708 ARRHENIUS [CHAP. 25 



Because of low density, particles of organic matter from land might spread 

 extensively over adjacent oceanic areas, a minor fraction being windborne (cf. 

 Fig. 21) and the majority transported into the ocean by rivers. Some pelagic 

 sediments, in areas near land with heavy tropical vegetation and protected by 

 marginal trenches (such as on the Pacific side of Middle America), may contain 

 several per cent of terrestrial organic detritus. 



The decomposing organic matter on the ocean floor rarely forms well defined 

 organic minerals. Weddellite (calcium oxalate dihydrate) and earlandite 

 (calcium citrate tetrahydrate) have been described by Bannister and Hey 

 (1936) from Antarctic sediment samples, together with authigenic gypsum. 

 Small amounts of strontium and barium substitute for calcium in the organic 

 crystals. Since later attempts to find these minerals in freshly collected Ant- 

 arctic sediments have failed, and as gypsum is often observed to form in drying 

 bottom samples from calcium carbonate and decomposing proteins, it is possible 

 that the weddellite and earlandite formed during storage of the samples. 



3. Productivity Control of Pelagic Sedimentation 



One may select approximately level areas of pelagic deposition where the 

 pressure factor and (at least during Pleistocene-Recent times) the temperature 

 factor influencing the post-depositional dissolution of calcite is small compared 

 to the production factor and can be separately estimated (Fig. 19). This 

 facilitates investigating the combined effects of organic production and total 

 rate of deposition on the concentration and rate of accumulation of calcium 

 carbonate. Such studies have been carried out in an approximately level area in 

 the east equatorial Pacific where the Equatorial Divergence maintains a high 

 organic productivity. 



The meridional distribution of the rate factors involved, the causative 

 circulation mechanism, and the ensuing sedimentary record are shown graphic- 

 ally in Fig. 36. The top diagram shows a north-south section through the surface 

 layer of the Pacific Ocean at approximately 130°W, The temperature distribu- 

 tion is indicated by the isotherms for 15, 17, 20, 25 and 27°C. The cold inter- 

 mediate water is represented by a dotted area, and the vertical circulation is 

 indicated by arrows. i Divergence of the trade-wind drift currents produces 

 upwelling along the equator of the nutrient-rich intermediate water, resulting 

 in a high rate of production of phytoplankton, sustaining an increased pro- 

 ductivity also of higher members of the food chain. At higher tropical latitudes 

 the lack of a mechanism enriching the euphotic layer keeps the productivity 

 low. 



The rate of production of skeletal calcium carbonate from coccolithophorids 

 and Foraminifera is illustrated in the middle diagram of Fig. 36, where the full 

 line curve marked "Pleistocene minimum rate of production (interglacial) " 

 corresponds to present-time conditions. The inferred rate of dissolution of 

 calcium carbonate before final burial of the fossils is indicated by the dashed 



1 The heavier shading marks the extension of the Cromwell Current (Knauss, 1960). 



