A large carbon pool in lake sediments over the arid/semiarid region,NW China

Jianghu Lan?Hai Xu?Bin Liu,2?Enguo Sheng,3?Jiangtao Zhao?Keke Yu,3

A large carbon pool in lake sediments over the arid/semiarid region,NW China

Jianghu Lan1?Hai Xu1?Bin Liu1,2?Enguo Sheng1,3?Jiangtao Zhao1?Keke Yu1,3

Carbon burial in lake sediments is an important component of the global carbon cycle.However,little is known about the magnitude of carbon sequestered in lake sediments over the arid/semiarid region of China(ASAC). In this study,we estimate both organic and inorganic carbon burial since～AD1800 based on nine lakes in ASAC, and discuss the most plausible factors controlling carbon burial.Our estimates show that the annual organic carbon burial rate(OCBR)ranges from 5.3 to 129.8 g cm-2year-1(weightedmeanof49.9g cm-2year-1),leadingtoastanding stock of 1.1–24.0 kg cm-2(weighted mean of 8.6 kg cm-2) and a regional sum of～108 Tg organic carbon sequestered since～AD1800.The annual inorganic carbon burial rate (ICBR)ranges from 11.4 to 124.0 g cm-2year-1(weighted mean of 48.3 g cm-2year-1),which is slightly lower than OCBR.The inorganic carbon standing stock ranges from 2.4 to 26.0 kg cm-2(weighted mean of 8.1 kg cm-2), resulting in a sum of～101 Tg regional inorganic carbon burial since～AD1800,which is slightly lower than the organic carbon sequestration.OCBR in ASAC shows a continuously increasing trend since～AD1950,which is possibly due to the high autochthonous and allochthonous primary production and subsequently high sedimentation rate in thelakes.This increasing carbon burial is possibly related to both climatic changes and enhanced anthropogenic activities,such as land use change,deforestation,and eutrophication in the lake.Furthermore,OCBR and ICBR are expected to continuously increase under the scenario of increasing precipitation and runoff and enhanced anthropogenic activities. Theresultsofthisresearchshowthattheburiedcarboninlake sediments of the ASAC region constitutes a signifcant and large carbon pool,which should be considered and integrated into the global carbon cycle.

Organic and inorganic carbon burials·Lake sediments·ASAC·Carbon cycle

1 Introduction

Inland water systems are important transfer points for balancing the global atmospheric,terrestrial biosphere,and oceanic carbon cycles(Kastowski et al.2011).Carbon dioxide is directly and indirectly drawn-down from the atmosphere by biomass production and rock weathering, and transported by rivers to the ocean and/or lakes in both organic and inorganic carbon forms(Kastowski et al. 2011).In addition,carbon dioxide is directly drawn down from the atmosphere by hydrophytes,and subsequently sequestered in large amounts into sediments in organic form(Sobek et al.2009).Based on Pareto distribution, Dean and Gorham(1998)suggested that the inland water carbon pools,including lakes,reservoirs,and wetlands, collectively cover less than 2%of Earth’s surface and constitute a carbon sink of about 300 Tg year-1,which is three times larger than ocean carbon pools.Downing et al. (2006)proposed that about 4.6 million km2of continental land(more than 3%)is covered by water,which is twiceas large as previously assumed.Therefore,carbon burial in lake sediments should be a substantial and an important component of the global carbon cycle(Dean and Gorham 1998;Cole et al.2007).Lake sediments play an important role in removing carbon from the actively cycling carbon pools(Sobek et al.2009),and account for a large proportion of previously‘‘missing’’carbon by processing large amounts of terrestrially derived carbon,although certain fractions of carbon burial are returned to the atmosphere by degassing(Battin et al.2009;Downing 2009).However, due to the small coverage of Earth’s land surface,little attention has been directed to the role of carbon burial in lakes within the global carbon cycle.

