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Distinction of marine and terrestrial origin of humic acids in North Sea surface sediments by absorption spectroscopy
Marine Geology 164 (2000) 173–181 www.elsevier.nl/locate/margo
Distinction of marine and terrestrial origin of humic acids in North Sea surface sediments by absorption spectroscopy U. Fooken a,b,*, G. Liebezeit a,b b
a Forschungszentrum Terramare, Schleusenstraße 1, D-26382 Wilhelmshaven, Germany FB Chemie, Carl von Ossietzky-Universita¨t, Postfach 2503, D-26111 Oldenburg, Germany
Received 16 December 1998; accepted 14 October 1999
Abstract A suite of humic acid isolates from terrestrial and marine soils and sediments have been analysed by UV/visible spectroscopy. Marine samples are characterised by a maximum at 407 nm which is lacking in terrestrial ones. The absorbance ratio of 270/407 nm (A2/A4) can be used to differentiate humic acid sources as is demonstrated for North Sea surface sediments. Here, the southern coastal samples had high A2/A4 ratios indicating higher terrestrial contributions to the humic acid pool. In contrast, samples from the northern North Sea had low A2/A4 ratios suggesting higher marine contributions. Analysis of core samples and an Eocene oil shale indicates that the A2/A4 ratio is unaffected by early diagenetic processes. q 2000 Elsevier Science B.V. All rights reserved. Keywords: North Sea; Marine sediment; Humic acid; Absorption spectroscopy
1. Introduction Humic acids are a group of naturally occurring compounds that due to their highly complex and heterogeneous molecular nature defy detailed structural characterisation. Instead, at least for soils and sediments, they are operationally defined via the technique used to isolate these compounds. Differences in bulk molecular characteristics have been described (e.g. Nissenbaum and Kaplan, 1972; Stuermer et al., 1978) which are thought to be related to the different building blocks making up humic substances. Lignin phenols have been identified as major part of terrestrial humic acids (e.g. Ertel et al., 1984; Wershaw et al., 1990; Rasyid et al., 1992). Moran * Corresponding author. E-mail addresses: [email protected] (U. Fooken), [email protected] (G. Liebezeit).
and Hodson (1994) reported that a major part of dissolved humic compounds on the US continental shelf originates from lignin derived compounds. Marine derived humic acids, on the other hand, are less aromatic and have higher carbohydrate and protein contents (e.g. Ertel and Hedges, 1983). As humic material may account for up to 80% of the organic carbon pool in marine sediments (Rashid, 1985, p. 64) there is considerable interest in identifying autochthonous or allochthonous sources of this class of organic compounds. For this purpose, stable carbon and nitrogen isotope ratios have been used (e.g. Stuermer et al., 1978; Malcolm, 1990). Aromaticity obtained from nuclear magnetic resonance measurements ( 1H and 13C) (e.g. Hatcher et al., 1980) has also been employed to differentiate between marine and terrestrial humic acids. Although absorption spectra of humic acids are usually featureless the use of absorbance ratios at
0025-3227/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0025-322 7(99)00133-4
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U. Fooken, G. Liebezeit / Marine Geology 164 (2000) 173–181
Fig. 1. Absorption spectra of birch lignin and humic acids from different sources.
465 to 665 nm has been proposed by to characterise the degree of humification (Schnitzer, 1971). In the present communication we analyse for a suite of North Sea humic acids whether another absorbance ratio, that of 270/407 nm, can be employed to distinguish between humic acids derived from terrestrial and marine sources.
2. Material and methods North Sea surface sediment samples were obtained with a Shipek grab in August 1992 with R/V Planet. Further sediments were sampled in May and June 1996 with R/Vs Senckenberg and Viktor Hensen in the southern North Sea and the German Bight. In October 1997 additional samples were taken in the northern North Sea with R/V Dana. A total of 136 samples from the North Sea was analysed. Samples from the Landsort Deep, Baltic Sea (core
NC 51, 58840.3 0 N, 018819.3 0 E, z 442 m; were obtained in September 1994 with R/V Ha˚kon Mosby. In addition a pooled sample from a sediment trap experiment in the Sea of Marmara (,63 mm fraction) was analysed. Samples from a Flores Sea core were used as openoceanic reference. This core was obtained during cruise 45 of R/V Sonne at 008800.3 0 S, 118810.7 0 E and z 2200 m: A mixed sample from 20 to 286 cm sediment depth was used. Sedimentation rates in the area are about 25 cm/ka (Buch et al., 1992). Thus, the minimum age of the deepest sample analysed is about 11000 years before present (BP). Terrestrial reference samples include Lower Saxonian forest (Neuenburger Urwald—mixed beech/oak) and agricultural (maize) soils and different Holocene peats obtained from coastal marsh cores in Lower Saxony ranging in age from recent to about 3600 years BP. (Dellwig et al., 1998). Weser sediment was sampled in the freshwater region of the river at
U. Fooken, G. Liebezeit / Marine Geology 164 (2000) 173–181
175
Fig. 2. (A) Absorption spectra of North Sea lioid and humic acid extracts. (B) Absorption spectra of soil lipid and humic acid extracts.
