Application of coral carbon isotopes to study river discharge and paleo-turbidity in the Gulf of Chiriquí, Panamá

Seasonality in rainfall and river discharge (Q) in Panamá is largely driven by meridional migrations of the Intertropical Convergence Zone. Droughts in Panamá, related to ITCZ shifts or El Niño events, can be severe, lowering Q and reducing the amount of water available to operate the Panamá Canal. This has forced the Panamá Canal Authority to reduce the maximum draft of passing ships, most recently in Spring 2019. During the wet season from May-November Q increases and coastal waters become more turbid. Photosynthetic rate in corals is sensitive to both water clarity and water depth. A recent compilation of mean hermatypic coral carbon isotope values (δ¹³C) and water depth by Linsley et al. (2019) identified a clear inverse, apparently linear, relationship between coral δ¹³C and coral water depth in the upper ~20m of the water column. The relationship is likely due to a decrease in photosynthetic rate with depth as light availability dwindles, also supported by earlier work. Increased turbidity also has potential to dampen photosynthesis as suspended sediments limit sunlight penetration. We present δ¹⁸O and δ¹³C records from a nearshore Porites coral core collected in March 2018 near the discharge point of a small river in Encenada Mona in the Gulf of Chiriquí, Panamá. With the δ¹³C-water depth relationship, mean coral δ¹³C suggests a growth depth of ~10m, twice as deep as the actual depth of 4.6m, suggesting Q-driven turbidity is masquerading as an increase in water depth. It is notable that δ¹⁸O, which seasonally oscillates with precipitation-driven Q, and time matched δ¹³C are well correlated (R=0.75, p<0.01). We believe the correlation suggests that here coral ¹³C/¹²C is sensitive to photosynthetic rate via seasonal turbidity, considering the regional hydroclimate. Ultimately, these proxies will help us decode paleo-coastal reef conditions over the past ~100 years, while refining their paired use to better understand flood and drought recurrence in Panamá.