[MTT51-P03] Boron isotope as tracer for seawater pH reconstruction and
submarine hydrothermal circulation
Keywords:boron isotope, ocean acidification, submarine hydrothermal ore deposits
Here I present case study by using boron isotope, to demonstrate potential of boron isotopic investigation.
1) Seawater pH reconstruction from coral record
Ocean acidification caused by anthropogenically elevated CO2 concentration in the atmosphere can pose a critical threat to calcifying marine organisms and coral reef ecosystems. However, because of temporally and spatially limited instrumental pH records, little is known about the actual long-term trend and natural variability of seawater pH during the past century. We present an annually resolved time series of a pH proxy record for 1940–1999 using boron-isotope composition (δ11B) in a modern massive Porites coral from Guam Island (NW Pacific). When superimposed onto interannualvariability, the data show a slightly decreasing trend of ~0.39‰ (equivalent to ~0.05–0.08 pH units for surface water pH) in the northwestern tropical Pacific since the mid-20th century (Shinjo et al., 2013).
2) Submarine hydrothermal circulation: interstitial water B isotope profile
Many active hydrothermal venting fields have been discovered at the spreading axes of the Okinawa Trough, a nascent back-arc basin. In order to explore sub-seafloor extent of hydrothermal fluid circulation, boron isotopic compositions of interstitial water (IW) of drilled core sediments by the D/V CHIKYU were analyzed. While IW at the reference site have δ11B similar to seawater ~70 m below sea floor (mbsf), IW from active venting site and site at 200 m far from venting site show depth profile lowering δ11B at deeper than ~40 mbsf. Low δ11B values are approaching to those of venting fluids on δ11B vs 1/B diagram, implying hydrothermal fluid extension to far from the venting site. These results confirm that boron isotope of IW is a powerful tracer for exploration of hydrothermal activity at young rift or back-arc basin system (Ishibashi et al., 2017).
1) Seawater pH reconstruction from coral record
Ocean acidification caused by anthropogenically elevated CO2 concentration in the atmosphere can pose a critical threat to calcifying marine organisms and coral reef ecosystems. However, because of temporally and spatially limited instrumental pH records, little is known about the actual long-term trend and natural variability of seawater pH during the past century. We present an annually resolved time series of a pH proxy record for 1940–1999 using boron-isotope composition (δ11B) in a modern massive Porites coral from Guam Island (NW Pacific). When superimposed onto interannualvariability, the data show a slightly decreasing trend of ~0.39‰ (equivalent to ~0.05–0.08 pH units for surface water pH) in the northwestern tropical Pacific since the mid-20th century (Shinjo et al., 2013).
2) Submarine hydrothermal circulation: interstitial water B isotope profile
Many active hydrothermal venting fields have been discovered at the spreading axes of the Okinawa Trough, a nascent back-arc basin. In order to explore sub-seafloor extent of hydrothermal fluid circulation, boron isotopic compositions of interstitial water (IW) of drilled core sediments by the D/V CHIKYU were analyzed. While IW at the reference site have δ11B similar to seawater ~70 m below sea floor (mbsf), IW from active venting site and site at 200 m far from venting site show depth profile lowering δ11B at deeper than ~40 mbsf. Low δ11B values are approaching to those of venting fluids on δ11B vs 1/B diagram, implying hydrothermal fluid extension to far from the venting site. These results confirm that boron isotope of IW is a powerful tracer for exploration of hydrothermal activity at young rift or back-arc basin system (Ishibashi et al., 2017).