In recent years,an increasing number of studies have focused on lake carbon burial(Kortelainen et al.2004;Alin and Johnson 2007;Downing et al.2008;Kastowski et al. 2011;Dong et al.2012),among which nearly all studies focused on organic carbon burial,while inorganic carbon burial was,in most cases,overlooked.In addition,very few studies have focused on lake carbon burial in China even though the lake areas are large(Dong et al.2012).In this study,we estimate the organic and inorganic carbon burial from nine lakes in the arid/semiarid region of China (ASAC)since～AD1800,and discuss the most plausible factors controlling carbon burial.

2 Background and methods

2.1 Background

The ASAC is located in northwestern China and has a total area of～2.55 million km2.It is situated in the temperate continental climatic zone,with annual temperatures ranging from 0 to 10°C.Precipitation over the ASAC region is mostly＜400 mm per year,which is much less than the corresponding evaporation (～2000 mm per year) (Table 1).These climatic conditions lead to low vegetative cover,vulnerable ecosystems,and strong physical weathering over most of the ASAC area.Six provinces and/or autonomous regions of China(Xinjiang,Gansu,Inner Mongolia,Ningxia,Shannxi,and Shanxi)are located within the ASAC.

2.2 Methods

In this study,nine well-studied lakes in ASAC were chosen for estimating carbon burial based on quality of published sedimentation rate and organic and/or inorganic carbon content(Fig.1).To assess the burial rate of organic and inorganic carbon in each lake,the total organic carbon (TOC),total inorganic carbon(TIC),sedimentation rate, and dry bulk density(DBD)of the sediment are required. Once these parameters are derived it is possible to assess the organic carbon burial rate(OCBR)and inorganic carbon burial rate(ICBR)based on the following equations:

We then proceed to calculate the total burial of organic carbon and inorganic carbon in the ASAC based on estimated lake surface area derived by the Pareto distribution method.In the following sections we derive each one of the required parameters.

2.2.1 Sedimentation rate

Assessment of carbon burial in lake sediments requires knowledge of the sedimentation rate.The linear interpolation/extrapolation of sedimentation rates used in this study is calculated using age-depth models,which are usually established by210Pb and/or137Cs(Fig.2).

The results show that the sedimentation rate may have increased since～AD1950.This rate change have played a signifcant role in carbon burial in lake sediments(see discussion below).

2.2.2 Dry bulk density

Estimating carbon burial in lake sediments requires DBD data;unfortunately,it is generally not reported in the literature.Therefore,we choose an empirical approach to estimate DBD.A large number of studies show that DBD of lakesedimentsiscloselyrelatedtoTOC.Forexample,based on six alpine lake sediment cores,Menounos(1997)found a signifcant,nonlinear relationship between TOC and DBD (DBD=1.881-0.385×ln(TOC);R2=0.89).Avnimelech et al.(2001)examined the correlation between TOC and DBD in six different submerged sediments,including lake,pond,river,and sea foor sediments,and found that DBD is inversely related to TOC content(DBD=1.776 -0.363×ln(TOC);R2=0.70).Using three lakes in Minnesota,Dean and Gorham(1998)showed that DBD of lake sediments is mainly dependent on TOC content (DBD=1.665×TOC-0.887).Taking into consideration theeffectofcompaction,Campbelletal.(2000)presentedan additional relationship between TOC and DBD.A complicationintheDBDestimationmayarisewhenTOCcontentis less than 5%,because the data used to construct the best ft line does not include low TOC,as mentioned by Kastowski et al.(2011).Overall,DBD may be overestimated or underestimatedwhenTOCisveryloworveryhigh(Kastowski et al.2011).Here,we combine different empirical relationships between TOC and DBD in lake sediments generatedfrom the former(Fig.3)and divide the estimated DBD into two segments:when TOC content ranges from 2%to 10%, we use Eq.(3):

Table 1 Selected limnological characteristics of the nine lakes

While when DBD is higher than 10%or lower than 2%,Eq.(4)is used:

However,when TOC is lower than a threshold value,a persistent increase in DBD should probably not occur with decreasing TOC content in lake sediments.Thus,DBD in lake sediments cannot appreciably be calculated by the equations proposed above.Extremely low TOC has often been reported in ASAC because of the low annual precipitation and high potential evaporation.Therefore,when the TOC content was less than 0.5%we use a DBD value of 2.5 g cm-3(Richerson et al.2008).

