4:15 PM - 4:30 PM
[MIS12-08] Local Seawater Mass Dynamics at the Middle of North Pacific Subtropical Gyre Over the Past 100 years through Geochemical Approach Using Modern Hawaiian Coral
Keywords:Coral skeletons, Hawaii, North Pacific Subtropical Gyre, Sr/Ca ratio, Nitrogen isotope, nutrient variability
Hawaii is located near the middle of the largest subtropical gyre far away from the ocean basin margins with sea surface temperature (SST) remaining relatively neutral with respect to ENSO internal climate variability. Also, the trade winds are known to associated with the local (colder windward / warmer leeward) climate difference. (Chavanne et al., 2002). Despite the strategical location for assessing both local and global climate change, the available instrumental measurements are limited to the modern era, and long-term, high-resolution temperature reconstructions from paleo-proxies lack spatial coverage.
Reef coral skeletons provides high-resolution (sub-monthly~) archives of changes of the past environment. Strontium/calcium ratio (Sr/Ca ratio) in aragonite coral skeletons have been known as robust proxy to reconstruct past seawater temperature. Nitrogen isotope ratio (δ15Ncoral) in organic matter within coral skeletons has been known as a tracer for investigating seawater dynamics (Yamazaki et al., 2011b). This study aims to reconstruct the seawater temperature and nutrient variability at Hawaii over the past 100 years using a coral Sr/Ca and δ15Ncoral.
Two long core samples of modern Porites. sp. were collected from two locations on Oahu Island (Makai Pier on the windward side and Ko’Olina on the leeward side). The annual density bands observed in X-ray photo of the coral slabs were used to determine maximum growth axis as analysis transects. Sr/Ca ratio was analyzed using inductivity-coupled plasma optimal emission (ICP-OES). Additionally, annual coral samples were subsampled for δ15Ncoral (‰ vs. air N2) analysis. Coral samples were demineralized in 1N hydrochloric acid, and after oxidizing residual organic matter, total nitrogen was oxidized to NO3- and then converted to N2O using the Chemical Conversion method. The δ15Ncoral was measured using a continuous flow mass spectrometry system. The analytical error was ±0.38‰ (0.5σ) based on repeated analysis for standard.
Seawater temperature was reconstructed back to 1908 for the windward (Makai Pier) and 1957 for the leeward (Ko’Olina). The correlation map between the reconstructed temperature for Makai Pier and the gridded SST product exhibited a distribution along the North Pacific Subtropical Gyre. The δ15Ncoral on the windward side had a tendency of decreasing with its timing of increasing seawater temperature, reflecting the increase in N2 fixation rate associated with seawater stratification. On the other hand, the leeward side exhibited distinct variability in seawater temperature compared to the windward side. When the leeward side temperature was colder than windward side, the δ15Ncoral on the leeward side increased. Furthermore, the variations in δ15Ncoral on the leeward side showed a significant correlation (r = 0.38, 3-year moving average, n = 60) with the NPGO index, suggesting vertical upwelling on the leeward side is associated with cooling and nutrient supply.
Reef coral skeletons provides high-resolution (sub-monthly~) archives of changes of the past environment. Strontium/calcium ratio (Sr/Ca ratio) in aragonite coral skeletons have been known as robust proxy to reconstruct past seawater temperature. Nitrogen isotope ratio (δ15Ncoral) in organic matter within coral skeletons has been known as a tracer for investigating seawater dynamics (Yamazaki et al., 2011b). This study aims to reconstruct the seawater temperature and nutrient variability at Hawaii over the past 100 years using a coral Sr/Ca and δ15Ncoral.
Two long core samples of modern Porites. sp. were collected from two locations on Oahu Island (Makai Pier on the windward side and Ko’Olina on the leeward side). The annual density bands observed in X-ray photo of the coral slabs were used to determine maximum growth axis as analysis transects. Sr/Ca ratio was analyzed using inductivity-coupled plasma optimal emission (ICP-OES). Additionally, annual coral samples were subsampled for δ15Ncoral (‰ vs. air N2) analysis. Coral samples were demineralized in 1N hydrochloric acid, and after oxidizing residual organic matter, total nitrogen was oxidized to NO3- and then converted to N2O using the Chemical Conversion method. The δ15Ncoral was measured using a continuous flow mass spectrometry system. The analytical error was ±0.38‰ (0.5σ) based on repeated analysis for standard.
Seawater temperature was reconstructed back to 1908 for the windward (Makai Pier) and 1957 for the leeward (Ko’Olina). The correlation map between the reconstructed temperature for Makai Pier and the gridded SST product exhibited a distribution along the North Pacific Subtropical Gyre. The δ15Ncoral on the windward side had a tendency of decreasing with its timing of increasing seawater temperature, reflecting the increase in N2 fixation rate associated with seawater stratification. On the other hand, the leeward side exhibited distinct variability in seawater temperature compared to the windward side. When the leeward side temperature was colder than windward side, the δ15Ncoral on the leeward side increased. Furthermore, the variations in δ15Ncoral on the leeward side showed a significant correlation (r = 0.38, 3-year moving average, n = 60) with the NPGO index, suggesting vertical upwelling on the leeward side is associated with cooling and nutrient supply.