Dreye; 12 km SE of Bremen. In addition the immature Eocene Messel oil shale was used for comparison. This shale was deposited under lacustrine conditions some 50 million years BP and did not experience any thermal stress (e.g. Habermehl and Hundrieser, 1983; Goth et al., 1988). Dead glasswort (Salicornia sp.) was sampled in the saltmarshes of Jade Bay, Lower Saxonian Wadden Sea. Lignin (alkali) was purchased from Aldrich, Deisenhofen. Additional samples from the East Frisian Wadden Sea have been described by de Wall (1995). Detailed information on the Skagerrak humic acid has been provided by Fengler et al. (1989, 1994). Samples were stored deep frozen prior to work-up. The ,63 mm fraction was obtained by wet sieving and centrifugation at 6000 rpm from about 1–2 kg of wet sediment. After drying at 708C the fine fraction
was treated with dilute hydrochloric acid to remove carbonates and methanol/dichloromethane 9:1 (v/v) to remove extractable lipids. After re-drying at 708C the remaining material was repeatedly extracted with 0.1 M NaOH until the extracts became colourless. No pyrophosphate was added as determination of phosphorus in the isolated humic acids was intended. The combined extracts were acidified to pH 1 with concentrated hydrochloric acid and left to stand overnight at 48C. The precipitate was centrifuged and re-dissolved in 0.1 M NaOH. The precipitation/dissolution procedure was repeated at least three times. Finally the raw humic acids obtained this way were dialysed against tap water for one week, followed by doubly distilled water for another week. The water was renewed daily. The purified humic acids were precipitated with hydrochloric acid, dried at 708C and ground in an agate mortar and pestle.
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Table 1 Selected element concentrations in humic acid isolates (ICP-OES data) (NU—Neuenburger Urwald, forest soil; SC 1—NW North Sea sediment) Sample
Cr
Cu
Mn
Ni
V
Zn
A2/A4
NU SC 1
89 52
128 499
297 55
21 46
123 51
173 65
4.26 2.38
A Kontron Uvikon 930 was used for measuring absorption spectra in the UV and visible range (230–700 nm). Humic acids were dissolved in 5% sodium carbonate solution (w/v).
3. Results and discussion Typical examples of HA absorption spectra are shown in Fig. 1. As expected and in correspondence with published data (e.g. Ertel and Hedges, 1983) these spectra are rather featureless and increase in absorbance more or less monotonously from 600 to 200 nm. A noticeable feature of marine samples is, however, the occurrence of a maximum at around 407 nm. This maximum is particularly evident in the Flores Sea sample. Povoledo et al. (1972) as well as Ertel and Hedges (1983) reported similar absorption characteristics of HAs from lacustrine and marine sources, respectively. The former authors related the absorption maximum at 407 nm to phaeopigments bound to humic acids. In our samples phaeopigments were present in the lipid fraction of both terrestrial and marine samples as indicated by the presence of the absorption bands at 660 and 407 nm (Fig. 2A). However, the corresponding HA spectra (Fig. 2B) show that these pigments occur more prominently in marine isolates suggesting a tighter bondage of these chlorophyll degradation products to marine HAs. Ishiwatari (1973) was able to remove the 407 nm shoulder from humic acid isolates from lake sediments by extensive methanol treatment while Ertel and Hedges (1983) did not succeed in removing this band from their marine isolates by benzene/methanol extraction. In our case, after repeated extraction with methanol the 407 nm maximum remained virtually unchanged. Minor changes were observed in the E4/E6 ratio of a North Sea isolate decreasing from 5.62 to 5.29 while
