Influence of Micro-Geomorphology on Sediment Grain Size Characteristics in the Lagoon Slopes of the Nansha Coral Reefs

Abstract

The lagoon slopes of coral atolls, which connect reef flats to lagoon basins, are key geomorphic zones where sediments are physically sorted and transported. These areas are essential for understanding post-reef sedimentary processes and the broader evolutionary dynamics of atolls. However, the influence of the microtopographic features of lagoon slopes on sediment grain size distribution remains unclear. Sediment grain size is a sensitive indicator of environmental and climatic changes and provides valuable insights into sediment transport patterns and depositional environment reconstructions. The spatial distribution of grain size is influenced by the proximity to sediment sources, material availability, and surrounding geomorphological conditions. This study focused on the lagoon slopes of the Shuquan and Jiaojian Reefs in the Nansha Islands, where surface sediment samples were collected and analyzed for grain size, along with depth data and underwater microtopography. The results indicated the following: (1) Surface sediments in the lagoon slopes of the Shuquan and Jiaojian Reefs are predominantly composed of sand-sized particles, with minimal silt and fine gravel content. Specifically, Shuquan Reef sediments comprised 94.55% sand-sized particles, 4.18% fine gravel, and 0.61% silt; the S1-S1' transect contained 93.96% sand, 4.04% fine gravel, and 1.65% silt; and the S2-S2' transect consisted of 91.73% sand, 6.84% fine gravel, and 1.17% silt. (2) The grain size characteristics of lagoon sands were primarily governed by the proximity to sediment sources and hydrodynamic conditions. A significant negative correlation was observed between median grain size and water depth, with grain size decreasing as depth increased. Most lagoon sands originated from reef flats or lagoon slopes, reflecting the diminishing sorting capacity of oceanic wave energy as it moves toward the lagoon center. The grain size distribution curves of lagoon sands are generally unimodal or bimodal, with a trend toward negative skewness. In deeper waters, the curves became sharper, indicating a transition from low-energy to high-energy depositional environments, where finer particles were replaced by coarser ones. Geomorphological changes within the lagoon, particularly in the transition zone from the lagoon slope to the inner reef flat, played a critical role in the grain size distribution. The bimodal distribution in Shuquan Reef was mainly attributed to coarse particles from lagoon pinnacles, whereas in Jiaojian Reef, the bimodal distribution was influenced by reduced dynamic sorting due to seagrass root stabilization. By comparing these distinct reef systems, this study elucidated the impact of localized geomorphic features, such as pinnacles and seagrass beds, on sediment sorting mechanisms. Understanding the grain size characteristics and distribution patterns of sand in natural microtopographic settings on the lagoon slopes of atolls is crucial for the rational use and scientific management of resources, with significant implications for land reclamation and reef engineering projects.

Keywords

• grain size
• micro-geomorphology
• lagoon
• atoll
• nansha islands