Millennial‐Scale Hydrothermal Variability at the Carlsberg Ridge Driven by Sea‐Level Change and Transient Magmatism

Hydrothermal systems at mid‐ocean ridges (MORs) mediate the transfer of heat and geochemical fluxes from the mantle to the hydrosphere, facilitating fluid‐rock interaction and metal cycling. While short‐term hydrothermal dynamics are well studied, the long‐term response of these systems at slow‐spreading ridges to glacial‐interglacial sea‐level change remains poorly constrained. Here, we measured Pb isotope ratios and trace metal concentrations (Co, Ni, Cr) in authigenic Fe–Mn oxyhydroxide coatings from sediment core SSD77‐GC03, recovered from the Carlsberg Ridge, to reconstruct hydrothermal activity over the past 49 kyr. Pb isotope compositions of basalt fragments were also analyzed to constrain the local hydrothermal end‐members. The Fe–Mn coating data delineate six discrete hydrothermal events (HY1–HY6), revealing two distinct modes of sea‐level forcing. Events HY4–HY6 (∼46–36 ka) follow the Marine Isotope Stage (MIS) 4 sea‐level lowstand and reflect a delayed, melt‐driven hydrothermal response, consistent with decompression‐induced mantle melting and subsequent magma ascent. In contrast, events HY1–HY3 coincided with rapid sea‐level falls during the MIS 3–2 transition and Last Glacial Maximum (LGM ∼ 24–22 ka), and reflect a hybrid mechanism in which fault‐enhanced permeability and transient magmatic heat sustained hydrothermal discharges. These dual mechanisms highlight the sensitivity of hydrothermal systems to both mantle melting and upper crustal stress reconfiguration driven by sea‐level change. During these events, trace metal enrichments in authigenic coatings indicate enhanced trace metal fluxes to the deep Indian Ocean. Our findings demonstrate that hydrothermal systems at slow‐spreading ridges respond dynamically to sea‐level forcing and modulate deep‐ocean chemistry over glacial‐interglacial cycles.