Natural Environmental Change: The Last 3 Million Years

By A. M. Mannion | Go to book overview

5

The record of environmental change in continental archives

5.1

Introduction

The continental deposits that initially inspired natural scientists to speculate on past environmental change were those left by glaciers and ice sheets. Together with observations in actively glaciated regions, such deposits inspired the natural scientists of the nineteenth century to formulate the glacial theory (Section 1.2). Today, the deposits left by ice sheets, etc. still attract considerable attention, as efforts to reconstruct the dimensions and impact of the many ice advances of the last 3×106 years continue. Geomorphological and lithological features both have a role to play in elucidating past environmental change.

Although only high latitudes, and, to some extent, middle latitudes, as well as high altitudes, were directly affected by glacial and periglacial activities, the considerable drop in global temperatures during the ice ages had ramifications for all Earth-surface features and processes. World biomes, for example, were quite different during ice ages, in relation to both their geographical location and their species composition, compared with their characteristics during interglacials. Similarly, coastlines and coastal features were very different during ice ages as compared with interglacials; the incarceration of a vast volume of water caused substantial falls in sea-level and major rises ensued as ice sheets melted. What are today continental shelf regions of the ocean floor were dry land during the ice ages.

Evidence for these environmental changes derives from a variety of continental archives. On the basis of physical, chemical and biological techniques, data on environmental change are being elicited to provide a local record of change, which also contributes to the regional and global tableaux. Such archives are diverse; they include lake sediments, peats, palaeosols, loess, carbonate deposits and packrat middens. Some of these archives cover long periods of time, as is the case with a number of lake sediments and loess deposits. Others are particularly relevant to the Holocene. The use of age-determination techniques, oxygen isotope stratigraphy and palaeomagnetic stratigraphy have facilitated correlation between diverse and distant continental records and between continental records and those from ocean sediments (Chapter 3) and ice cores (Chapter 4).

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