Criteria of climatic change in the inorganic components of marine sediments

Abstract

The most commonly used criteria in marine sediments for detecting climatic changes are the remains of organisms and the position of the shorelines, for these two types of criteria can have a relatively quick response to climatic change. The inorganic components of marine sediments, however, also provide useful criteria. On the inner continental shelf where the best correlation should be found between modern terrigenous marine sediments and modern climates, sediment texture is the main criterion. Where land ice reaches the sea, gravel may be deposited, but much of the inner shelf in polar climates receives abundant mud, containing a small amount of clay minerals. From tropical humid climates abundant mud is delivered composed mainly of clay minerals, but knowledge of their composition is required, because the largest rivers do not have a dominance of tropical sediment products. In arid climates and midlatitude moderate rainfall climates, inner shelf sand is indicative, although it also possibly reflects the common entrapment of mud in estuaries and the presence of the middle latitude cyclone belt in which storms remove the fine material present on the inner shelf. Climate also controls extensive carbonate deposits. In deep-sea sediments composition contains more important criteria than texture. Some criteria appear to be reliable for various aspects of modern climates and therefore should be useful in detecting climatic changes. These criteria include the size, surface texture, and mineralogical and chemical composition of eolian transported material downwind of arid lands; global dust in latitudinal bands of atmospheric circulation; volcanic ash downwind of geologically instantaneous events; surface texture of quartz grains and the abundance of terrigenous material in pelagic sediments as indication of glaciation; chlorite from a polar climate; kaolinite from a tropical climate, and inorganically precipitated calcium carbonate in enclosed seas. Less definitive criteria are possibly the rate of turbidity current activity, iron-rich layers in the sediment, sedimentation from the nepheloid zone, construction of features by bottom currents, organic matter content, and sedimentation rate. Speculations include the intensity of benthic faunal reworking of sediment. Using these criteria it is possible to identify the sediment products of the extreme climates: polar, tropical rainy, and dry (desert), and thereby to infer the existence of these climates. The moderate climates apparently are not so easily detected. The criteria also indicate the nature of the water, wind, and ice processes delivering the sediment products to the sea. Extreme values in the frequency or magnitude of the climate-associated processes have great significance in the supplying of terrigenous material, and changes in these extreme values could produce salient changes in the sedimentary sequence. The criteria of climatic change might well be considered criteria of change in extreme values of the processes.