Changing Sub‐Surface Chemistry Resulting From a 26‐Million‐Year Unconformity: Porewater Chemistry From IODP Site U1553 in the South Pacific

Whereas changes in pore water chemistry are known to impact carbonate sediment geochemistry, little is known about the impact of long unconformities on carbonate alteration. IODP Site 378‐U1553 on the southern Campbell Plateau, with a 26‐million‐year, erosional unconformity, provides a key location for examining the impact of long‐term unconformities on sub‐surface chemistry and carbonate archives. This study examined 76 interstitial water samples for sulfate sulfur and oxygen isotopes, as well as 28 bulk carbonate samples for carbonate associated sulfate (CAS) sulfur isotopes, to quantify the effects of this unconformity on the sub‐surface redox chemistry of the Site. The current state of the system suggests limited influence of redox processes on the CAS archive. Manganese reduction reaches 30 mbsf, with a decrease in manganese reduction between 20 and 30 mbsf. Below 30 mbsf, the system transitions to iron reduction to a depth of approximately 140 mbsf where sulfate reduction begins. Dissolved sulfate sulfur and oxygen isotope values suggest repeated oxygenation of sulfides. The CAS record from the Site deviates from previously published seawater values. However, the lack of a relationship between the dissolved sulfate and CAS records suggests most of the alteration of the CAS record likely occurred before the unconformity when the carbonate sediments were more reactive. This further supports the CAS record as a relatively robust archive, withstanding most post‐depositional mechanisms of carbonate alteration. Plain Language Summary The chemical composition of carbonate minerals in marine sediments are commonly used as archives for studying past climate and ocean conditions. Most studies examining carbonate environments use study sites with continuous, or nearly continuous, sediment deposition. Less is known about how long periods of extended erosion, or no deposition, impact the carbonate archive's long‐term stability. This study examined sulfur isotopes in sulfate dissolved in pore water and sulfate from the carbonate sediments at IODP Site 378‐U1553 on the southern Campbell Plateau. This site was selected as it has an approximately 26‐million‐year period of strong erosion resulting in no sediment deposition. The long‐term erosional interval led to dissolved species from more recent bottom water diffusing into the previously buried Oligocene sediments. Here we provide evidence that the carbonate minerals at the Site are relatively unaffected by the long