change and for predicting future climate change. A recent synthesis suggests that the increase in global-mean surface temperature in response to a doubling of the atmospheric carbon dioxide concentration, termed 'climate sensitivity', is between 1.5 and 6.2 degrees C (5-95 per cent likelihood range), but some evidence is inconsistent with this range. Moreover, most estimates of climate sensitivity are based on records of climate change over the past few decades to thousands of years, when carbon dioxide concentrations and global temperatures were similar to or lower than today, so such calculations tend to underestimate the magnitude of large climate-change events and may not be applicable to climate change under warmer conditions in the future. Here we estimate long-term equilibrium climate sensitivity by modelling carbon dioxide concentrations over the past 420 million years and comparing our calculations with a proxy record. Our estimates are broadly consistent with estimates based on short-term climate records, and indicate that a weak radiative forcing by carbon dioxide is highly unlikely on multi-million-year timescales. We conclude that a climate sensitivity greater than 1.5 degrees C has probably been a robust feature of the Earth's climate system over the past 420 million years, regardless of temporal scaling.     

Eocene bipolar glaciation associated with global carbon cycle changes

The transition from the extreme global warmth of the early Eocene 'greenhouse' climate approximately 55 million years ago to the present glaciated state is one of the most prominent changes in Earth's climatic evolution. It is widely accepted that large ice sheets first appeared on Antarctica approximately 34 million years ago, coincident with decreasing atmospheric carbon dioxide concentrations and a deepening of the calcite compensation depth in the world's oceans, and that glaciation in the Northern Hemisphere began much later, between 10 and 6 million years ago. Here we present records of sediment and foraminiferal geochemistry covering the greenhouse-icehouse climate transition. We report evidence for synchronous deepening and subsequent oscillations in the calcite compensation depth in the tropical Pacific and South Atlantic oceans from approximately 42 million years ago, with a permanent deepening 34 million years ago. The most prominent variations in the calcite compensation depth coincide with changes in seawater oxygen isotope ratios of up to 1.5 per mil, suggesting a lowering of global sea level through significant storage of ice in both hemispheres by at least 100 to 125 metres. Variations in benthic carbon isotope ratios of up to approximately 1.4 per mil occurred at the same time, indicating large changes in carbon cycling. We suggest that the greenhouse-icehouse transition was closely coupled to the evolution of atmospheric carbon dioxide, and that negative carbon cycle feedbacks may have prevented the permanent establishment of large ice sheets earlier than 34 million years ago.     

Variable effects of nitrogen additions on the stability and turnover of soil carbon

Soils contain the largest near-surface reservoir of terrestrial carbon and so knowledge of the factors controlling soil carbon storage and turnover is essential for understanding the changing global carbon cycle. The influence of climate on decomposition of soil carbon has been well documented, but there remains considerable uncertainty in the potential response of soil carbon dynamics to the rapid global increase in reactive nitrogen (coming largely from agricultural fertilizers and fossil fuel combustion). Here, using 14C, 13C and compound-specific analyses of soil carbon from long-term nitrogen fertilization plots, we show that nitrogen additions significantly accelerate decomposition of light soil carbon fractions (with decadal turnover times) while further stabilizing soil carbon compounds in heavier, mineral-associated fractions (with multidecadal to century lifetimes). Despite these changes in the dynamics of different soil pools, we observed no significant changes in bulk soil carbon, highlighting a limitation inherent to the still widely used single-pool approach to investigating soil carbon responses to changing environmental conditions. It remains to be seen if the effects observed here-caused by relatively high, short-term fertilizer additions-are similar to those arising from lower, long-term additions of nitrogen to natural ecosystems from atmospheric deposition, but our results suggest nonetheless that current models of terrestrial carbon cycling do not contain the mechanisms needed to capture the complex relationship between nitrogen availability and soil carbon storage.     

Geochemical evidence for widespread euxinia in the later Cambrian ocean

Widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long-term inhibitor of evolutionary radiation on early Earth. In recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later Cambrian oceans, possibly influencing evolutionary events during this time. Physical evidence of widespread anoxia in Cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored. Here we present sulphur isotope records from six globally distributed stratigraphic sections of later Cambrian marine rocks (about 499 million years old). We find a positive sulphur isotope excursion in phase with the Steptoean Positive Carbon Isotope Excursion (SPICE), a large and rapid excursion in the marine carbon isotope record, which is thought to be indicative of a global carbon cycle perturbation. Numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large-scale anoxic and sulphidic (euxinic) conditions. Independently, molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia. These results identify the SPICE interval as the best characterized ocean anoxic event in the pre-Mesozoic ocean and an extreme example of oxygen deficiency in the later Cambrian ocean. Thus, a redox structure similar to those in Proterozoic oceans may have persisted or returned in the oceans of the early Phanerozoic eon. Indeed, the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in Cambrian oceans.     

Increased thermohaline stratification as a possible cause for an ocean anoxic event in the Cretaceous period

Ocean anoxic events were periods of high carbon burial that led to drawdown of atmospheric carbon dioxide, lowering of bottom-water oxygen concentrations and, in many cases, significant biological extinction. Most ocean anoxic events are thought to be caused by high productivity and export of carbon from surface waters which is then preserved in organic-rich sediments, known as black shales. But the factors that triggered some of these events remain uncertain. Here we present stable isotope data from a mid-Cretaceous ocean anoxic event that occurred 112 Myr ago, and that point to increased thermohaline stratification as the probable cause. Ocean anoxic event 1b is associated with an increase in surface-water temperatures and runoff that led to decreased bottom-water formation and elevated carbon burial in the restricted basins of the western Tethys and North Atlantic. This event is in many ways similar to that which led to the more recent Plio-Pleistocene Mediterranean sapropels, but the greater geographical extent and longer duration (approximately 46 kyr) of ocean anoxic event 1b suggest that processes leading to such ocean anoxic events in the North Atlantic and western Tethys were able to act over a much larger region, and sequester far more carbon, than any of the Quaternary sapropels.     

