COASTAL SEDIMENTS OF PACIFIC ATOLLS: WHAT DO WE NEED TO KNOW?

Coastal sediments are an important resource for habitation, tourism and construction in all tropical Pacific nations. As sediments are immediately affected by environmental change and human activities, understanding their dynamics and budgets is vital to predicting the responses of the systems to changes and to managing the coastal and nearshore environment.

Our knowledge of many processes varies; some are well understood but others need much study. The starting point is sediment composition, with relative proportions of the important components known from some areas; determining these is not always straightforward and may require sedimentological and geochemical techniques. Most sediments are biogenic carbonates so knowledge of the biology of a range of organisms, and especially rate of carbonate precipitation, is crucial. Growth rates and carbonate production are well known for some groups for some areas (e.g. coral average 1200 g/m-2/y-1) but less so for others (Halimeda 13-2000 g/m-2/y-1, foraminifera 40-5000 g/m-2/y-1). Most studies are of productivity of species per unit area (often areas favourable for growth or amenable to study) but total productivity across the entire system is more important. Species distribution is also important, and total productivity probably varies regionally.

Mechanical and bio-erosion processes and rates determine how much of the carbonate produced reaches the nearshore system. There are few reliable studies of this and the relative importance of normal and catastrophic events needs to be assessed. Depositional processes, including transport directions and mechanisms, beach abrasion and burial diagenesis, also need further study. It is important to determine the “normal” residence time of each type of clast in each part of the system. For example, if the residence time of a foraminiferan test on a beach is 50 years and if these contribute 60 % of the sediment (as is common), then loss of the living foraminifera will cause loss of 12 % of the beach per decade through attrition alone, without climatic change.

Parameters vary significantly across the Pacific region and understanding the sediment budget at one location will not meet all needs. However, until the “normal” processes are understood, reactions of the systems to global climate and other change cannot be quantified.