Productivity patterns in the equatorial Pacific over the last 30,000 years

Abstract

The equatorial Pacific traverses a number of productivity regimes, from the highly productive coastal upwelling along Peru to the near gyre-like productivity lows along the international dateline, making it an ideal target for investigating how biogeochemical systems respond to changing oceanographic conditions over time. However, conflicting reconstructions of productivity during periods of rapid climate change, like the last deglaciation, render the spatiotemporal response of equatorial Pacific productivity ambiguous. In this study, surface productivity since the last glacial period (30,000 years ago) is reconstructed from seven cores near the Line Islands, central equatorial Pacific, and integrated with productivity records from across the equatorial Pacific. Three coherent deglacial patterns in productivity are identified: (1) a monotonic glacial-Holocene increase in productivity, primarily along the Equator, associated with increasing nutrient concentrations over time; (2) a deglacial peak in productivity~15,000 years ago due to transient entrainment of nutrient rich southern-sourced deep waters; and (3) possible precessional cycles in productivity in the eastern equatorial Pacific that may be related to Intertropical Convergence Zone migration and potential interactions with El Niño-Southern Oscillation dynamics. These findings suggest that productivity was generally lower during the glacial period, a trend observed zonally across the equatorial Pacific, while deglacial peaks in productivity may be prominent only in the east.

Deep‐Sea Oxygen Depletion and Ocean Carbon Sequestration During the Last Ice Age

“…In both CEP cores we find lower preserved fluxes of opal and xsBa during the LGP than during the postglacial period (Figures a and b), consistent with findings throughout the CEP indicating that export production during the last ice age was less than during the Holocene (Costa et al, ). In contrast to these inorganic proxies, the preserved fluxes of C 37 alkenones, produced by coccolithophoridae (Volkman et al, ) and of 24‐methylcholesta‐5,22E‐dien‐3 β ‐ol (brassicasterol), produced by pennate diatoms (Rampen et al, ) and other taxa (Volkman et al, ), during the LGP are as much as an order of magnitude greater than during the postglacial period (Figures c and d).…”

“…At MANOP Site C (0.878°N, 138.955°W), near the location of our TT013 cores (Figure and Table ), Prahl et al () described eightfold greater accumulation of C 37 alkenones during the last glacial maximum compared to the Holocene, similar to our results (Figure c), as well as fifteenfold greater accumulation of dinosterol, a compound produced by dinoflagellates. Therefore, with similar results from three sites (TT013‐PC18, TT013‐PC72, and MANOP Site C) and for lipid biomarkers representing three major taxonomic groups (diatoms, prymnesiophytes, and dinoflagellates, represented by brassicasterol, C 37 alkenones, and dinosterol, respectively), combined with widespread evidence for lower export production in the CEP during the LGP (this work, Costa et al, , ; Winckler et al, ), one can be confident that the order of magnitude greater accumulation of these lipid biomarkers during the LGP reflects greater preservation of OM and not simply a shift from one dominant phytoplankton taxon to another.…”

230Th Normalization: New Insights on an Essential Tool for Quantifying Sedimentary Fluxes in the Modern and Quaternary Ocean

“…Along the equatorial Pacific, Holocene mass fluxes average about 1 g/cm 2 kyr, with a latitudinal gradient that mirrors the decreasing productivity trend with increasing distance from the nutrient‐rich zone of equatorial upwelling. For example, at the Line Islands (approximately −160°E), Holocene mass fluxes along a latitudinal transect of nine sites steadily decrease from ~1.8 g/cm 2 kyr at the equator (0.2°S) to 0.8 g/cm 2 kyr at the northernmost site (7.0°N) (Costa et al, , ; Jacobel et al, ), a trend that is not captured in age model‐based mass accumulation rates. The equatorial Pacific also manifests a distinct zonal distance effect (supporting information Figure S6), with the lowest mass fluxes occurring in the central equatorial Pacific (~0.5 g/cm 2 kyr) and increasing more or less monotonically toward the continental margins.…”

“…This situation creates a concentration gradient in the water column that in turn generates a dispersive transport (advection + eddy diffusion) of the affected element toward the high‐particle‐flux zone, a process called boundary scavenging, as it was first identified at continental boundaries (Bacon et al, ). Boundaries are now defined more broadly, and they can include productivity gradients such as those driven by upwelling in the central equatorial Pacific (e.g., Costa et al, ), which can occur far from any continental margin.…”

Paleoproductivity and Stratification Across the Subarctic Pacific Over Glacial‐Interglacial Cycles

“…In this study, we generate new records of productivity ( 231 Pa/ 230 Th, opal flux, and excess silica flux) from the East Subarctic Pacific (approximately 45°N, 230°E, 2,700 m) over the last two glacial cycles. Disagreement between various productivity proxies is not uncommon and may lead to ambiguous paleo‐productivity trends (e.g., in the equatorial Pacific; Anderson & Winckler, ; Costa et al, ), and in the Subarctic (Costa et al, ; Serno et al, ). Because 231 Pa/ 230 Th is the productivity proxy least susceptible to preservation biases, it may provide the most reliable reconstruction of paleoproductivity while allowing the qualitative assessment of the impact of preservation and diagenesis on other proxies, particularly opal, in the Subarctic Pacific.…”