Seasonal palaeoenvironmental variability during pre-industrial times revealed by a Southern Hemisphere Tridacna shell

The end of the Little Ice Age (LIA) represents a key pre-industrial climate interval, marking the final major cooling phase before widespread anthropogenic warming after the mid-nineteenth century. However, high-resolution evidence for this transition remains dominated by Northern Hemisphere archives, limiting understanding of how low-latitude marine environments in the Southern Hemisphere responded to late-LIA climate variability. Here, we analysed a Tridacna gigas shell that lived for approximately 30 years during the early 1800s to provide a high-resolution 30-year snapshot of environmental variability at Jiigurru in the northern Great Barrier Reef (GBR), tropical Southern Hemisphere. Stable isotope records (δ 18 O, δ 13 C) from both the inner and outer shell layers were applied to reconstruct SST, precipitation variability, and local ENSO-like events. Our results suggest that cooling associated with the LIA extended to the northern GBR. Under a constant modern SSS-derived δ 18 O Seawater baseline, reconstructed SSTs were ~ 0.9–1.2 °C lower than modern instrumental records, a monthly SSS-corrected sensitivity reconstruction reduced this estimate to ~0.6–0.9 °C. These results suggest that late-LIA cooling at this tropical Southern Hemisphere site was smaller in magnitude than many Northern Hemisphere reconstructions. Shell δ 13 C exhibited pronounced seasonal variability, consistent with the precipitation-related hydroclimatic seasonality. Statistical analyses reveal pronounced two- to three-year periodicities in both SST and hydroclimatic variability, with more frequent El Niño-like events in the late 1700s and stronger La Niña-like conditions in the early 1800s. This 30-year Tridacna record therefore provides a rare seasonal marine archive for evaluating Southern Hemisphere climate variability during the terminal LIA.