Rolf E. Sonnerup

Reconstructing the oceanic 13C Suess Effect

The anthropogenic δ13C change for the time period 1968 to 1991 was determined based on calculations of the preformed 13C/12C of dissolved inorganic carbon (DIC) distributions on isopycnal surfaces in the main thermocline of the Pacific, North Atlantic and South Indian Oceans. The time rate of change of preformed δ13C (the 13C Suess effect) along isopycnals was calculated using CFC-derived water ages and yields a time history of the surface water δ13C change at the isopycnal outcrop location. The surface ocean Suess effect recorded on isopycnals decreased with increasing outcrop latitude from approximately −0.2‰ decade−1 within the subtropics to around −0.1‰ decade−1 in the subpolar oceans. In the Pacific Ocean these surface δ13C change rate reconstructions agree, both in magnitude and meridional trend, with direct observations of surface ocean δ13C changes reported from time series measurements and from comparisons of surface water δ13C of DIC measurements in 1970 and 1993. A global ocean average surface δ13C rate of change of −0.15 ± 0.04 ‰ decade−1 is determined, which is slightly smaller than a previous time series data and model-based estimate (−0.171‰ decade−1 , [Bacastow etal., 1996]). Depth integrations of the 13C reconstructions in the Pacific, Indian, and Atlantic Oceans, when combined with these previous individual depth profile comparisons and Geochemical Ocean Sections Study bomb 14C inventories [Quay et al, 1992], imply a global depth-integrated δ13C change rate of −9.7 ± 2.4‰ m yr−1 over the time period 1970–1990. These results imply a net oceanic CO2 uptake rate of 1.9 ± 0.9 Gt C yr−1 over the time period 1970–1990 when applied to an atmospheric 13CO2 and 12CO2 budget.

Anthropogenic chlorofluorocarbons in the Black Sea and the Sea of Marmara

Measurements of the distributions of two chlorofluorocarbons, CCl3F (CFC-11) and CCl2F2 (CFC-12), made during the June/July 1988 R. V. Knorr cruise in the Black Sea indicate that CFC-11 is non-conservative relative to CFC-12 in the strongly reducing anoxic waters. A multi-layer model combining horizontal/vertical renewal and vertical diffusion processes was tuned to the distributions of CFC-12 and salinity in suboxic and anoxic waters and used to estimate the residence times of subsurface waters and first-order in situ removal rate constants for CFC-11. The model-calculated residence times were B5 yr in the suboxic zone (80–120 m) and increased to B625 yr at 500 m. The first-order CFC-11 in situremoval coefficients were 0.52 70.19 yr � 1 for the depth range 160–480 m. Based on the observed CFC-12 concentrations, the residence time of cold intermediate layer water (B50–100 m) was estimated to be o5 yr. The residence time of the deep water (100–450 m) in the Sea of Marmara was estimated to be 12–19 yr using a box model tuned to CFC-12, implying an in situ oxygen utilization rate of 10–18mmol kg � 1 yr � 1 . r 2002 Elsevier Science Ltd. All

The Indian Ocean 13C Suess Effect

The δ13C of dissolved inorganic carbon (DIC) decrease (the 13C Suess effect) in the Indian Ocean was calculated using a multiparameter linear regression comparison of the 1978 Geochemical Ocean Sections Study and 1995 World Ocean Circulation Experiment δ13C of DIC, hydrographic, and nutrient data. The surface ocean δ13C decrease rate along 80° and 115°E ranged from approximately −0.1‰ decade−1 at 55°S to a maximum of approximately −0.18‰ decade−1 at ∼ 35°S and decreased northward to around −0.13‰ decade−1 at the equator. Using a global extrapolation based on ocean model results [Bacastow et al., 1996] and previous δ13C changes calculated from 1970 to 1993 station reoccupations in the Pacific Ocean and based on preformed δ13C versus chlorofluorocarbon age trends in the Indian, Pacific, and Atlantic Oceans [Sonnerup et al., 1999], an ocean-wide average surface ocean δ13C rate of change of −0.15 to −0.17‰ decade−1 was estimated. The average depth-integrated δ13C change rate between 1978 and 1995 along 80° and 115°E was −6.9 ± 0.5‰ m yr−1 from 58°S to 5°N.