In cases where no TOC data were available,we calculated TOC data from organic matter content(OM) (Eq.5)—(Dean 1974,1999)or loss on ignition at 550°C (LOI550)(Eq.6)—(Santisteban et al.2004):

TIC was estimated in two ways:loss on ignition at 950°C(LOI950)using—(Santisteban et al.2004):

or calculated by dividing carbonate content by 8.33(Dean 1999),which is the proportion of carbon in carbonate.

2.2.3 Total lake surface area

Evaluation of carbon burial in lake sediments of ASAC requires an estimate of the total lake surface area.Previous estimates suggested that large lakes dominate the global lakeareawith a totallakesurfaceareaof about 2–2.8×106km2(Meybeck 1995;Kalff 2001;Shiklomanov and Rodda 2003).Downing et al.(2006)addedsmall water bodies and using the Pareto distribution proposed that the global lake area is～4.6×106km2.

Ma et al.(2010,2011)and Lehner and Do¨ll(2004)provided comprehensive lake surface data in China.Based on satellite images from CBERS CCD and Landsat TM/ETM, Maetal.(2010,2011)suggestedthattherearepresently2693 natural lakes in China with an area larger than 1 km2.The lakedistribution,asproposedbyLehnerandDo¨ll(2004)can generally be described by the size-frequency function:

where Nais the number of lakes larger and/or equal to a specifc lake size A,and a and b are ftted equation parameters. In this study,we mainly analyze lakes in ASAC larger than 1 km2and neglect those smaller than 1 km2due to lack of data.Based on comprehensive lake data in this area (Ma et al.2010,2011),we found that the number of lakes in ASAC follows the Pareto distribution,where the number of lakes increases exponentially with decreasing lake area (Fig.4a)described by:

And the total surface area of this region also can be obtained from Ma et al.(2010,2011),who implied that there is a total lake surface area of about 12,590 km2,with 514 lakes＞1 km2and 19 lakes＞100 km2(Figs.1,4b).

Fig.2 The210Pb and137Cs activities in part of the selected lakes.a Erbinur Lake(Ma et al.2011;Wu et al.2009).b Bosten Lake(Chen et al. 2006).c Daihai Lake(Lan et al.2011).d Wulungu Lake(Liu et al.2008a,b).e Jili Lake(Jiang et al.2010)

Fig.3 The empirical relationship between total organic carbon (TOC)content and dry bulk density(DBD)in lake sediments.The square dotted lines are results from Menounos(1997),Avnimelech et al.(2001),Campbell et al.(2000),and Dean and Gorham(1998). The red dotted line is applied for this study,stacked from the formers

3 Results:estimating the carbon burial rate

Based on the parameters calculated above and using Eqs.1 and 2,the results show that OCBR ranges from 5.3 to 129.8 g cm-2year-1with a weighted mean of 49.9 g cm-2year-1,resulting in an organic carbon standing stock of about 1.1–24.0 kg cm-2(weighted mean 8.6 kg cm-2)and a total regional organic carbon sequestration of～108 TgC since～AD1800 for lakes larger than 1 km2in ASAC(Table 2).

The ICBR in ASAC was calculated as ranging between 11.4 and 124.0 g cm-2year-1,with a weighted mean of 48.3 g cm-2year-1resulting in an inorganic carbon standing stock of2.4–26.0 kg cm-2(weighted mean 8.1 kg cm-2)and a TIC sequestration of～101 TgC in lake sediments throughout the whole arid/semiarid area, both of which are slightly smaller than the corresponding organic carbon burial(Table 2).

