Planktonic Foraminifera: Selective solution and the lysocline

Abstract

Pacific Ocean waters appear to be undersaturated with calcium carbonate at all depths except in the uppermost few hundred meters. This leads to continuous destruction of almost all calcareous sediment exposed on the Pacific Ocean floor. Samples derived from sediment and from plankton were exposed for four months on a taut wire buoy in the central Pacific, in order to assess the effects of solution on foraminiferal death assemblages. The rate of destruction varied for different species and for different variants within species. Sediment assemblages therefore should tend to become enriched with resistant (non-spinose) species and with opaque (usually thick-shelled), zero and negative forms, i.e., specimens with small terminal chambers. Results obtained by laboratory experiments compare well with those obtained from the solution experiments in the field. The distribution of resistant forms in surface sediment samples from the East Pacific Rise shows that there exists a level of rapid solution increase (lysocline) in this area. The surface of the lysocline is at approximately 4,000 m depth in the tropics on the western side of the East Pacific Rise. The lysocline surface slopes upward toward Antarctica and toward South America and apparently becomes less well defined in high latitudes and near the continent. The calcium carbonate compensation depth results from a balance of rates of solution and of rates of supply of calcareous matter and usually lies well below the lysocline. The existence of a lysocline implies an associated oxygen minimum roof, and is bound to both active bottom water production and excess calcite supply by planktonic organisms.