A novel proxy for terrestrial organic matter in diments bad on
branched and isoprenoid tetraether lipids
Ellen C.Hopmans *,Johan W.H.Weijers,Enno Schefuß1,Lydie Herfort,
Jaap S.Sinninghe Damste
´,Stefan Schouten Department of Marine Biogeochemistry and Toxicology,Royal Netherlands Institute for Sea Rearch (NIOZ),P .O.Box 59,1790AB Den Burg,
Texel,The Netherlands
Received 4February 2004;received in revid form 29April 2004;accepted 7May 2004
Abstract
We propo a novel tracer for terrestrial organic carbon in diments bad on the analysis of tetraether lipids using high-performance liquid chromatography/mass spectrometry (HPLC/MS).Analysis of terrestrial soil and peats shows that branched tetraether lipids are predominant
in terrestrial environments in contrast to crenarchaeol,the characteristic membrane lipid of non-thermophilic crenarchaeota,which is especially abundant in the marine and lacustrine environment.Bad on the findings,an index was developed,the so-called Branched and Isoprenoid Tetraether (BIT)index,bad on the relative abundance of terrestrially derived tetraether lipids versus crenarchaeol.This BIT index was applied to surface diments from the Angola Basin,where it was shown to trace the outflow of the Congo River.Furthermore,analys of particulate organic matter from the North Sea showed relatively higher BIT indices in water column particulate organic matter near large river inputs.A survey of globally distributed marine and lacustrine surface diments shows that the BIT index in the environments correlates with the relative fluvial input of terrestrial organic material making this index generally applicable.The new proxy allows the rapid asssment of the fluvial input of terrestrial organic material in immature diments up to 100Ma old.D 2004Elvier B.V .All rights rerved.
Keywords:terrestrial organic matter;glycerol dialkyl glycerol tetraethers;proxy;marine diments;lacustrine diments;peats
1.Introduction
Large amounts of terrestrial organic carbon are annually transported from the continents to the oceans mainly by fluvial transport or,in lower amounts,by aeolian dust.They reprent a large source for organic
carbon in the marine environment (ca.4Â1014g C/year;[1])and are thus an important part of the global carbon cycle.Information on the modes and distances of transport of terrestrial carbon allows the reconstruc-tion of,for example,the proximity to continents and wind strengths.Hence,detailed knowledge on (past)variations in transport of terrestrial carbon to marine environments is of importance for reconstructing (past)carbon cycles.
Typically,the relative amounts of terrestrial or-ganic matter in marine diments are determined by analyzing the 13C contents and C/N ratio of bulk
0012-821X/$-e front matter D 2004Elvier B.V .All rights rerved.doi:10.1016/j.epsl.2004.05.012
*Corresponding author.Tel.:+31-222-369432;fax:+31-222-319674.
E-mail address:hopmans@nioz.nl (E.C.Hopmans).1
Prent address:Marine Geosciences,University of Bremen,PO Box 330440,Bremen 28334,German
y.
/locate/epsl
Earth and Planetary Science Letters 224(2004)107–
116
organic matter([2]and references cited therein). Prent-day terrestrial organic matter tends to have a more13C-depleted content and a higher C/N ratio than marine phytoplanktonic organic matter.How-ever,variations in13C contents of phytoplanktonic organic matter can be ,[3])and terrestrial organic matter containing an important fraction of C4-plants can have substantially enriched13C values (e.g.,[3]).Furthermore,early diagenesis can signifi-cantly alter C/N ratios by,for example,lective loss of amino acids.Hence,this approach can easily lead to erroneous interpretations of(changes in)the rela-tive amounts of terrestrial organic matter in marine diments.