the A2/A4 ratio changed from 1.59 to 1.56. This suggests a sorptive binding mechanism for chlorophyll degradation products in terrestrial HAs compared to covalent binding in marine ones. As humic acids form complexes with metals (e.g. Ashley, 1996; Wasserman et al., 1998) which can influence absorption characteristics of humic acids we tested our isolates for possible metal interferences. Treatment of 10 mg peat and North Sea HAs in 20 ml 5% sodium carbonate solution with copper sulphate in concentrations ranging from 10 24 to 10 27 mol/l did not result in any changes of the absorption spectra. Furthermore, despite large differences in element contents between a forest soil and a North Sea sediment HA (Table 1) the corresponding A2/A4 ratios still reflect the terrestrial and marine origin, respectively. Thus, interference by complexation with metals does not play a role in determining A2/A4 ratios. To test whether temporal changes might affect the A2/A4 ratio a core from the Norwegian Trough (60800.25 0 N, 004815.260 0 E, z 209 m was analysed down to 3.5 m sediment depth. No sedimentation rate information is available for this core. As, however, rates decrease from the central to the western Skagerrak and into the Norwegian Trough from .4 to ,1 mm/a (Liebezeit, 1991; van Weering et al., 1993) it can be assumed that the sedimentation rate at the core location is also ,1 mm/a. This would correspond to a minimum age of about 3500 years BP. Neither the E4/E6 5:36 ^ 0:11; n 8 nor the A2/A4 ratio 2:55 ^ 0:04; n 8 change appreciably over this time span suggesting that early diagenetic effects do not play a role in controlling these ratios. Despite its high age the Messel oil shale sample had an A2/A4 ratio which was not significantly different from modern freshwater sediments (Weser). This indicates that this ratio can be used even for fossil samples provided their thermal history is known. Lignin-derived contributions can be suspected from the shoulder at about 280 nm in terrestrial samples (Fig. 2). This together with the fact that marine humic compounds contain tightly bound phaeopigments might be employed to differentiate between HAs of marine and terrestrial origin. In analogy to the E4/E6 ratio absorbance ratios at 270/407 nm were calculated. These show distinct differences between marine and terrestrial samples (Fig. 3). While the
U. Fooken, G. Liebezeit / Marine Geology 164 (2000) 173–181
177
Fig. 3. E4/E6 (upper) and A2/A4 (lower) ratios of humic acids from various sources. North Sea, German Bight and Wadden Sea values are mean values for all samples from these regions.
E4/E6 ratios, with the exception of the Salicornia, peat and lignin samples, do not show regular differences between marine and terrestrial humic acids there is a clear tendency in the A2/A4 ratios. Here, terrestrial samples have consistently higher values (.3.4) than marine ones. Within the latter group the Flores Sea sample has the lowest ratio of 1.9 while samples from coastal areas such as the Wadden Sea have intermediate values between 2.4 and 3.1. This distinction becomes even clearer in a plot of A2/A4 vs. E4/E6 ratios (Fig. 4). Again terrestrial samples are distinguished from the Flores Sea sample while samples from coastal areas fall into an intermediate region. It is evident that the separation of marine and terrestrial samples is largely due to the
A2/A4 ratio while the E4/E6 ratio does not provide source information. When this approach is applied to the North Sea HA isolates most samples fall into the intermediate region indicating that both marine and terrestrial sources are present in these sediments (Fig. 5). Only one sample falls into the terrestrial range while the E4/E6 ratios of samples in the marine range show some scatter with values ranging from 3.8 to 6 with somewhat lower A2/ A4 ratios than the Flores Sea marine reference. This suggests that marine sources need to be better constrained with respect to this ratio. The A2/A4 ratio can now be used to establish the relative contributions of humic acids with a more terrestrial character to North Sea surface sediments (Fig. 6). Although the samples that provided enough
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Fig. 4. A2/A4 vs. E4/E6 for humic acids from different sources. North Sea, German Bight and Wadden Sea values are mean values for all samples from these regions.
Fig. 5. A2/A4 vs. E4/E6 plot for all North Sea (diamonds) and Wadden Sea (crosses) samples.
U. Fooken, G. Liebezeit / Marine Geology 164 (2000) 173–181
Fig. 6. Areal distribution of A2/A4 ratios in humic acids from North Sea surface sediments.