The hydrologic cycle in deep-time climate problems

Hydrology refers to the whole panoply of effects the water molecule has on climate and on the land surface during its journey there and back again between ocean and atmosphere. On its way, it is cycled through vapour, cloud water, snow, sea ice and glacier ice, as well as acting as a catalyst for silicate-carbonate weathering reactions governing atmospheric carbon dioxide. Because carbon dioxide affects the hydrologic cycle through temperature, climate is a pas des deux between carbon dioxide and water, with important guest appearances by surface ice cover.     

Limited carbon storage in soil and litter of experimental forest plots under increased atmospheric CO2

The current rise in atmospheric CO2 concentration is thought to be mitigated in part by carbon sequestration within forest ecosystems, where carbon can be stored in vegetation or soils. The storage of carbon in soils is determined by the fraction that is sequestered in persistent organic materials, such as humus. In experimental forest plots of loblolly pine (Pinus taeda) exposed to high CO2 concentrations, nearly half of the carbon uptake is allocated to short-lived tissues, largely foliage. These tissues fall to the ground and decompose, normally contributing only a small portion of their carbon content to refractory soil humic materials. Such findings call into question the role of soils as long-term carbon sinks, and show the need for a better understanding of carbon cycling in forest soils. Here we report a significant accumulation of carbon in the litter layer of experimental forest plots after three years of growth at increased CO2 concentrations (565 microl l(-1)). But fast turnover times of organic carbon in the litter layer (of about three years) appear to constrain the potential size of this carbon sink. Given the observation that carbon accumulation in the deeper mineral soil layers was absent, we suggest that significant, long-term net carbon sequestration in forest soils is unlikely.     

A 'snowball Earth' climate triggered by continental break-up through changes in runoff

Geological and palaeomagnetic studies indicate that ice sheets may have reached the Equator at the end of the Proterozoic eon, 800 to 550 million years ago, leading to the suggestion of a fully ice-covered 'snowball Earth'. Climate model simulations indicate that such a snowball state for the Earth depends on anomalously low atmospheric carbon dioxide concentrations, in addition to the Sun being 6 per cent fainter than it is today. However, the mechanisms producing such low carbon dioxide concentrations remain controversial. Here we assess the effect of the palaeogeographic changes preceding the Sturtian glacial period, 750 million years ago, on the long-term evolution of atmospheric carbon dioxide levels using the coupled climate-geochemical model GEOCLIM. In our simulation, the continental break-up of Rodinia leads to an increase in runoff and hence consumption of carbon dioxide through continental weathering that decreases atmospheric carbon dioxide concentrations by 1,320 p.p.m. This indicates that tectonic changes could have triggered a progressive transition from a 'greenhouse' to an 'icehouse' climate during the Neoproterozoic era. When we combine these results with the concomitant weathering effect of the voluminous basaltic traps erupted throughout the break-up of Rodinia, our simulation results in a snowball glaciation.     

Changes in deep-water formation during the Younger Dryas event inferred from 10Be and 14C records

Variations in atmospheric radiocarbon (14C) concentrations can be attributed either to changes in the carbon cycle--through the rate of radiocarbon removal from the atmosphere--or to variations in the production rate of 14C due to changes in solar activity or the Earth's magnetic field. The production rates of 10Be and 14C vary in the same way, but whereas atmospheric radiocarbon concentrations are additionally affected by the carbon cycle, 10Be concentrations reflect production rates more directly. A record of the 10Be production-rate variations can therefore be used to separate the two influences--production rates and the carbon cycle--on radiocarbon concentrations. Here we present such an analysis of the large fluctuations in atmospheric 14C concentrations, of unclear origin, that occurred during the Younger Dryas cold period. We use the 10Be record from the GISP2 ice core to model past production rates of radionuclides, and find that the largest part of the fluctuations in atmospheric radiocarbon concentrations can be attributed to variations in production rate. The residual difference between measured 14C concentrations and those modelled using the 10Be record can be explained with an additional change in the carbon cycle, most probably in the amount of deep-water formation.     

红树林碳埋藏过程对海平面上升、气候变化和人类活动的响应

红树林具有很高的初级生产力和沉积速率,能够在沉积过程中埋藏封存大量有机碳,在全球碳循环中扮演着重要角色。本文总结有关红树林碳埋藏特征和沉积过程对气候变化、海平面上升和人类活动响应的相关文献研究。红树林土壤碳埋藏储量主要取决于红树林根系分布深度,碳埋藏速率则主要受沉积速率控制。全球变暖导致亚热带冬季低温事件减少,以及降雨量的增多,都会促进红树林扩张,有利于红树林碳埋藏。在短期内,海平面上升可以通过增加滩面沉积速率和有机碳含量的方式,达到保持或提高红树林碳埋藏速率的效果。但是从长期来看,不断增加的海平面上升速率会对泥沙供应较少或潮差较小地区的红树林造成严重威胁。人类的不合理活动则会造成红树林面积和碳埋藏储量的大幅减少。因此,为保护红树林并发挥其碳库潜力,应采取有效措施保证红树林沉积的物源供给,以维持红树林滩面高程增加速率,并在适宜地区进行人工红树林栽种。