Depth-transport functions and erosion-deposition diagrams as indicators ofslope inclination and time-averaged traction forces: applications in tropical reef environments

The trigonometric relationship between slope inclination, the horizontally acting time-averaged traction force and the vertical depth of transport all ows the estimation of one factor, when both others are known. Depth-transpo rt functions can be deduced by comparing the depth distributions of living organisms and their skeletal remains, and this paper simplifies this compar ison using foraminifera in which a single test represents an individual. Di fferences in distribution parameters between living individuals and empty t ests allow depth-transport functions to be determined; these functions diff er between species at a single transect according to the varying buoyancies of the tests. Within a single species, differences in depth-transport func tions between locations are based on either slope inclination or traction i ntensities. After establishing a mean depth-transport function by averaging species-characteristic functions, the time-averaged traction force acting on the studied transect can be calculated. Transport intensities are also e stimated using an erosion-deposition diagram that combines the relative fre quency distributions of living individuals and empty tests. The proportion of 'eroded', 'parautochthonous' and 'allochthonous' tests mirrors the influ ence of both slope inclination and traction force for the deposition of emp ty tests. To test the model, six species of symbiont-bearing benthic forami nifers were investigated at two transects in front of a NW Pacific coral re ef. One transect is distinguished by a strong slope flattening below the st eep reef slope (30 m), whereas further steepening characterizes the equival ent part in the other transect. These differences are mirrored in the depth -transport functions as well as in the erosion-deposition diagrams of all s pecies. The time-averaged traction forces differ in intensities between tra nsects, because of the position of the reef front with respect to the predo minant wind direction. However, the form of the functions is identical and distinguished by an increase from the surface to 35 m depth, followed by a decrease down to 105 m. This can be explained by successive onshore and off shore forces acting on the shallow slope, such as the tropical cyclones tha t cross the region every summer.