|Isotopic and molecular characterization of particulate organic matter in coastal waters: thesis submitted in partial fulfillment of the requirements for the degree of PhD in Sciences|
Megens, L. (2000). Isotopic and molecular characterization of particulate organic matter in coastal waters: thesis submitted in partial fulfillment of the requirements for the degree of PhD in Sciences. PhD Thesis. Rijksuniversiteit Groningen: Groningen. 160 pp.
This thesis reports on investigations of the stable and radioactive carbon isotope distributions in sedimentary and suspended particulate organic matter (POM) in coastal waters with the attempt to determine the origin of the POM. POM in coastal waters -estuaries or coastal seas -can have a (i) marine origin, i.e. the ultimate source of the POM is marine phytoplankton, or (ii) a terrestrial origin, as rivers and winds transport POM from the continents to the sea. The rivers alone transport annually more particulate organic carbon to the oceans than the total amount of organic carbon buried in all marine sediments. This means that terrestrial POM is largely mineralized once it enters the sea. In order to understand what is the fate of terrestrial POM in the sea it is necessary to be able to distinguish between terrestrial and marine POM. One method widely used is to analyze the abundance ratio between the two stable isotopes of carbon - 13C and 12C, conventionally expressed as the relative deviation from that of a standard (delta13C). In general there is a difference between this delta13C value in (marine) phytoplankton (delta13C around -21 promille) and terrestrial plants (delta13C around -27 promille), due to a phenomenon called isotopic fractionation: the carbon isotopic composition of a POM sample is a measure of fractional contribution of terrestrial and marine organic matter. However, there are some factors that complicate the use of bulk delta13C values: 1) delta13C values of phytoplankton (and plants) varies, depending on a number of (environmental) factors; 2) different organic compounds in the same organism have different delta13C values. Generally, carbohydrates and proteins have higher delta13C values than lipids and other cell components. These compounds can degrade at different rates, causing the delta13C of the total organic matter mixture to change in time depending on environmental conditions. This thesis is the result of an extensive study of the 14C abundance as an additional indicator of the origin of POM. The relative 14C abundance (14a), related to the age of organic matter, has been proposed as a possibly useful tracer, because in many rivers POM is mainly derived from erosion of old peat deposits and soils. Because conventionally 14C abundances are corrected for isotopic fractionation (normalized 14a values), the so corrected values are equal for all the components in short living organisms. Therefore, selective degradation, changing the delta13C, will not affect the 14a of organic matter of a single source. Apart from 14C as a second indicator of the POM source of the bulk sample, the isotopic analyses were applied to different constituents of total POM, assuming that these fractions are more homogeneous with respect to stability and that, consequently, the delta13C value would be much less affected by selective degradation. To obtain additional information about the nature of the POM the samples were also subjected to pyrolysis-gas chromatography-mass spectrometry. The first two chapters of this thesis are dealing with the variation in carbon isotopic distributions with particle size in recent sediments from the North-East Pacific off the coast of the state Washington (USA) and from the Ems-Dollard estuary in North-West Europe between the Netherlands and Germany. In the sediment samples from ca. 25 cm deep on the slope and in the so-called Cascadia Basin off the Washington coast, size fractionated by SPLITT-separation, small variations in carbon isotopic compositions are observed, up to 5% for 14a and 1.3 promille for delta13C. There are no clear trends common to the three samples studied. The bulk samples were also extracted with hot water. For the two deep samples