An alternative approach is the analysis of specific molecular tracers for terrestrial organic matter and u them as a proxy for the terrestrial contribution to total dimentary organic matter[2].Long-chain
odd-car-bon-numbered n-alkanes are the most widely found terrestrial compounds in marine diments and are derived from the surface waxes of terrestrial higher plant leaves[4].The compounds can be washed from the leaves by rain or eroded with soils and transported by rivers to the coastal marine environ-ment.In addition,dust can ablate the waxes from leaves,thereby transporting them through the atmo-sphere to locations far from the ,[5,6]). Long-chain n-alkanes are therefore found in both coastal and open ocean sites[2].The stable carbon isotopic composition of the n-alkanes allows to deconvolute sources of the n-alkanes and the recon-struction of vegetation belts on continents through ,[7,8]).Long-chain even-carbon-numbered fatty acids and n-alcohols are also derived from higher plants and are mainly fluvially ,[9]). Specific triterpenoids such as oleanene and taraxerol allow tracing specific plant inputs from gymnosperms and mangroves,,[10]).Terrestrial organic components not directly amenable to gas chromatographic analysis are lignin and cutin,bio-polymers which occur abundantly in vascular plant tissues[11]and are mainly fluvially , [12,13]).Through cuprous oxide degradation,reaction products can be obtained from the biopolymers, which are typical for different plants and tissues(e references in[2]).By determining their stable carbon isotopic composition,inferences can be made on the relative inputs of C3and C4plants in marine ,[12,13]).Finally,reconstruction of terres-trial input is also often ba
d on the relative abundance of C29sterols compared to C27and C28 sterols[14]since C29sterols are dominant in terres-trial plants.However,C29sterols have also shown to be ubiquitously occurring in marine ,[15]) and thus may not reprent a pure terrestrial signal in marine diments.
A large range of molecular proxies for terrestrial organic matter is thus available.However,quantifica-tion of the relative inputs of terrestrial carbon is difficult due to large variations in concentrations of compounds in the different plant materials.Also, terrestrially derived biomarkers have different degra-dation rates both compared to each other and to marine-derived compounds.For instance,we recently showed that prervation factors of long-chain n-alkanes,typical markers for terrestrial input,are substantially higher than tho for long-chain alke-nones,typical markers for prymnesiophyte algae [16,17].Hence,relative changes in the amounts of terrestrial n-alkanes compared to marine compounds may not only be due to changes in terrestrial contri-bution,but also to changing oxygen exposure times.
An alternative approach to reconstruct terrestrial input into the marine environment is not to u a tracer derived from higher plants but from organisms thriv-ing in soils and peats.For instance,Prahl et al.[18] ud diploptene in Washington coastal diments as a tracer for soil organic carbon.Although this worked quite well in the diments,general application of this tracer is made
difficult by the fact that a number of marine bacteria also make diploptene or precursor compounds from which diploptene can be formed (e.g.,[19,20]).
The development of a high-performance liquid chromatography/mass spectrometry(HPLC/MS)tech-nique for the analysis of glycerol dialkyl glycerol tetraethers(GDGTs)[21]enabled us to recognize a group of non-isoprenoidal GDGTs(structures I,II, and III in Fig.1)that was recently identified with2D NMR techniques after isolation from a Dutch peat [22].Its biological origin is as yet unclear,but a survey of recent diments indicates that it is derived from organisms living in the terrestrial environment [23].In addition,we identified a structurally related isoprenoid GDGT of marine planktonic archaea,‘‘crenarchaeol’’(structure IV,Fig.1;[24]).This
E.C.Hopmans et al./Earth and Planetary Science Letters224(2004)107–116 108
compound occurs abundantly and ubiquitously in marine and lake diments (e.g.,[23,25,26]),the marine water column (e.g.,[27,28])and the only available uni-archaeal ‘‘culture’’of the marine pelagic crenarchaeota,Cenarchaeum symbiosum [24].In ma-rine diments,this biomarker is,together with GDGT V (a less specific GDGT but also predominantly derived from planktonic archaea),probably the single most abundant component [23].
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Here,we show that the amount of branched GDGTs compared to crenarchaeol in marine and lacustrine diments,quantified in the so-called Branched and Isoprenoid Tetraether (BIT)index,is correlated with the relative amount of fluvial terres-trial input.This gives a new approach to reconstruct the fate of soil organic carbon,and thereby fluvial terrestrial input,in marine and lacustrine environ-ments bad on the analysis of GDGTs.
