Decadal changes in the CaCO₃ saturation state along 179°E in the Pacific Ocean

Abstract

[1] To assess degrees of ocean acidification, we mainly investigated decadal changes in the saturation state of seawater with respect to aragonite (W arg ), which is a more vulnerable mineral form of CaCO 3 , along the 179 E meridian (WOCE P14N) in the Pacific Ocean. We found a maximum decrease of W arg of À0.48 (À0.034 a À1 ) at 200-300 dbar (isopycnal surfaces of 24.0-25.8 kg m À3 ) at 20 N. Between 1993 and 2007, the saturation horizon rose by 17 dbar (1.2 dbar a À1 ) at latitudes 10 N-50 N. Although DW arg mostly reflected changes in normalized dissolved inorganic carbon (DnC T ), it was larger than could be explained by anthropogenic CO 2 storage alone. Decomposition of DnC T revealed that DW arg was enhanced by approximately 50% by a non-anthropogenic CO 2 contribution represented by changes in apparent oxygen utilization. Our results suggest that ocean acidification can be temporarily accelerated by temporal changes in oceanic conditions. Citation: Murata, A., and S. Saito (2012), Decadal changes in the CaCO 3 saturation state along

Ocean acidification from 1997 to 2011 in the subarctic western North Pacific Ocean

“…In the western subarctic gyre, the significant increase in TA at a rate of 0.5 ± 0.1 µmol kg −1 yr −1 on the 26.9σ θ surface (Table 2) can account for the slower decline in and CO 2− 3 concentration in subsurface waters. In fact, the observed decrease rate of CO 2− 3 on the 26.9σ θ surface (-0.36 µmol kg −1 yr −1 ) is slower than the rate calculated by using increasing DIC and constant TA (-0.49 µmol kg −1 yr −1 ); this calculated rate predicts a faster decline of (-0.007 aragonite yr −1 , -0.010 calcite yr −1 ) than the observed declines, which is in good agreement with rates in the subarctic region along 179 • E and 152 • W (Murata and Saito, 2012;Feely et al, 2012). Because one component of TA and DIC is CO 2− 3 , increases in TA slow the decrease rates of aragonite and calcite in intermediate waters.…”

“…Ocean acidification of surface and subsurface waters of the North Pacific Ocean has been clearly documented in time series and repeat hydrography data (direct basin-wide observations) over the past two decades (e.g., Dore et al, 2009;Byrne et al, 2010;Midorikawa et al, 2010, Ishii et al, 2011Feely et al, 2004;Feely et al, 2012;Murata and Saito, 2012). The observed pH changes in the surface ocean are consistent with those predicted based on equilibration of atmospheric CO 2 with the seawater (about −0.002 yr −1 ) (Dore et al, 2009;Byrne et al, 2010;Midorikawa et al, 2010;Ishii et al, 2011).…”

“…In our data, collected during 1997-2011, AOU varies on a shorter than bi-decadal cycle and the AOU increase on 26.9σ θ (1.8 ± 0.4 µmol kg −1 yr −1 ) was twice that from 1968 to 1998 in the Oyashio region near the western subarctic gyre (0.8 ± 0.3 µmol kg −1 yr −1 ) (Ono et al, 2001). Considering this context and the stoichiometric ratio of carbon to oxygen from organic matter decomposition, the minimum possible contribution of non-anthropogenic CO 2 (0.8 × 117/170 = 0.55 µmol kg −1 yr −1 ) is the same as the anthropogenic CO 2 uptake (0.5 ± 0.4 µmol kg −1 yr −1 ) in the western subarctic gyre, which is the same the uptake rate in intermediate water along 152 • W during 1991-2006 (Byrne et al, 2010) and 179 • E during 1993(Murata and Saito, 2012. Thus, the enhanced acidification (pH decrease) between 200 and 300 m depth in this region reflects both anthropogenic CO 2 and non-anthropogenic CO 2 contribution, which will not affect the spatial distributions of acidification rates (Murata and Saito, 2012).…”

“…5, Table 1). These subsurface rates are slower than rates in subtropical mode water (-0.034 aragonite yr −1 ) or in subsurface waters of the subarctic region (north of 40 • N) along 179 • E and 152 • W (about -0.007 aragonite yr −1 and -0.010 calcite yr −1 , Murata and Saito, 2012;Feely et al, 2012). In the western subarctic gyre, the significant increase in TA at a rate of 0.5 ± 0.1 µmol kg −1 yr −1 on the 26.9σ θ surface (Table 2) can account for the slower decline in and CO 2− 3 concentration in subsurface waters.…”

“…In subsurface water, pH decreases (-0.003 yr −1 at Station ALOHA (22.75 • N; 158 • W; Dore et al (2009) and 0.004 yr −1 in North Pacific Intermediate Water; Byrne et al (2010)) have been enhanced by the accumulation of anthropogenic CO 2 and by natural variability related to temporal changes in physical and biogeochemical processes such as ocean ventilation, and remineralization of organic matter related to apparent oxygen utilization (AOU). The downward transport of anthropogenic CO 2 taken up by the oceans since the preindustrial era has caused shoaling of the CaCO 3 saturation horizons of both aragonite and calcite in the North Pacific (Feely and Chen, 1982;Feely et al, 2004Feely et al, , 2012Murata and Saito, 2012). These results are caused by an increase of H + (i.e., a decrease of pH) and a concurrent decrease of CO 2− 3 concentration, and will impact many marine calcifying species in surface and subsurface water of North Pacific (e.g., coccolithophores, foraminifera, and pteropods).…”