Ecology of the past

COMMENT ON THE USE OF NITROGEN ISOTOPES IN PALAEOLIMOLOGICAL STUDIES
As a component of my doctoral research, I am examining nitrogen (N) isotopes within sediments obtained from Lake Bosumtwi (West Africa). Below I review and comment on the key uses and limitations of using N isotopes to interpret past environmental change with particular reference to lake sediments. Discussion is based on the key text by Talbot (2001).

REFERENCE
Talbot, M.R. 2001. Nitrogen isotopes in palaeolimnology. Tracking environmental change using lake sediments. Volume 2. Physical and geochemical methods (ed. by W.M. Last and J.P. Smol), pp. 401-439. Kluwer Academic Press, Dordrecht.

NOTE: This text is avaliable to Open University students as an ebook via the library

NITROGEN AND CARBON IN SEDIMENTS
The elements nitrogen and carbon (C) each have two stable isotopes (same number of protons and electrons with a differing number of neutons). Using mass spectrometry, the relative abundance of the isotopes can be determined and the isotopic composition is expressed in delta notation relative to a standard. For example, nitrogen has two stable isotopes ¹⁴N and ¹⁵N and its isotopic composition is expressed relative to air:

Carbon has two stable isotopes ¹²C and ¹³C and its isotopic composition is expressed relative to Pee Dee Belemnite (a Cretaceous marine fossil resembling a squid):

The δ¹⁵N and δ¹³C ratios of bulk organic matter (OM) sourced from lake sediments can be measured through time (down core) and can be used to infer changes in the provenance of the organic matter found within the sediment (whether of aquatic or terrestrial source). Changes in δ¹³C and δ¹⁵N can also provide information on nutrient cycling, lake mixing regimes and water column stability (e.g. Meyers and Ishiwatari, 1993; Thornton and McManus, 1994, Talbot and Johannessen, 1992; Mackie et al., 2007; Brodie et al., 2011).

Elemental C:N ratios

• Lake sediments can contain OM derived from many sources: such as aquatic plants, algae, terrestrial plants and/or soil humus. The sources contain different amounts of N.  Examination of the C:N ratio within sediment,  in conjunction with δ¹³C and δ¹⁵N, can provide supporting information on the origin of the organic matter and nutrient sources. A low C:N ratio (3-9) is indicative of protein rich, lignin poor OM sourced from phytoplankton (lake sourced algae). Conversely, high a C:N ratio (>20) is dominated by protein poor, lignin rich terrestrial biomass. Intermediate C/N ratio of 10-20 indicates a mix of aquatics and terrestrial OM (Brodie et al., 2011).
• The interpretation of the sources of OM can be difficult; for example, soil OM may have a high N content, with C/N values close to that of planktonic OM due to the presence of nitrogen fixing microorganisms around plant roots leading to N enrichment (Talbot, 2001).

δ¹⁵N

• Differences in the relative proportion of the two isotopes partitioned between several reservoirs (N-isotope fractionation) arise as a result of various inorganic and biochemical processes (see Table 1 of Talbot, 2001). The typical δ¹⁵N values for the major N pools are shown Talbot (2001).
• Various problems arise when attempting to interpret δ¹⁵N isotope data. N-isotope ratios can be altered by diagenetic processes as proteins and lipids are able to decompose rapidly (Talbot, 2001). Additionally, in some cases inorganically bound N is present in clay rich lacustrine sediment which is difficult to distinguish between exchangeable and fixed N (Talbot, 2001).

Nitrogen isotopes can be used as a palaeoenvironmental proxy; however, the results can be equivocal. This ambiguity can be removed by using additional proxies such as C-isotopes, smear slides and C/N ratios and together, they can provide unique and important palaeoenvironmental information.

REFERENCES
Brodie, C.R., Casford, J.S.L., Lloyd, J.M., Leng, M.J., Heaton, T.H.E., Kendrick, K.P., Yongqiang, Z., 2011. Evidence for bias in C/N, δ¹³C and δ¹⁵N values of bulk organic matter, and on environmental interpretation, from a lake sedimentary sequence by pre-analysis acid treatment methods. Quaternary Science Reviews 30, 3076-3087.

Mackie, E.A.V., Lloyd, J.M., Leng, M.J., Bentley, M.J., Arrowsmith, C., 2007. Assessment of d δ¹³C and C/N ratios in bulk organic matter as palaeosalinity indicators in Holocene and Late glacial isolation basin sediments, northwest Scotland. J. Quat. Sci 22 (6), 579-591.

Meyers, P.A., Ishiwatari, R., 1993. Lacustrine organic geochemistry e an overview of indicators of organic matter and diagenesis in lake sediments. Org. Geochem. 20 (7), 867-900.

Talbot, M. R. & T. Johannessen, 1992. A high resolution palaeoclimatic record for the last 27,500 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustrine organic matter. Earth and Planet. Sci. Let. 110, 23-37.

Talbot, M.R. 2001. Nitrogen isotopes in palaeolimnology. Tracking environmental change using lake sediments. Volume 2. Physical and geochemical methods (ed. by W.M. Last and J.P. Smol), pp. 401-439. Kluwer Academic Press, Dordrecht.

Thornton, S.F., McManus, J., 1994. Applications of organic carbon and nitrogen stable isotope and C/N ratios as source indicators of organic matter provenance in estuarine systems: evidence from the Tay estuary. Scotland. Estuar. Coast. S. Sci. 38, 219-233.