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A mechanistic investigation of the coral Mn/Ca-based trade-wind proxy at Kiritimati
Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2022-05-04 , DOI: 10.1016/j.gca.2022.04.030
Alice U. Chapman , Diane M. Thompson , Stephan R. Hlohowskyj , Jessica E. Carilli , Gwyneth Gordon , Tyler J. Goepfert , Hussein R. Sayani , Thomas M. Marchitto , Kim M. Cobb

Tropical Pacific trade-wind behavior is linked to the El Niño-Southern Oscillation and Pacific Decadal Variability, which modulate the rate of climate change. Despite their importance, high-resolution trade-wind observations span only the past 30–40 years and are too sparse to assess decadal wind variability and long-term trends. Previous work demonstrated that reef-building corals growing at the tropical Pacific island of Tarawa (2°N, 165°E) exhibit spikes in the manganese-to-calcium ratio (Mn/Ca) of their skeleton in response to a reversal of trade winds (i.e., westerly winds). Records of Mn/Ca from long-lived corals therefore hold great promise as indicators of past trade-wind variability. However, at other nearby islands with west-facing lagoons, there is a lag between westerly winds and coral Mn/Ca spikes and a significant difference in the magnitude of spikes between corals. To address uncertainties in how winds are recorded by coral Mn/Ca, we assess the reservoirs of Mn in the sediment, sediment pore spaces (porewater), and water column of Kiritimati’s lagoon and inland lakes (1.9°N, 157.5°W). We find that insoluble dustborne Mn, once buried in lagoon sediments, becomes reduced and more soluble, leading to its accumulation in the sediment porewater. This Mn reservoir is then released into the water column when strong westerly wind events cause sufficient water-column mixing to reach lagoonal sediments. While this mechanism is consistent with what was previously proposed at Tarawa, the concentration of dissolved porewater Mn at Tarawa is nearly 20 times greater than at Kiritimati. We attribute this difference to the water depth of the sediment core from which porewater was sampled and the time elapsed between the most recent westerly wind event and core sampling, which both influence the “recharge time” of the porewater Mn reservoir. As such, lagoon bathymetry and morphology modulate lagoon water and sediment porewater Mn concentrations, which impact how westerly winds imprint their signal onto coral Mn/Ca. Armed with an improved understanding of the mechanism behind this coral Mn/Ca-trade wind relationship, we can better assess the reliability of this coral proxy through space and time and identify optimal sites for Mn/Ca-based wind reconstructions, paving the way for critical new insights into the role of winds in future climate change.



中文翻译:

基里蒂马蒂珊瑚锰/钙基信风代理的机理研究

热带太平洋信风行为与调节气候变化速率的厄尔尼诺-南方涛动和太平洋年代际变率有关。尽管它们很重要,但高分辨率信风观测仅跨越过去 30 至 40 年,而且太稀少,无法评估年代际风变率和长期趋势。以前的工作表明,在热带太平洋塔拉瓦岛(2°N,165°E)生长的造礁珊瑚在贸易逆转时表现出骨骼的锰钙比(Mn/Ca)峰值风(即西风)。因此,来自长寿珊瑚的 Mn/Ca 记录很有希望作为过去信风变化的指标。然而,在附近其他有朝西泻湖的岛屿上,西风和珊瑚 Mn/Ca 尖峰之间存在滞后,并且珊瑚之间尖峰的幅度存在显着差异。为了解决珊瑚 Mn/Ca 如何记录风的不确定性,我们评估了 Kiritimati 泻湖和内陆湖泊(1.9°N,157.5°W)的沉积物、沉积物孔隙空间(孔隙水)和水柱中的 Mn 储层。我们发现,一旦埋藏在泻湖沉积物中,不溶性尘埃 Mn 会减少并且更易溶解,导致其在沉积物孔隙水中积累。当强烈的西风事件导致足够的水柱混合到达泻湖沉积物时,该锰储层随后被释放到水柱中。虽然这种机制与之前在 Tarawa 提出的机制是一致的,但 Tarawa 的溶解孔隙水 Mn 浓度几乎是 Kiritimati 的 20 倍。我们将这种差异归因于采样孔隙水的沉积岩心的水深以及最近一次西风事件与岩心采样之间的时间间隔,这两者都影响孔隙水 Mn 储层的“补给时间”。因此,泻湖水深测量和形态调节泻湖水和沉积物孔隙水 Mn 浓度,这会影响西风如何将其信号印在珊瑚 Mn/Ca 上。有了对这种珊瑚 Mn/Ca 贸易风关系背后机制的更好理解,我们可以通过空间和时间更好地评估这种珊瑚代理的可靠性,并确定基于 Mn/Ca 的风重建的最佳地点,为对风在未来气候变化中的作用的重要新见解。

更新日期:2022-05-04
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