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of the ,63 mm fraction to obtain HA isolates do not provide a complete areal coverage of the investigated area it is evident that there is a trend towards a more marine signal from the southern North Sea, especially the Elbe estuary, towards the central and northern North Sea. The results described above compare well with other data such as stable carbon and nitrogen isotope ratios (Liebezeit et al., 1999) and IR spectroscopic measurements (Fooken and Liebezeit, 1999) of these isolates.
4. Conclusions As the analytical technique used here for a differentiation between humic acid sources is easy to handle and does not require extensive instrumentation or special training it allows a rapid assessment of large sample sets. Although so far clearly defined marine sedimentary end members have not been described in the literature our data indicate that the maximum found at 407 nm can be used to differentiate between terrestrial and marine contributions to the humic acid pool of marine sediments. Undoubtedly more work is needed to fully constrain the A2/A4 ratios of humic acids from both marine and terrestrial sources. Nevertheless, the data presented above indicate that the approach used here can provide a first rapid information on the source of a major fraction of organic matter in marine sediments.
Acknowledgements We are indebted to Hartmut Janßen and Martin Steffens for sediment sampling in 1992. The help of the crews of R/Vs Planet and Victor Hensen is gratefully acknowledged. Matthias Piontek, Esther Kraft and Thorsten Piper helped with sample preparation and extraction. ICP-OES data were kindly supplied by Bernhard Schnetger, University of Oldenburg. Critical comments by H. Chamley, M.A. Moran and an anonymous reviewer on an earlier version of the manuscript were useful in improving the text.
References Ashley, J.T.F., 1996. Adsorption of Cu(II) and Zn(II) by estuarine, riverine and terrestrial humic acids. Chemosphere 33, 2175– 2187. Buch, B., Degens, E.T., Kempe, S., 1992. Sediment characteristics and hydrochemistry of Saleh Bay (Indonesia)—first results. Mittlg. Geol.—Pala¨ontol. Inst. Univ. Hamburg 70, 59–86. Dellwig, O., Gramberg, D., Vetter, D., Watermann, F., Barckhausen, J., Brumsack, H.-J., Gerdes, G., Liebezeit, G., Rullko¨tter, J., Scholz-Bo¨ttcher, B.M., Streif, H., 1998. Geochemical and microfacies characterization of a Holocene depositional sequence in northwest Germany. Org. Geochem. 29, 1687– 1699. Ertel, J.R., Hedges, J.I., 1983. Bulk chemical and spectroscopic properties of marine and terrestrial humic acids, melanoidins and catechol-based synthetic polymers. In: Christman, R.F., Gjessing, E.T. (Eds.). Aquatic and Terrestrial Humic Materials, Ann Arbor Science, Ann Arbor, MA, pp. 143–162. Ertel, J.R., Hedges, J.I., Perdue, E.M., 1984. Lignin signature of aquatic humic substances. Science 223, 485–487. Fengler, G., Haupt, E.T.K., Liebezeit, G., 1989. Humic substances in Holocene sediments of the Skagerrak (NE North Sea) and the Elbe river. Science Tot. Environ. 81 (82), 335–342. Fengler, G., Grossmann, D., Kersten, M., Liebezeit, G., 1994. Trace metals in humic acids from recent Skagerrak sediments. Mar. Poll. Bull. 28, 143–147. Fooken, U., Liebezeit, G., 1999. An IR study of humic acids in soils and sediments. J. Conf. Abs. 4, 620. Goth, K., de Leeuw, J.W., Pu¨ttmann, W., Tegelaar, E.W., 1988. Origin of Messel oil shale kerogen. Nature 336, 759–761. Habermehl, G., Hundrieser, H.J., 1983. Fossile Relikte der ¨ lschiefer. Naturwissenschaften “Wasserblu¨te” im Messeler O 70, 566. Hatcher, P.G., Rowan, R., Matingly, M.A., 1980. 1H and 13C NMR of marine humic acids. Org. Geochem. 2, 77–85. Ishiwatari, R., 1973. Chemical characterization of fractionated humic acids from lake and marine sediments. Chem. Geol. 12, 113–126. Liebezeit, G., 1991. Kohlenhydrate in marinen Sinkstoffen und Sedimenten—Umsetzungen und Biomarkerkriterien. Habilitationsschrift, Fachbereich Geowiss., Univ. Hamburg, 158 pp. Liebezeit, G., Hannemann, F., Fooken, U., Bo¨ttcher, M.E., Voss, M., 1999. Stable isotope (C, N, S) signatures of humic acids to differentiate between marine and terrestrial organic matter sources in marine sediments. J. Conf. Abs. 4, 620. Malcolm, R.L., 1990. The uniqueness of humic substances in each of soil, stream and marine environments. Anal. Chim. Acta 232, 19–30. Moran, M.A., Hodson, R.E., 1994. Dissolved humic substances of vascular plant origin in a coastal marine environment. Limnol. Oceanogr. 39, 762–771. Nissenbaum, A., Kaplan, I.R., 1972. Chemical and isotopic evidence for the in situ origin of marine humic substances. Limnol. Oceanogr. 17, 570–582. Povoledo, D., Murray, D., Pitze, M., 1972. Pigments and lipids in the humic acids of some Canadian Lake sediments. In:
