Possible Early Diagenetic Alteration of Palaeo Proxies

The search for palaeo tracers in the last decade has resulted in a number of potentially useful tracers for past temperature/salinity, productivity, pCO₂(aq), nutrients, etc. Recently, it has become evident that some limitations exist on the application of these tracers. Early diagenetic processes may considerably alter the primary signals. The diagenetic fates of palaeo proxies may vary for different environments, which can lead to preferential preservation, decomposition or relocation of particular tracers. Consequently, one must be very cautious when applying a palaeo-tracer that is sensitive to early diagenetic alteration. In this paper we present data from the Madeira Abyssal Plain, the eastern Mediterranean, and Kau Bay that clearly illustrate some of these alterations: 1.Diagenetic changes in some organic proxies (% C, C/N, δ13C, δ15C, UK₃₇). Origin and diagenetic history of sedimentary organic matter may result in the preferential preservation of some compounds. As a consequence the organic matter content and its composition may be altered dramatically upon postdepositional decomposition.2.Variations in the sedimentary Ba content (un)related to palaeoproductivity. Parameters other than productivity alone can determine the rain rate of Ba and consequently, its distribution in sediments. In addition, hydrothermal activity and anoxic mobilization coupled with reprecipitation are known to produce relatively high levels of Ba, unrelated to productivity.3.(Diagenetic) Mn-spikes at palaeo-productivity changes. Although the occurrence of Mn spikes has been related to changes in palaeoproductivity, and to sudden changes of the ocean water from suboxic to oxic conditions, most of these spikes can as well be explained by the early diagenetic mobilization of Mn from within sediments, deposited under non-steady state conditions such as those fostered by the emplacement of turbidites, a sudden change in sediment or organic C accumulation rate, or a change in bottomwater oxygenation.4.Diagenetic dissolution/precipitation of carbonates. Not only the preferential dissolution of some biogenic carbonate, but also the precipitation of carbonate (both by early diagenesis) may lead to variations in species- and isotopic composition that are unrelated to ‘primary signals’. Despite these limitations, palaeogeochemical tracers can provide significant and highly useful insights into oceanographic processes, provided their use is properly tempered.