2.Methodology 2.1.Samples
Surface diment samples reprenting the Congo River plume were collected during the RV Tyro crui in the eastern South Atlantic during fall of 1989[29].The box-cores were stored frozen at À20j C and the uppermost 1–1.5cm were ud for analys.The two samples from the estuary of the Congo River,‘Anker 24’and ‘Anker 26’,were taken as grab samples [30]and stored as dried diment in dark polyethylene五加茸血口服液
containers at room temperature before analys.Par-ticulate organic matter from the southern North Sea was sampled during the CRENS crui with the RV Pelagia in February 2002by filtration of 40l surface waters using 0.7-A m GFF filters.Particulate organic matter of the Wadden Sea water was sampled near the Mok Bay by filtration of 20l surface waters using 0.7-A m GFF filters.Atmospheric d
ust was sampled off the coast of Southwestern Africa according to the proce-dure described by Schefußet al.[8].Soil was collect-ed from ca.10cm depth in a deciduous forest near De Koog on the island of Texel,The Netherlands.2.2.GDGT analysis
Intact GDGTs were identified by high-performance liquid chromatography/atmospheric pressure positive ion chemical ionization mass spectrometry (HPLC/APCI-MS)[21].Briefly,most diments were ultra-sonically extracted three times with methanol,three times with dichloromethane (DCM)/methanol (1:1,v/v),and three times with DCM and all extracts were combined to obtain a total extract.In most cas,this total extract was subquently parated using a col-umn packed with Al 2O 3to obtain an apolar and a polar fraction using hexane/dichloromethane (9:1,v/v)and dichloromethane/methanol (1:1,v/v)as eluents,re-spectively.An aliquot of either the dried total extract or the polar fraction was dissolved by sonication (10min)in hexane/propanol (99:1,v/v).The resulting suspension was centrifuged (1min,3500rpm)and the supernatant filtered through a 0.45-A m,七色花原文
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Fig.1.Structures of some GDGTs prent in marine diments,lakes,and terrestrial soils and peats.
E.C.Hopmans et al./Earth and Planetary Science Letters 224(2004)107–116109
diameter PTFE filter prior to injection.Conditions for HPLC/MS analys of the purified extracts were mod-ified from Hopmans et al.[21].Analys were per-formed using an HP(Palo Alto,CA,USA)1100ries LC-MS equipped with an auto-injector and Chemsta-tion chromatography manager software.Separation was achieved on an Prevail Cyano column(2.1Â150 mm,3A m;Alltech,Deerfield,IL,USA),maintained at 30j C.Injection volumes varied from1to5A l. Tetraethers were eluted isocratically with99%A and 1%B for5min,followed by a linear gradient to1.8%B in45min,
where A is hexane and B is propanol.Flow rate was0.2ml/min.After each analysis,the column was cleaned by back-flushing hexane/propanol(90:10, v/v)at0.2ml/min for10min.Detection was achieved using atmospheric pressure positive ion chemical ion-ization mass spectrometry(APCI-MS)of the eluent. Conditions for APCI-MS were as follows:nebulizer pressure60psi,vaporizer temperature400j C,drying gas(N2)flow6l/min and temperature200j C,capil-lary voltageÀ3kV,corona5A A(f3.2kV).Positive ion spectra were generated by scanning m/z950–1450 in1.9s.Relative GDGT distributions were determined by integrating the summed peak areas in the respective [M+H]+and[M+H+1]+(protonated molecule and first isotope peak,respectively)traces of the GDGTs.
3.Results and discussion
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3.1.Occurrence of branched GDGTs
All peat samples investigated contained,in addi-tion to the ubiquitous and non-specific GDGT V,high amounts of branched GDGTs I–III.Interestingly,a soil sample obtained from a small deciduous forest located on the southwestern part of Texel(the Nether-lands)contained branched GDGTs I–II only(Fig.
2A).The abnce of GDGT V in this soil suggests that the organisms that biosynthesize the branched GDGTs do not biosynthesize isoprenoid GDGTs such as GDGT V.Hence,in peats where G
DGT V is found together with branched GDGTs I–III,the isoprenoid GDGT is likely derived from a different source,most likely methanogenic archaea.This confirms the find-ings of Pancost et al.[31],who found different stable carbon isotopic compositions for the branched and isoprenoid GDGTs.The prence of the branched GDGTs in the Texel soil suggests that this class of compounds can be ud as a tracer for soil and peat organic matter.
The branched GDGTs are also often found in coastal marine,open marine,and lacustrine diments. This could indicate that the compounds are not only produced by organisms living in soils and peats,but also in marine and lake waters.However,analys of water column samples in the Arabian Sea and the Black Sea distantly located from coasts and river
input Fig.2.HPLC/MS ba peak chromatograms of tetraether lipids in (A)soil from a deciduous tree forest,Texel,(B)surface diment from the Mok Bay,Texel,the Netherlands,(C)water column sample of the Wadden Sea and(D)core top sample from the Congo River basin.Roman numbers refer to structures in Fig.1.
