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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. 

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

Abstract Planktonic calcifying organisms play a key role in regulating ocean carbonate chemistry and atmospheric CO₂. 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 (CaCO₃) standing stock, with coccolithophore calcite comprising ~90% of total CaCO₃production, and pteropods and foraminifera playing a secondary role. We show that pelagic CaCO₃production is higher than the sinking flux of CaCO₃at 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 CaCO₃production derived from satellite observations/biogeochemical modeling versus estimates from shallow sediment traps. We suggest future changes in the CaCO₃cycle and its impact on atmospheric CO₂will largely depend on how the poorly-understood processes that determine whether CaCO₃is remineralised in the photic zone or exported to depth respond to anthropogenic warming and acidification. 

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. 

Abstract North Pacific atmospheric and oceanic circulations are key missing pieces in our understanding of the reorganization of the global climate system since the Last Glacial Maximum. Here, using a basin‐wide compilation of planktic foraminiferal δ¹⁸O, we show that the North Pacific subpolar gyre extended ~3° further south during the Last Glacial Maximum, consistent with sea surface temperature and productivity proxy data. Climate models indicate that the expansion of the subpolar gyre was associated with a substantial gyre strengthening, and that these gyre circulation changes were driven by a southward shift of the midlatitude westerlies and increased wind stress from the polar easterlies. Using single‐forcing model runs, we show that these atmospheric circulation changes are a nonlinear response to ice sheet topography/albedo and CO₂. Our reconstruction indicates that the gyre boundary (and thus westerly winds) began to migrate northward at ~16.5 ka, driving changes in ocean heat transport, biogeochemistry, and North American hydroclimate. 

Abstract The cycling of biologically produced calcium carbonate (CaCO₃) in the ocean is a fundamental component of the global carbon cycle. Here, we present experimental determinations of in situ coccolith and foraminiferal calcite dissolution rates. We combine these rates with solid phase fluxes, dissolved tracers, and historical data to constrain the alkalinity cycle in the shallow North Pacific Ocean. The in situ dissolution rates of coccolithophores demonstrate a nonlinear dependence on saturation state. Dissolution rates of all three major calcifying groups (coccoliths, foraminifera, and aragonitic pteropods) are too slow to explain the patterns of both CaCO₃sinking flux and alkalinity regeneration in the North Pacific. Using a combination of dissolved and solid‐phase tracers, we document a significant dissolution signal in seawater supersaturated for calcite. Driving CaCO₃dissolution with a combination of ambient saturation state and oxygen consumption simultaneously explains solid‐phase CaCO₃flux profiles and patterns of alkalinity regeneration across the entire N. Pacific basin. We do not need to invoke the presence of carbonate phases with higher solubilities. Instead, biomineralization and metabolic processes intimately associate the acid (CO₂) and the base (CaCO₃) in the same particles, driving the coupled shallow remineralization of organic carbon and CaCO₃. The linkage of these processes likely occurs through a combination of dissolution due to zooplankton grazing and microbial aerobic respiration within degrading particle aggregates. The coupling of these cycles acts as a major filter on the export of both organic and inorganic carbon to the deep ocean.