U. Fooken, G. Liebezeit / Marine Geology 164 (2000) 173–181 Povoledo, D., Goltermann, H.L. (Eds.). Humic Substances— Their Structure and Function in the Biosphere, Pudoc, Wageningen, p. 233. Rashid, M.A., 1985. Geochemistry of Marine Humic Compounds, Springer, Berlin 300 pp. Rasyid, U., Johnson, W.D., Wilson, M.A., Hanna, J.V., 1992. Changes in organic structural group composition of humic and fulvic acids in sediments from similar geographical but different depositional environments. Org. Geochem. 18, 521–529. Schnitzer, M., 1971. Characterization of humic constituents by spectroscopy. In: McLaren, A.D., Skujins, J. (Eds.), Soil Biochemistry. Marcel Dekker, New York, Vol. 2, pp. 60– 95. Stuermer, D.H., Peters, K.E., Kaplan, I.R., 1978. Source indicators of humic substances and proto-kerogen: stable isotope ratios,
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elemental composition and electron spin resonance. Geochim. Cosmochim. Acta 42, 989–997. de Wall, I., 1995. Schwermetalle in Huminsa¨uren von Oberfla¨chensedimenten des Niedersa¨chsischen Wattenmeeres, Diploma Thesis, Fachhochschule Emden, pp. 75. Wasserman, J.C., Oliveira, F.B.L., Bidarra, M., 1998. Cu and Fe associated with humic acids in sediments of a tropical coastal lagoon. Org. Geochem. 28, 813–822. van Weering, T.C.E., Berger, G.W., Okkels, E., 1993. Sediment transport, resuspension and accumulation rates in the northeastern Skagerrak. Mar. Geol. 111, 269–285. Wershaw, R.L., Pinckney, D.J., Llaguno, E.C., Vincente-Beckett, V., 1990. NMR characterization of humic acid fractions from different Philippine soils and sediments. Anal. Chim. Acta 232, 31–42.
Distinction of marine and terrestrial origin of humic acids in North Sea surface sediments by absorption spectroscopy U. Fooken a,b,*, G. Liebezeit a,b b
a Forschungszentrum Terramare, Schleusenstraße 1, D-26382 Wilhelmshaven, Germany FB Chemie, Carl von Ossietzky-Universita¨t, Postfach 2503, D-26111 Oldenburg, Germany
Received 16 December 1998; accepted 14 October 1999
Abstract A suite of humic acid isolates from terrestrial and marine soils and sediments have been analysed by UV/visible spectroscopy. Marine samples are characterised by a maximum at 407 nm which is lacking in terrestrial ones. The absorbance ratio of 270/407 nm (A2/A4) can be used to differentiate humic acid sources as is demonstrated for North Sea surface sediments. Here, the southern coastal samples had high A2/A4 ratios indicating higher terrestrial contributions to the humic acid pool. In contrast, samples from the northern North Sea had low A2/A4 ratios suggesting higher marine contributions. Analysis of core samples and an Eocene oil shale indicates that the A2/A4 ratio is unaffected by early diagenetic processes. q 2000 Elsevier Science B.V. All rights reserved. Keywords: North Sea; Marine sediment; Humic acid; Absorption spectroscopy
1. Introduction Humic acids are a group of naturally occurring compounds that due to their highly complex and heterogeneous molecular nature defy detailed structural characterisation. Instead, at least for soils and sediments, they are operationally defined via the technique used to isolate these compounds. Differences in bulk molecular characteristics have been described (e.g. Nissenbaum and Kaplan, 1972; Stuermer et al., 1978) which are thought to be related to the different building blocks making up humic substances. Lignin phenols have been identified as major part of terrestrial humic acids (e.g. Ertel et al., 1984; Wershaw et al., 1990; Rasyid et al., 1992). Moran * Corresponding author. E-mail addresses: [email protected] (U. Fooken), [email protected] (G. Liebezeit).
