Biomarkers as palaeoceanographic proxies.

In palaeoceanography the term biomarker is used to name organic molecules found in sediments, initially produced by a variety of organisms either on land or in the aquatic environment. A key characteristic of biomarkers is that after their biosynthesis, and the death of the source organisms, they survive deposition to sediments in a recognizable form in terms of their original structure and sterical configuration (that is, spatial distribution of the atoms). They can thus be considered chemical fossils. The usefulness of organic components as palaeoproxies largely depends on their resilience to early degradation processes during sedimentation and after incorporation into the sediment. The most common biomarkers used as climate proxies belong to a few compound types, namely C37 alkenones and n-alkyl lipids. The key factors that make them suitable as proxies are that they are extremely common in sediments, their sources are identified, and their study provides information on key environmental variables. Biomarkers may be transported to sediments as part of the remains of the original organism (for example, leaf debris, marine snow), its digested remains (for example, faecal pellets), or adsorbed to mineral particles (that is, ballast minerals: silicate and carbonate biominerals, and dust), which eventually settle on the ocean bottom through gravity. The chapter describes the various approaches at distinct stages in their development and application