Salinity and hydrology reconstructions using B/Ga proxy records in the Pearl River Estuary (China)

Salinity plays a critical role in the hydrological conditions and environmental changes in marginal marine settings. However, bulk-sediment paleosalinity proxies require further refinement, and their sensitivity in detecting changes in ancient sediments needs to be better constrained. In this study, we evaluate the applicability of a boron (B)/gallium (Ga) salinity proxy to an analysis of sediments of the Pearl River Estuary (PRE). PRE bottom water salinity analyses featured a well-defined gradual salinity gradient of 8.6 psu (practical salinity unit) to 32.2 psu, making PRE an ideal archive to study the response of salinity proxies to salinity change. Our results show a significant positive linear correlation (r = 0.796, P < 0.01) detected between B/Ga in surface sediments (from 2.90 to 4.76) and salinity in bottom water, suggesting the sensitivity of B/Ga proxy for salinity reconstruction in marginal seas and estuarine environments. Additionally, we find significant correlations between salinity, grain size, and chemical weathering proxies, revealing that grain size and chemical weathering can be affected by terrestrial–marine environmental transitions. We propose using the salinity proxy B/Ga to help distinguish whether changes in grain size and chemical weathering are caused by the depositional environment changes, particularly in estuaries with complex environmental variations. Furthermore, B/Ga records from Holocene F01 core, located near the PRE, align with diatom-based salinity reconstructions and precipitation records from southern China, supporting its reliability as a salinity proxy and its potential to capture monsoonal precipitation variability through freshwater input changes. These findings highlight the sensitivity and validity of the B/Ga ratio as an indicator of salinity and hydrology in marginal marine environments.