4 Discussion

Carbon stored in atmospheric,oceanic,and terrestrial ecosystems are important parameters of the climate system and have thus received widespread attention(Fang et al. 2001;Sabine et al.2004;Canadell et al.2007;Bloom et al. 2010;Bond-Lamberty and Thomson 2010;Frank et al. 2010;Pan et al.2011;Ballantyne et al.2012).When constructing the various components of the carbon cycle,it is assumed that the oceans take up approximately one third of the CO2arising from fossil-fuel emissions,with the remaining two thirds divided between terrestrial ecosystems and the atmosphere.However,there still appears to be a missing carbon sink,which may be accounted for in terrestrial ecosystems(Siegenthaler and Sarmiento 1993). Thus it is crucial to assess the contributions of the various parts of the terrestrial ecosystems to the sequestration of carbon in order to better our understanding of the process that might occur under a regime of global warming. Models predict that in northwestern China,precipitation will increase as a consequence of global warming and enhance the water cycle(Shi et al.2007).The consequence of this process to the carbon cycle will be enhanced carbon transport to lakes from their catchments and subsequent escape to the atmosphere or sequestering in lake sediments (Molot and Dillon 1996).

4.1 OCBR trend in ASAC since～AD1800

OCBR in most ASAC lakes,excluding Ebinur Lake,has generally increased since～AD1950(Fig.5a).However, the temporal variations among those lakes are considerably different.

To obtain a more comprehensive understanding of carbon burial in ASAC,we considered in more detail the impacts on carbon burial of precipitation,proportion of cropland and forest,population,and sedimentation rate.

Fig.4 Total lake number of each lake size class in ASAC(a)and total lake surface area(b)

Table 2 Organic and inorganic carbon burial in lake sediments of ASAC

Fig.5 a OCBR trends in lake sediments over the arid/ semiarid,NW China,since～AD 1800.Bosten Lake(blue line), Erbinur Lake(pink line),Daihai Lake(wine line),Hongjiannao Lake(purple line),Jili Lake (yellow line)and Wulungu Lake (green line).b Precipitation reconstructed from tree-ring δ18O(red line)(Liu et al.2004, 2008a,b)and tree-ring width (blue line)(Zhang et al.2013)

Reconstructed precipitation patterns based on tree-ring width(Zhang et al.2013)and δ18O(Liu et al.2004,2008a, b)show considerable variation beforeAD1970s,and a signifcant increase thereafter(Fig.5b).Moreover,a stimulate model also shows an increasing trend of precipitation since～AD1975(Shi et al.2007).Both results may roughly indicate that increased precipitation corresponds with enhanced OCBR,attributed to high primary production and chemical weathering under increased precipitation and temperature.Thus,precipitation is plausibly a predominate factor for organic carbon burial.

Furthermore,the cropland area in Xinjiang province over the past 100 years has largely increased with the total population in China,especially afterAD1950(Ge and Dai 2005),which dramatically changed the land use and land cover.For example,the cropland area in Xinjiang province increased from about 17×104hm2in the 1920s to about 27×104hm2in the 1950s,and then rapidly increased to larger than 60×104hm2in 15 years(Ge and Dai 2005). The total forest area in ASAC substantially decreased during the period 1800–1950(Ge and Dai 2005;He et al. 2007)due to deforestation by anthropogenic activity.Itwas,however,slightly increased after 1950 and sharply reforested since 1975 due to governmental policy.Enhanced cropland area and reduced forest area due to anthropogenic activity may release organic carbon to the atmosphere which previously was stored in soils and forests(Ge et al.2008).On the other hand,higher sedimentation rates in lakes in areas with enhanced anthropogenic activity in their catchments suggest that organic carbon stored in the forest and soil and inorganic carbon stored in soil may be transported by rivers to lakes from catchments and then buried in lake sediments at higher rates.More organic and inorganic carbons are also sequestered by enhanced hydrophytesdueto eutrophication in lakes. Therefore,increased OCBR and ICBR in recent lake sediments may be indirectly driven by in-catchment anthropogenic activity and directly enhanced by autochthonous hydrophyte blooms.

