Lagoon Morphology: A Key Component of the Coral Mn/Ca-based Trade-wind Proxy

Abstract:

Tropical Pacific trade winds influence global climate by modulating subsurface ocean heat uptake on decadal and interannual timescales. For example, weakening easterly trade winds and intermittent bursts of westerly wind disrupt the zonal pressure gradient in the Pacific and lead to anomalous sea surface warming, a major characteristic of an El Niño event. With uncertainty surrounding the future of El Niño behavior in a warming climate, historical wind patterns can inform our understanding of future climate variability. However, wind observations in this critical region date back only 30-40 years, which is insufficient for the analysis of interannual, let alone decadal, variability.

The manganese-to-calcium ratio (Mn/Ca) of skeletal material accreted by corals growing at islands with west-facing lagoons was recently discovered to reflect the behavior of tropical Pacific trade winds over the 20^th century. These trade winds are thought to transport Mn-laden dust to the islands, where it accumulates in lagoon sediments. Episodic westerly wind events (WWEs) during El Niño years mechanically mix the lagoon and remobilize Mn-rich porewater from these sediments, which then can be incorporated into the coral skeleton.

This promising coral proxy was first developed at the equatorial atoll of Tarawa, and has since been reproduced using corals from Kiritimati and Butaritari. A comparison of the distribution of Mn across its key reservoirs (seawater, sediment, and porewater) at Tarawa and Kiritimati are consistent with the proposed mechanism of this proxy, with a step-wise accumulation of Mn from seawater, to sediment, to porewater. Nevertheless, the water Mn reservoir at Kiritimati demonstrates considerable spatial variability, with elevated Mn concentrations at the mouth of the lagoon relative to other lagoon regions. Here, we investigate the roles that lagoon morphology and depth play in elevating and transporting dissolved Mn, and the implications for the magnitude, timing, and reproducibility of the Mn/Ca wind signal subsequently captured in corals.

Diagnosing the cause of spatially variable Mn concentrations in the lagoon will improve our understanding of the innerworkings of this coral Mn/Ca-based trade-wind proxy, and will also help identify ideal sites for future coral-based wind reconstructions.