and Hodson (1994) reported that a major part of dissolved humic compounds on the US continental shelf originates from lignin derived compounds. Marine derived humic acids, on the other hand, are less aromatic and have higher carbohydrate and protein contents (e.g. Ertel and Hedges, 1983). As humic material may account for up to 80% of the organic carbon pool in marine sediments (Rashid, 1985, p. 64) there is considerable interest in identifying autochthonous or allochthonous sources of this class of organic compounds. For this purpose, stable carbon and nitrogen isotope ratios have been used (e.g. Stuermer et al., 1978; Malcolm, 1990). Aromaticity obtained from nuclear magnetic resonance measurements ( 1H and 13C) (e.g. Hatcher et al., 1980) has also been employed to differentiate between marine and terrestrial humic acids. Although absorption spectra of humic acids are usually featureless the use of absorbance ratios at
0025-3227/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0025-322 7(99)00133-4
174
U. Fooken, G. Liebezeit / Marine Geology 164 (2000) 173–181
Fig. 1. Absorption spectra of birch lignin and humic acids from different sources.
465 to 665 nm has been proposed by to characterise the degree of humification (Schnitzer, 1971). In the present communication we analyse for a suite of North Sea humic acids whether another absorbance ratio, that of 270/407 nm, can be employed to distinguish between humic acids derived from terrestrial and marine sources.
2. Material and methods North Sea surface sediment samples were obtained with a Shipek grab in August 1992 with R/V Planet. Further sediments were sampled in May and June 1996 with R/Vs Senckenberg and Viktor Hensen in the southern North Sea and the German Bight. In October 1997 additional samples were taken in the northern North Sea with R/V Dana. A total of 136 samples from the North Sea was analysed. Samples from the Landsort Deep, Baltic Sea (core
NC 51, 58840.3 0 N, 018819.3 0 E, z 442 m; were obtained in September 1994 with R/V Ha˚kon Mosby. In addition a pooled sample from a sediment trap experiment in the Sea of Marmara (,63 mm fraction) was analysed. Samples from a Flores Sea core were used as openoceanic reference. This core was obtained during cruise 45 of R/V Sonne at 008800.3 0 S, 118810.7 0 E and z 2200 m: A mixed sample from 20 to 286 cm sediment depth was used. Sedimentation rates in the area are about 25 cm/ka (Buch et al., 1992). Thus, the minimum age of the deepest sample analysed is about 11000 years before present (BP). Terrestrial reference samples include Lower Saxonian forest (Neuenburger Urwald—mixed beech/oak) and agricultural (maize) soils and different Holocene peats obtained from coastal marsh cores in Lower Saxony ranging in age from recent to about 3600 years BP. (Dellwig et al., 1998). Weser sediment was sampled in the freshwater region of the river at
U. Fooken, G. Liebezeit / Marine Geology 164 (2000) 173–181
175
Fig. 2. (A) Absorption spectra of North Sea lioid and humic acid extracts. (B) Absorption spectra of soil lipid and humic acid extracts.