Gudasz et al.(2010)found that the mineralization of organic carbon in lake sediments is strongly and positively related to temperature,suggesting that carbon burial would decrease with increased temperature.However,they did not consider the enhancement of anthropogenic activity and the possibility of increased autochthonous and allochthonous primary production with global warming.In our opinion,no matter what complex processes are involved,the burial of carbon in lake sediments can be regarded as a net sink,and sequestration will increase with increasing carbon burial rate.This is also consistent with the results of other studies(Hollander et al.1992;Dean and Gorham 1998;Downing et al.2008;Kastowski et al.2011; Dong et al.2012;Heathcote and Downing 2012).Shi et al. (2007)simulated an increase by 19%in precipitation and more than 10%in runoff in northwestern China under future climate for doubled CO2concentrations using a regional climate model—RegCM2.Anthropogenic activity will probably increase with increasing population.Under such circumstances,carbon burial rates would be expected to increase by about 20%,corresponding to OCBR and ICBR in ASAC conservatively increasing to 59.9 and 57.9 gm-2year-1,respectively.

4.2 Spatial patterns of carbon burial in ASAC

Although spatial patterns in lake carbon burial rates over the ASAC region are considerably different,some general features do stand out.For example,carbon burial rates in the eastern region are generally higher than those in the western region.Our estimates show that the OCBR in Daihai Lake,Hongjiannao Lake,and Hulun Lake in the eastern ASAC are 114.2,129.8 and 45.5 gm-2year-1,respectively;while ICBR estimates for those lakes are 124.0, 111.2,and 22.4 gm-2year-1,respectively.These are higher than the corresponding values in the western region.We assume that the east–west variation of carbon burial rate results primarily from variations in the mode of mean annual precipitation in China(Fig.1)and the strength of anthropogenic activity.Annual precipitation is much higher in the eastern than in the western ASAC(Fig.1), which would lead to larger carbon sequestration and strong weathering in the eastern catchment ecosystems.And eastern China is more densely populated,and thus the impact of anthropogenic activity may be much stronger.

There are,however,some exceptions,such as Bosten Lake and Chaiwopu Lake.Both OCBR and ICBR in these lakes are higher than other lakes in the same climatic region and even higher than lakes in wetter regions.We propose that these lakes may be subjected to stronger anthropogenic impacts than other lakes(Zheng et al.2012). In a recent study,Anderson et al.(2013,2014)proposed that contemporarily OCBR in European lakes have increased at least four to fve fold due to stronger anthropogenic impacts,not climate change.Thus,a second feature of carbon burial pattern in ASAC is that OCBR and ICBR are much higher in areas subjected to more intense anthropogenic activity.Furthermore,the two features of carbon burial rates indicate that a large carbon pool in ASAC is collectively driven by climatic change and enhanced anthropogenic activity.

4.3 Comparison with other regions

ToassesstheimportanceofOCBRinASAC,wecomparethe values calculated in this work with OCBR in other regions in China and around the world.The OCBR in ASAC ranges from 5.3 to 129.8 g cm-2year-1with a weighted mean of 49.9 g cm-2year-1since～AD1800.Ourresultssuggestthat organic carbon sequestered by lakes with area＞1 km2in ASAC(～73 Tg organic carbon)is much higher than that of theYangtzefoodplain(～41 Tgorganiccarbon)(Dongetal. 2012)in the same period(both since～AD1850).Although annual precipitation at the Yangtze foodplain is generally three to four times higher than at ASAC,anthropogenic activity at the Yangtze foodplain may also be much stronger than at ASAC and the total area of lakes＞1 km2in the Yangtze foodplain(about16,476 km2)isalsoslightly higher thaninASAC(about12,590 km2)(Dong etal.2012).OCBR in European lakes in the uppermost 20 cm sediments is 10.6 g cm-2year-1on average(Kastowski et al.2011).In agriculturallyeutrophicimpoundments,OCBRrangedfroma lowof148 g cm-2year-1toahighof17,000 g cm-2year-1(Downingetal.2008),whichismuchhigherthanotherreports and reinforces the importance of anthropogenic activity to carbon burial.Based on carbon mass balance studies of 20 small forested catchments and seven lakes in Ontario, Molot and Dillon(1996)calculated that approximately18–31 Tg year-1of organic and inorganic carbon are stored in boreal lake sediments.In one lake—Lake Chaohu on the Yangtze foodplain—OCBR was about 5–30 m-2year-1, resulting in standing stocks of 1.15 kg cm-2sinceAD1850 (Dongetal.2012).InalakeintheUnitedStates’stateofIowa, OCBR increased to 200 gm-2year-1following agricultural development(Heathcote and Downing 2012).OCBR for the top 10 cm of eutrophic Lake Greifen,Switzerland,is 50–60 gm-2year-2(DeanandGorham1998;Hollanderetal. 1992),which also supports the importance of anthropogenic activitytocarbonburial.Onaglobalscale,DeanandGorham (1998)suggest that global OCBR in the largest lakes(area＞5000 km2)and smaller lakes is 5 and 72 gm-2year-1,respectively.Subsequently,the total global organic carbon burialinlakesis42 Tg year-1.AllofthissuggeststhatASAC isalargecarbonpool,althoughtheprimaryproductioninthis region may be less than other regions.

