Pacific Anthropogenic Carbon Between 1991 and 2017

Abstract

We estimate anthropogenic carbon (Canth) accumulation rates in the Pacific Ocean between 1991 and 2017 from 14 hydrographic sections that have been occupied two to four times over the past few decades, with most sections having been recently measured as part of the Global Ocean Ship‐based Hydrographic Investigations Program. The rate of change of Canth is estimated using a new method that combines the extended multiple linear regression method with improvements to address the challenges of analyzing multiple occupations of sections spaced irregularly in time. The Canth accumulation rate over the top 1,500 m of the Pacific increased from 8.8 (±1.1, 1σ) Pg of carbon per decade between 1995 and 2005 to 11.7 (±1.1) PgC per decade between 2005 and 2015. For the entire Pacific, about half of this decadal increase in the accumulation rate is attributable to the increase in atmospheric CO2, while in the South Pacific subtropical gyre this fraction is closer to one fifth. This suggests a substantial enhancement of the accumulation of Canth in the South Pacific by circulation variability and implies that a meaningful portion of the reinvigoration of the global CO2 sink that occurred between ~2000 and ~2010 could be driven by enhanced ocean Canth uptake and advection into this gyre. Our assessment suggests that the accuracy of Canth accumulation rate reconstructions along survey lines is limited by the accuracy of the full suite of hydrographic data and that a continuation of repeated surveys is a critical component of future carbon cycle monitoring.

Contrasting marine carbonate systems in two fjords in British Columbia, Canada: Seawater buffering capacity and the response to anthropogenic CO2 invasion

“…Both fjords displayed lower TCO 2_Anth content relative to offshore waters, which was consistent with regional differences in buffering capacity and Revelle factor [13,82]. For instance, estimated TCO 2_Anth in the surface layer (Table 1) was lower compared to offshore Pacific surface water (~65 μmol kg -1 , [62,63]) where Revelle factors are typically lower than in coastal regions [83]). Deep layer mean TCO 2_Anth (Table 1) was similarly below recent estimates for near-bottom shelf waters (39 μmol kg -1 , [84]) and the California Undercurrent off the Washington coast (36 μmol kg -1 , [69]).…”

“…Given the trajectory of atmospheric pCO 2 , sub-surface water would have experienced lower atmospheric pCO 2 than modern surface water and therefore contains lower TCO 2_Anth content (e.g. [62,63]) We considered the surface layer to vertically mix sufficiently to interact with current atmospheric pCO 2 (Figs 2 and 3).…”

Technical note: Interpreting pH changes

“…The third opportunity to improve clarity concerns the interpretation of changes with depth, such as those between repeat hydrography line occupations or model time steps. Recently, the magnitude of chemical changes between repeat hydrographic sections has been inferred using various linear regression techniques (Carter et al, 2019;Chen et al, 2017;Chu et al, 2016;Williams et al, 2015;Woosley et al, 2016) and water mass characterization approaches Ríos et al, 2015), with results often plotted in terms of pH. Most ocean regions exhibit a larger range of pH in the upper 1000 m of the water column (∼ 7.4-8.5; Lauvset et al, 2016Lauvset et al, , 2020 than across surface waters of the open ocean (∼ 7.7-8.5; Lauvset et al, 2016;Fassbender et al, 2017).…”

Rapid deep ocean deoxygenation and acidification threaten life on Northeast Pacific seamounts

“…Only the tallest summits (Union and Dellwood, Table 1) are shallow enough to experience mixing with the relatively young (5–10 years) thermocline which is ventilated in the Northwest Pacific (NWP; Ueno & Yasuda, 2003; Whitney et al., 2007). Anthropogenic carbon is expected in this water (Carter et al., 2019; Wakita, Watanabe, Murata, Tsurushima, & Honda, 2010) and drives a reduction in Ω (Figure 2). The oxygen trends in this zone (~120–700 m depth in the NEP) are consistent with similar observations on common density surfaces near the ventilation region (Nakanowatari, Ohshima, & Wakatsuchi, 2007; Sasano et al., 2015) and previous local analyses (Crawford & Peña, 2016; Whitney et al., 2007).…”

“…Similarly, Ω horizons between the top of the OMZ and the permanent pycnocline, where significant DIC accumulation is expected (Carter et al, 2019), are shoaling rapidly ( Figure 2). Both Ω Ar = 0.7 and Ω Ca = 1 horizons exhibit strong trends despite the rapid deepening of isopycnals in that depth range (~2 m/year for 300-350 m; Figure 2b).…”