Dreye; 12 km SE of Bremen. In addition the immature Eocene Messel oil shale was used for comparison. This shale was deposited under lacustrine conditions some 50 million years BP and did not experience any thermal stress (e.g. Habermehl and Hundrieser, 1983; Goth et al., 1988). Dead glasswort (Salicornia sp.) was sampled in the saltmarshes of Jade Bay, Lower Saxonian Wadden Sea. Lignin (alkali) was purchased from Aldrich, Deisenhofen. Additional samples from the East Frisian Wadden Sea have been described by de Wall (1995). Detailed information on the Skagerrak humic acid has been provided by Fengler et al. (1989, 1994). Samples were stored deep frozen prior to work-up. The ,63 mm fraction was obtained by wet sieving and centrifugation at 6000 rpm from about 1–2 kg of wet sediment. After drying at 708C the fine fraction
was treated with dilute hydrochloric acid to remove carbonates and methanol/dichloromethane 9:1 (v/v) to remove extractable lipids. After re-drying at 708C the remaining material was repeatedly extracted with 0.1 M NaOH until the extracts became colourless. No pyrophosphate was added as determination of phosphorus in the isolated humic acids was intended. The combined extracts were acidified to pH 1 with concentrated hydrochloric acid and left to stand overnight at 48C. The precipitate was centrifuged and re-dissolved in 0.1 M NaOH. The precipitation/dissolution procedure was repeated at least three times. Finally the raw humic acids obtained this way were dialysed against tap water for one week, followed by doubly distilled water for another week. The water was renewed daily. The purified humic acids were precipitated with hydrochloric acid, dried at 708C and ground in an agate mortar and pestle.
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U. Fooken, G. Liebezeit / Marine Geology 164 (2000) 173–181
Table 1 Selected element concentrations in humic acid isolates (ICP-OES data) (NU—Neuenburger Urwald, forest soil; SC 1—NW North Sea sediment) Sample
Cr
Cu
Mn
Ni
V
Zn
A2/A4
NU SC 1
89 52
128 499
297 55
21 46
123 51
173 65
4.26 2.38
A Kontron Uvikon 930 was used for measuring absorption spectra in the UV and visible range (230–700 nm). Humic acids were dissolved in 5% sodium carbonate solution (w/v).
3. Results and discussion Typical examples of HA absorption spectra are shown in Fig. 1. As expected and in correspondence with published data (e.g. Ertel and Hedges, 1983) these spectra are rather featureless and increase in absorbance more or less monotonously from 600 to 200 nm. A noticeable feature of marine samples is, however, the occurrence of a maximum at around 407 nm. This maximum is particularly evident in the Flores Sea sample. Povoledo et al. (1972) as well as Ertel and Hedges (1983) reported similar absorption characteristics of HAs from lacustrine and marine sources, respectively. The former authors related the absorption maximum at 407 nm to phaeopigments bound to humic acids. In our samples phaeopigments were present in the lipid fraction of both terrestrial and marine samples as indicated by the presence of the absorption bands at 660 and 407 nm (Fig. 2A). However, the corresponding HA spectra (Fig. 2B) show that these pigments occur more prominently in marine isolates suggesting a tighter bondage of these chlorophyll degradation products to marine HAs. Ishiwatari (1973) was able to remove the 407 nm shoulder from humic acid isolates from lake sediments by extensive methanol treatment while Ertel and Hedges (1983) did not succeed in removing this band from their marine isolates by benzene/methanol extraction. In our case, after repeated extraction with methanol the 407 nm maximum remained virtually unchanged. Minor changes were observed in the E4/E6 ratio of a North Sea isolate decreasing from 5.62 to 5.29 while
the A2/A4 ratio changed from 1.59 to 1.56. This suggests a sorptive binding mechanism for chlorophyll degradation products in terrestrial HAs compared to covalent binding in marine ones. As humic acids form complexes with metals (e.g. Ashley, 1996; Wasserman et al., 1998) which can influence absorption characteristics of humic acids we tested our isolates for possible metal interferences. Treatment of 10 mg peat and North Sea HAs in 20 ml 5% sodium carbonate solution with copper sulphate in concentrations ranging from 10 24 to 10 27 mol/l did not result in any changes of the absorption spectra. Furthermore, despite large differences in element contents between a forest soil and a North Sea sediment HA (Table 1) the corresponding A2/A4 ratios still reflect the terrestrial and marine origin, respectively. Thus, interference by complexation with metals does not play a role in determining A2/A4 ratios. To test whether temporal changes might affect the A2/A4 ratio a core from the Norwegian Trough (60800.25 0 N, 004815.260 0 E, z 209 m was analysed down to 3.5 m sediment depth. No sedimentation rate information is available for this core. As, however, rates decrease from the central to the western Skagerrak and into the Norwegian Trough from .4 to ,1 mm/a (Liebezeit, 1991; van Weering et al., 1993) it can be assumed that the sedimentation rate at the core location is also ,1 mm/a. This would correspond to a minimum age of about 3500 years BP. Neither the E4/E6 5:36 ^ 0:11; n 8 nor the A2/A4 ratio 2:55 ^ 0:04; n 8 change appreciably over this time span suggesting that early diagenetic effects do not play a role in controlling these ratios. Despite its high age the Messel oil shale sample had an A2/A4 ratio which was not significantly different from modern freshwater sediments (Weser). This indicates that this ratio can be used even for fossil samples provided their thermal history is known. Lignin-derived contributions can be suspected from the shoulder at about 280 nm in terrestrial samples (Fig. 2). This together with the fact that marine humic compounds contain tightly bound phaeopigments might be employed to differentiate between HAs of marine and terrestrial origin. In analogy to the E4/E6 ratio absorbance ratios at 270/407 nm were calculated. These show distinct differences between marine and terrestrial samples (Fig. 3). While the
U. Fooken, G. Liebezeit / Marine Geology 164 (2000) 173–181
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Fig. 3. E4/E6 (upper) and A2/A4 (lower) ratios of humic acids from various sources. North Sea, German Bight and Wadden Sea values are mean values for all samples from these regions.