Transit time distributions and oxygen utilization rates from chlorofluorocarbons and sulfur hexafluoride in the Southeast Pacific Ocean

Chlorofluorocarbons-11 (CFC-11), CFC-12, and sulfur hexafluoride (SF6) were measured during the December 2007 to February 2008 CLIVAR/Repeat Hydrography (RH) P18 section along ∼103°W in the Southeast Pacific Ocean. Transit-time distributions (TTDs) of 1-D transport that matched all three tracers were consistent with high Peclet number flow ventilating the subtropical mode water and the main subtropical thermocline (30°S–42°S, 200–800 m). In the subtropics, TTDs with predominantly advective transport predicted decadal increases in CFC-12 and CFC-11 consistent with those observed comparing 1994 WOCE with 2007/2008 CLIVAR/RH data, indicating steady ventilation in this region, and consistent with the near-zero changes observed in dissolved oxygen. The mean transport timescales from the tracer-tuned TTDs were used to estimate apparent oxygen utilization rates (OURs) on the order of 8–20 μmol kg−1 yr−1 at ∼200 m depth, attenuating to ∼2 μmol kg−1 yr−1 typically by 500 m depth in this region. Depth-integrated over the thermocline, these OURs implied carbon export rates from the overlying sea surface on the order of ∼1.8 moles C m−2 yr−1 from 30°S to 45°S, 2–2.5 moles C m−2 yr−1 from 45°S to 52°S, and 2.5–3.5 moles C m−2 yr−1 from 52°S to 60°S.

Anthropogenic δ 13 C changes in the North Pacific Ocean reconstructed using a multiparameter mixing approach (MIX)

A multiparameter mixing approach, ‘MIX’, for determining oceanic anthropogenic CO2 was used to reconstruct the industrial-era change in the 13C/12C of dissolved inorganic carbon (δ13C of DIC) along the 1992 165°E WOCE P13N section in the North Pacific Ocean. The back-calculation approach was tested against a known anthropogenic tracer, chlorofluorocarbon-11 (CFC-11), and also by reconstructing an ocean general circulation model’s (OGCM) anthropogenic δ13C change. MIX proved accurate to ±10% against measured CFC-11, but only to ± ±~25% reconstructing the OGCM’s δ13C change from 1992 model output. The OGCM’s CFC distribution was also poorly reconstructed using MIX, indicating that this test suffers from limitations in the OGCM’s representation of water masses in the ocean. The MIX industrial-era near-surface (200 m) δ13C change reconstructed from the WOCE P13N data ranged from .0.8° in the subtropics (15-30°N), to -0.6‰ in the tropics (10°N), and -0.4 to -0.2‰ north of 40°N. Depthintegrated changes along 165°E were -400‰.m to -500‰.m at low latitudes, and were smaller (-200‰.m) north of 40°N. The MIX North Pacific δ13C change is consistent with the global anthropogenic CO2 inventory of 118 ± 17 Pg from ΔC*

Anthropogenic CO2 accumulation and uptake rates in the Pacific Ocean based on changes in the 13C/12C of dissolved inorganic carbon

The anthropogenic CO2 accumulation rate for the Pacific Ocean was estimated from the decrease in δ13C of the dissolved inorganic carbon measured on six World Ocean Circulation Experiment cruises during the 1990s and repeated during Climate Variability and Predictability in the 2000s. A mean depth-integrated anthropogenic δ13C change of −83 ± 20‰ m decade−1 was estimated for the basin by using the multiple linear regression approach. The largest anthropogenic δ13C decreases occurred between 40°S and 60°S, whereas the smallest decreases occurred in the Southern Ocean and subpolar North Pacific. A mean anthropogenic CO2 accumulation rate of 0.41 ± 0.13 mol C m−2 yr−1 (0.82 ± 0.26 Pg C yr−1) was determined based on observed δ13C changes and is in agreement with previous observation- and model-based estimates. The mean dissolved inorganic carbon DIC13 inventory change of −178 ± 43‰ mol m−2 decade−1 was primarily the result of air-sea CO2 exchange acting on the measured air-sea δ13C disequilibrium of ~ −1.2 ± 0.1‰. Regional differences between the DIC13 inventory change and air-sea 13CO2 flux yielded net anthropogenic CO2 uptake rates (independent of ΔpCO2) that ranged from ~0 to 1 mol m−2 yr−1 and basin-wide mean of 1.2 ± 1.5 Pg C yr−1. High rates of surface ocean DIC increase and δ13C decrease observed in the Drake Passage (53°S–60°S) support above average anthropogenic CO2 accumulation since 2005. Observed δ13C changes in the Pacific Ocean indicate that ocean transport significantly impacted the anthropogenic CO2 distribution and illustrate the utility of δ13C as a tracer to unravel the processes controlling the present and future accumulation of anthropogenic CO2 in the ocean.

A Synoptic View of the Ventilation and Circulation of Antarctic Bottom Water from Chlorofluorocarbons and Natural Tracers

Antarctic Bottom Water (AABW) is the coldest, densest, most prolific water mass in the global ocean. AABW forms at several distinct regions along the Antarctic coast and feeds into the bottom limb of the meridional overturning circulation, filling most of the global deep ocean. AABW has warmed, freshened, and declined in volume around the globe in recent decades, which has implications for the global heat and sea level rise budgets. Over the past three decades, the use of tracers, especially time-varying tracers such as chlorofluorocarbons, has been essential to our understanding of the formation, circulation, and variability of AABW. Here, we review three decades of temperature, salinity, and tracer data and analysis that have led to our current knowledge of AABW and how the southern component of deep-ocean ventilation is changing with time.