E.C.Hopmans et al./Earth and Planetary Science Letters224(2004)107–116 110
[27,28]did not reveal the prence of branched GDGTs.In addition,analysis of the GDGTs in surface diment of the Mok Bay,a small inlet connected to the Wadden Sea,revealed high amounts of branched GDGTs I–III together with crenarchaeol(Fig.2B).In contrast,analysis of water filtrates from the Wadden Sea revealed GDGT V and crenarchaeol as the dom-inant GDGTs(Fig.2C).The results strongly suggest that branched GDGTs are derived from organisms living in soils and peats and that they can be ud as tracers for soil organic matter[22,23].
宝繁体3.2.Branched isoprenoid tetraether index
Bad on the obrvations,it is clear that the branched GDGTs prent in marine and lacustrine diments are derived from transport of terrestrial material.Crenarchaeol is the dominant compound in water column samples,in contrast to peats where crenarchaeol is only a minor ,repr
e-nting<1%of total GDGTs(Weijers et al.,unpub-lished results).Thus,crenarchaeol in diments is mainly derived from aquatic input.Hence,the two structurally similar types of GDGTs,branched GDGTs and crenarchaeol,reprent terrestrial and marine organic matter sources,respectively.Bad on this,we propo an index bad on the relative abundance of the two types of GDGTs which is able to trace the relative amount of terrestrial organic carbon in open ocean,coastal marine,and lake di-ments.This Branched and Isoprenoid Tetraether(BIT) index is bad on the relative abundance of branched GDGTs,reprenting terrestrial organic matter,and crenarchaeol,reprenting aquatic organic matter,and is defined as follows:
BIT¼½IþIIþIII
ð1Þ
2021七夕The roman numerals refer to the GDGTs indicated in Fig.1.GDGT V was excluded from this novel index as it can originate from both marine and terrestrial sources[23].In addition,GDGT V and some of its cyclized derivatives are produced by methanogenic ,[32])and anaerobic methane-oxidizing ,[33]).This BIT index,defined as above, can reach values ranging from0,reprenting no branched GDGTs,to1,reprenting no crenarchaeol.
To test the novel index,it was applied to core top diments from the Angola Basin(eastern tropical Atlantic)near the mouth of the Congo River(Fig. 2D).Schefußet al.[34]previously showed,bad on organic carbon contents and bulk stable carbon isoto-pic composition(Fig.3a),the extent of the fluvially transported terrestrial organic matter deposited by the Congo River in the Angola Basin.A contour plot of the BIT index clearly follows the plume of the Congo River outflow into the Angola Basin(Fig.3b),with BIT values ranging from0in open marine surface diments to0.91for the diments taken directly at the mouth of the Congo River.This plot is similar to the contour plot of the bulk stable carbon isotopic compositions of organic matter with13C depleted values near the Congo River mouth(Fig.3b;[34]). In contrast,the contour plot is quite different from that of the abundance of aeolian transported n-alkanes, which shows a plume-like distribution below the main trade-wind trajectory originating from the coast of Sothwestern Africa(Fig.3c;[34]).This suggests that the BIT index does not trace aeolian transported terrestrial organic matter,but that the index primarily reflects the fluvial transport of terrestrial organic matter.Further confirmation was obtained by the analysis of atmospheric dust obtained by air filtration near the west coast of Central Africa.HPLC/MS analysis revealed that GDGTs were below the detec-tion level,suggesting that they are not transported through the atmosphere.
The BIT index was further tested via the analysis of particulate organic matter obtained from surface waters in the southern part of the North Sea.BIT values varied from0to0.17(Table1).Interestingly, the highest BIT indices were found in surface waters at and south of the Frisian front,an area where different water mass of the North Sea converge. The waters had significantly lower salinities testi-fying to a significant input of river water plume mainly from the rivers Thames and Humber[35]. Thus,in the southern North Sea,the BIT index also traces fluvial input of terrestrial organic matter.
3.3.Global survey of BIT index
To test the general applicability of the BIT index, we calculated BIT values for a range of Holocene marine and lake diments,peats,and a soil.The
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