E4/E6 ratios, with the exception of the Salicornia, peat and lignin samples, do not show regular differences between marine and terrestrial humic acids there is a clear tendency in the A2/A4 ratios. Here, terrestrial samples have consistently higher values (.3.4) than marine ones. Within the latter group the Flores Sea sample has the lowest ratio of 1.9 while samples from coastal areas such as the Wadden Sea have intermediate values between 2.4 and 3.1. This distinction becomes even clearer in a plot of A2/A4 vs. E4/E6 ratios (Fig. 4). Again terrestrial samples are distinguished from the Flores Sea sample while samples from coastal areas fall into an intermediate region. It is evident that the separation of marine and terrestrial samples is largely due to the
A2/A4 ratio while the E4/E6 ratio does not provide source information. When this approach is applied to the North Sea HA isolates most samples fall into the intermediate region indicating that both marine and terrestrial sources are present in these sediments (Fig. 5). Only one sample falls into the terrestrial range while the E4/E6 ratios of samples in the marine range show some scatter with values ranging from 3.8 to 6 with somewhat lower A2/ A4 ratios than the Flores Sea marine reference. This suggests that marine sources need to be better constrained with respect to this ratio. The A2/A4 ratio can now be used to establish the relative contributions of humic acids with a more terrestrial character to North Sea surface sediments (Fig. 6). Although the samples that provided enough
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Fig. 4. A2/A4 vs. E4/E6 for humic acids from different sources. North Sea, German Bight and Wadden Sea values are mean values for all samples from these regions.
Fig. 5. A2/A4 vs. E4/E6 plot for all North Sea (diamonds) and Wadden Sea (crosses) samples.
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Fig. 6. Areal distribution of A2/A4 ratios in humic acids from North Sea surface sediments.
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of the ,63 mm fraction to obtain HA isolates do not provide a complete areal coverage of the investigated area it is evident that there is a trend towards a more marine signal from the southern North Sea, especially the Elbe estuary, towards the central and northern North Sea. The results described above compare well with other data such as stable carbon and nitrogen isotope ratios (Liebezeit et al., 1999) and IR spectroscopic measurements (Fooken and Liebezeit, 1999) of these isolates.
4. Conclusions As the analytical technique used here for a differentiation between humic acid sources is easy to handle and does not require extensive instrumentation or special training it allows a rapid assessment of large sample sets. Although so far clearly defined marine sedimentary end members have not been described in the literature our data indicate that the maximum found at 407 nm can be used to differentiate between terrestrial and marine contributions to the humic acid pool of marine sediments. Undoubtedly more work is needed to fully constrain the A2/A4 ratios of humic acids from both marine and terrestrial sources. Nevertheless, the data presented above indicate that the approach used here can provide a first rapid information on the source of a major fraction of organic matter in marine sediments.
Acknowledgements We are indebted to Hartmut Janßen and Martin Steffens for sediment sampling in 1992. The help of the crews of R/Vs Planet and Victor Hensen is gratefully acknowledged. Matthias Piontek, Esther Kraft and Thorsten Piper helped with sample preparation and extraction. ICP-OES data were kindly supplied by Bernhard Schnetger, University of Oldenburg. Critical comments by H. Chamley, M.A. Moran and an anonymous reviewer on an earlier version of the manuscript were useful in improving the text.
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