Seasonal and long‐term dynamics of the upper ocean carbon cycle at Station ALOHA near Hawaii

Abstract

[1] Long-term trends and average seasonal variability in the upper ocean carbon cycle are investigated at Station ALOHA, the site of the U.S. JGOFS Hawaii Ocean Time series program (HOT), on the basis of a 14-year time series (1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002) . More than half of the rates of change in sDIC and oceanic pCO 2 , and most of the change in 13 C/ 12 C, are attributed to the uptake of isotopically light anthropogenic CO 2 from the atmosphere. The residual trends appear to be caused mainly by a regional change in the net freshwater budget, perhaps associated with a regime change of the North Pacific climate system near 1997. Computed oceanic pCO 2 is below atmospheric pCO 2 for nearly the entire year, leading to an annual mean surface ocean pCO 2 undersaturation of about 18 matm, and to an annual uptake of CO 2 from the atmosphere, which we compute to be 1.0 ± 0.1 mol m À2 yr À1 . We estimate that about 30% of this flux relates to the uptake of anthropogenic CO 2 , and the remainder to biologically mediated export of organic carbon. Using a modified version of the diagnostic model of Gruber et al. [1998], constrained by d 13 C oc , we infer net community production of organic carbon (NCP) to be the dominant process generating the observed seasonal variability in sDIC. The annual integral of NCP, 2.3 ± 0.8 mol m À2 yr À1 , is comparable to previous estimates of biological production in the subtropical North Pacific. Annually integrated fluxes of air-sea gas exchange and NCP at Station ALOHA are each about two thirds of those computed for the upper ocean near Bermuda using similar methods of estimation [Gruber et al., 1998. However, the seasonal amplitudes of sDIC and d 13 C oc near Hawaii are only half as large as near Bermuda, because air-sea gas exchange and NCP tend to oppose each other near Hawaii, but reinforce each other near Bermuda.

Physical and biogeochemical modulation of ocean acidification in the central North Pacific

“…4) is related mainly to factors governing the relative input and output fluxes of DIC. Air-sea exchange, mixing, and entrainment, horizontal transport, and biological drawdown and export all contribute to the net DIC balance (15,16,18). We see in Fig.…”

“…Once these waters are isolated from the atmosphere, biological remineralization of organic carbon at depth may continue to impact their DIC content in a nonconservative manner during their transit to ALOHA. Organic carbon export from the euphotic zone, via gravitational settling of particles and by downward turbulent mixing of dissolved compounds, has varied considerably over our study period (16,18,23). About 60% of this passively exported carbon is converted back to DIC through microbiological respiration between 150-300 m, and another 20% is remineralized between 300-500 m (23,24).…”

“…The 8-year record from Tatoosh Island on the North American west coast revealed a more rapid pH decline, but this data set is complicated by very strong seasonal and even daily variability several times greater than corresponding pH variability at the open ocean sites (13). Although the factors driving seasonal and interannual variability in surface DIC, TA, and pCO 2oce at the oceanic time-series stations have been the foci of numerous reports (11,12,(14)(15)(16)(17)(18), less attention has been given to surface pH variability at these sites (12), and their subsurface pH has not yet been discussed.…”

Coral reefs and changing seawater carbonate chemistry

“…Because coral reef ecosystems are confined to the upper mixed layer of the ocean, they will be readily exposed to these shifts in the ocean carbonate equilibrium. (Figure 3) [Bates, 2001;Keeling et al, 2004].…”

Variability of oceanic carbon cycle in the North Pacific from seasonal to decadal scales

“…On interannual time scales, studies of the carbon cycle are limited to a few sites where long-term time series of observations are available, such as the Hawaii Ocean Time-series (HOT) [Dore et al, 2003;Brix et al, 2004;Keeling et al, 2004], the Ocean Station Papa [Signorini et al, 2001;Wong et al, 2002], the South East Asia Time-Series Station (SEATS) [Sheu et al, 2010], and a few sections in the western North Pacific [Midorikawa et al, 2005]. Takahashi et al…”