4.4 A possible carbon sink from parts of the buried inorganic carbon

Little attention has been previously given to the inorganic carbon cycle in lakes,which is partly due to the understanding that inorganic carbon is mechanically transported from catchment to lake and/or directly inputted from precipitation(Dillon and Molot 1997;Urban et al.2005),but not directly from atmospheric CO2.However,part of the dissolved inorganic carbon(DIC),which comes from both carbonate and silicate weathering and/or directly from hydration with atmosphere(Liu et al.2010,2011;Liu 2011),may be kept in the water systems for a prolonged time span,and can be regarded as a temporary carbon sink. In addition,more than 70%of riverine DIC is directly and/ or indirectly withdrawn from atmospheric CO2,and portions of the DIC in lake waters may potentially precipitate as alkaline minerals and be buried in lake sediments,as suggested by our most recent estimate(Xu et al.2013). Therefore,although we do not know exactly how much of the inorganic carbon in lake sediments is a carbon sink, some portion may eventually form a carbon sink and balance the global carbon cycle.

Recently,based on three methods,Wang et al.(2012) determined the soil organic carbon(SOC)and soil inorganic carbon(SIC)in northwest China,and showed that all three methods produce consistently low values for SOC and high values for SIC.These results also imply the potential importance of inorganic carbon for lake carbon burial.The combined organic and inorganic carbon burial in the lakes in ASAC constitutes a large carbon sink and deserve more attention and integration into the global carbon cycle.

5 Summary

We estimated the carbon burial in lake sediments over the ASAC region,and showed that carbon burial in lake sediments since～AD1800 amounted to～209 TgC,with organic and inorganic carbon burials～108 and～101 TgC,respectively.Since～AD1950,OCBR shows increasing trends, which are possibly related to climatic changes and enhanced anthropogenic activities.If global warming continues,the increased precipitation and runoff over the ASAC region may lead to increased OCBR and ICBR in lake sediments of ASAC.These constitute a large carbon pool in the ASAC region and need to be given more attention in the future.

AcknowledgmentsThe authors are grateful to Yoni Goldsmith (Lamont–Doherty Earth Observatory,Columbia University)for suggestions on this manuscript.This work is jointly funded by the Chinese Academy of Science(CAS)Strategic Priority Research Program (Grant No.XDA05120404),the National Basic Research Program of China(No.2013CB955903),the State Key Laboratory of Loess and Quaternary Geology(No.SKLLQG1406)and the Western Light Talent Culture Project of CAS.

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Received:8 October 2014/Revised:21 January 2015/Accepted:9 March 2015/Published online:27 March 2015

?Science Press,Institute of Geochemistry,CAS and Springer-Verlag Berlin Heidelberg 2015

? Jianghu Lan lanjh@ieecas.cn;lanjh115@163.com

1State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment,Chinese Academy of Sciences,Xi’an 710061,China

2College of Resources and Environment,Zunyi Normal College,Zunyi 563002,China

3University of Chinese Academy of Sciences,Beijing 100049, China