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The observation of extremely low radiocarbon content / old radiocarbon ages (>4000 years old) in the intermediate-depth ocean during the last ice age draws attention to our incomplete understanding of ocean carbon cycling. For example, glacial-interglacial seawater 14C anomalies near the Gulf of California have been explained by both the advection from a 14C-depleted abyssal source and local geologic carbon flux. To provide insight to this the origin of the seawater 14C anomalies, we have produced several new records of glacial-interglacial intermediate water (i.e., 14C, δ11B, δ18O, and δ13C) in waters that are “upstream” and “downstream” of the Gulf of California. These observations plus geochemical modeling allow us to: (1) Answer whether the old seawater 14C ages are advected or produced locally; (2) Identify the approximate chemical make-up of this carbon; and (3) Consider the role of known sedimentary processes in this carbon flux to the ocean. (Note that several sites have age model controls based on terrestrial plant 14C ages, providing more confidence in our results.) Our new measurements and modeling indicate that the well-established >4000-year-old seawater 14C anomalies observed near known seafloor volcanism in the Gulf of California are not present “upstream,” indicating that this carbon flux results from a “local” geologic carbon. Furthermore, based on our new benthic foraminifera δ11B measurements, this local carbon Blux does not appear to affect seawater pH. Finally, we suggest several potential geologic carbon source(s) that could explain the anomalously old seawater 14C ages, the relatively unremarkable changes in seawater δ13C, and the essentially negligible change in seawater pH. 

Abstract Planktonic calcifying organisms play a key role in regulating ocean carbonate chemistry and atmospheric CO 2 . Surprisingly, references to the absolute and relative contribution of these organisms to calcium carbonate production are lacking. Here we report quantification of pelagic calcium carbonate production in the North Pacific, providing new insights on the contribution of the three main planktonic calcifying groups. Our results show that coccolithophores dominate the living calcium carbonate (CaCO 3 ) standing stock, with coccolithophore calcite comprising ~90% of total CaCO 3 production, and pteropods and foraminifera playing a secondary role. We show that pelagic CaCO 3 production is higher than the sinking flux of CaCO 3 at 150 and 200 m at ocean stations ALOHA and PAPA, implying that a large portion of pelagic calcium carbonate is remineralised within the photic zone; this extensive shallow dissolution explains the apparent discrepancy between previous estimates of CaCO 3 production derived from satellite observations/biogeochemical modeling versus estimates from shallow sediment traps. We suggest future changes in the CaCO 3 cycle and its impact on atmospheric CO 2 will largely depend on how the poorly-understood processes that determine whether CaCO 3 is remineralised in the photic zone or exported to depth respond to anthropogenic warming and acidification. 

Abstract Ocean ventilation, or the transfer of tracers from the surface boundary layer into the ocean interior, is a critical process in biogeochemical cycles and the climate system. Here, we assess steady‐state ventilation patterns and timescales in three models of ocean transport: a 1 global configuration of the Nucleus for European Modeling of the Ocean (NEMO), a recent 2 solution of the Ocean Circulation Inverse Model (OCIM), and a 2 solution of the Total Matrix Intercomparison (TMI). We release artificial dyes in six surface regions of each model and compare equilibrium dye distributions as well as ideal age distributions. We find good qualitative agreement in large‐scale dye distributions across the three models. However, the distributions indicate that TMI and OCIM are more diffusive than NEMO. A shallow bias of North Atlantic ventilation in NEMO contributes to a stronger presence of the North Atlantic dye in the mid‐depth Southern Ocean and Pacific. This isopycnal communication between the North Atlantic surface and the mid‐depth Pacific is very slow, however, and NEMO simulates a maximum age in the North Pacific (NP) about 900 years higher than the data‐constrained models. Overly slow NP ventilation persists across NEMO sensitivity experiments encompassing our current best knowledge of diapycnal and isopycnal mixing, pointing to biases in subarctic Pacific dynamics. This study provides a synoptic picture of deep ocean ventilation and a framework for assessing its representation in general circulation models. 

A compilation of radiocarbon measurements is used to characterize deep-sea overturning since the last ice age. 

Modeling emission lines from the millimeter to the UV and producing synthetic spectra is crucial for a good understanding of observations, yet it is an art filled with hazards. This is the proceedings of “Walking the Line”, a 3-day conference held in 2018 that brought together scientists working on different aspects of emission line simulations, in order to share knowledge and discuss the methodology. Emission lines across the spectrum from the millimeter to the UV were discussed, with most of the focus on the interstellar medium, but also some topics on the circumgalactic medium. The most important quality of a useful model is a good synergy with observations and experiments. Challenges in simulating line emission are identified, some of which are already being worked upon, and others that must be addressed in the future for models to agree with observations. Recent advances in several areas aiming at achieving that synergy are summarized here, from micro-physical to galactic and circum-galactic scale.