977 / 2024-09-19 23:06:55

The potential impacts of tephra diagenesis on Rb and Fe isotope systems in pelagic deep-sea sediments

Rubidium isotope,Iron isotope,pelagic deep-sea sediment,Tephra,Diagenesis

Volcanic materials constitute a significant proportion of deep-sea sediments, particularly near collisional tectonic boundaries. Tephra within marine sediments is usually fine-grained and easily altered to form authigenic minerals, influencing regional or even global elemental cycling. However, potential impacts of tephra diagenesis in pelagic deep-sea sediments are less understood. Here, two sediment cores from abyssal basin (water depth > 5000 m) close to the Mariana arc were performed with mineralogical, geochemical and Rb-Fe isotope analysis. Leaching experiments were also conducted to better understand the characteristics of labile fractions. Combining with literature data of over 2400 sediment samples from 121 sites in the Western Pacific Ocean, our results suggest that tephra addition generally results in lower bulk K/Al, Rb/Al and Cr/Al with similar δ⁸⁷Rb to the UCC, while low-temperature alteration significantly elevates the bulk K/Al, Rb/Al and δ⁸⁷Rb with phillipsite and smectite formations. The δ⁸⁷Rb of diagenetic products are still lower than the modern seawater, indicating tephra diagenesis as an isotopically light sink for oceanic Rb budgets. The bulk Fe/Al and δ⁵⁶Fe remain almost unchanged and within the range of tephra and mineral dust, indicating new Fe in this region mainly derived from tephra/dust inputs. Labile Fe fractions decrease from 53% to 28% with increasing bulk K/Al and δ⁸⁷Rb. The δ⁵⁶Fe of labile fractions also decrease accordingly, indicating Fe redistributions during tephra/dust diagenesis. Overall, we suggest that tephra/dust diagenesis in pelagic deep-sea sediments can influence oceanic Rb and Fe cycling and their isotope compositions in sediments.