Chapter 8 Tropical Margins 8 . 1 An Overview of Tropical Margins

On our planet, the ocean margins between the Tropic of Capricorn (23◦ 27′ S) and the Tropic of Cancer (23◦ 27′ N) are directly below the sun and are consequently warmer than other parts of the earth. Higher solar energy inputs are about the only rational (but still contested) explanation for the orders of magnitude greater biological species diversity (Allen et al., 2002; Algar et al., 2006) and hydrological and biogeochemical cycling speeds observed in the equatorial and tropical regions of the world, land and sea (Aller et al., 1996). Although extreme aridity is found in the tropics, this latitudinal band also contains the world’s wettest regions (SE Asia, the Amazon, the Congo/Zaire, and some equatorial islands), where less than one quarter of the world land surface provides over half of the freshwater and riverine sediment input to the world ocean (Nittrouer et al., 1994). The cycling of water among the land, sea and atmosphere in the tropics is a major driving engine for natural (ENSO, PDO, AMO, Indian Ocean Dipole) and anthropogenic (greenhouse) climate change and ocean circulation of heat, salt and water vapour (Broecker, 1998). The giant atmospheric Hadley–Walker cells and the western Pacific warm pool in the Indo-Pacific equatorial regions appear to affect climate around the world, as well as ecosystem services to global health and human food harvests (Sun et al., 2006; Juillet-Leclerc et al.,