Variability in tropical thermocline nutrient chemistry on the glacial/interglacial timescale

Loubere, Paul; Richaud, Mathieu; Mireles, Susan

doi:10.1016/j.dsr2.2007.01.005

Cited by 18 publications

(29 citation statements)

References 62 publications

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“…Nevertheless, these data indicate a substantial change in the depth and possibly formation of SOIW during the deglaciation with a deeper and more southward formed SOIW. At the same time at roughly 18 ka, the reestablishment of deep EUC waters with a mainly southern‐sourced water mass occurred (Loubere et al, , ). A δ 13 C study on thermocline‐dwelling Neogloboquadrina dutertrei demonstrates that the onset of deglacial δ 13 C minima events in the EEP occur simultaneously with the initiation of Southern Ocean warming (Spero & Lea, ).…”

Section: Discussionmentioning

confidence: 99%

Alternating Influence of Northern Versus Southern‐Sourced Water Masses on the Equatorial Pacific Subthermocline During the Past 240 ka

et al. 2017

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The eastern equatorial Pacific (EEP) is a key area to understand past oceanic processes that control atmospheric CO2 concentrations. Many studies argue for higher nutrient concentrations by enhanced nutrient transfer via Southern Ocean Intermediate Water (SOIW) to the low‐latitude Pacific during glacials. Recent studies, however, argue against SOIW as the primary nutrient source, at least during early Marine Isotope Stage 2 (MIS 2), as proxy data indicate that nutrients are better utilized in the Southern Ocean under glacial conditions. New results from the subarctic Pacific suggest that enhanced convection of nutrient‐rich Glacial North Pacific Intermediate Water (GNPIW) contributes to changes in nutrient concentrations in equatorial subthermocline water masses during MIS 2. However, the interplay between SOIW versus GNPIW and its influence on the nutrient distribution in the EEP spanning more than one glacial cycle are still not understood. We present a carbon isotope (δ13C) record of subthermocline waters derived from deep‐dwelling planktonic foraminifera Globorotaloides hexagonus in the EEP, which is compared with published δ13C records around the Pacific. Results indicate enhanced influence of GNPIW during MIS 6 and MIS 2 compared to today with largest contributions of northern‐sourced intermediate waters during glacial maxima. These observations suggest a mechanistic link between relative contributions of northern and southern intermediate waters and past EEP nutrient concentrations. A switch from increased GNPIW (decreased SOIW) to diminished GNPIW (enhanced SOIW) influence on equatorial subthermocline waters is recognized during glacial terminations and marks changes to modern‐like conditions in nutrient concentrations and biological productivity in the EEP.

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Section: Discussionmentioning

confidence: 99%

Alternating Influence of Northern Versus Southern‐Sourced Water Masses on the Equatorial Pacific Subthermocline During the Past 240 ka

et al. 2017

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“…The glacial‐interglacial climate cycles are imprinted on biogeochemical changes recorded in the sediments of the ETP. Variations in nutrient supply, export productivity and water column denitrification are related to both this regional interplay of biological and physical processes [ Toggweiler et al , 1991] and to remote, global scale climate [ Lea et al , 2006], circulation [ M. Kienast et al , 2006; Meissner et al , 2005; Toggweiler et al , 1991], and biogeochemical [ Loubere et al , 2007; Robinson et al , 2007; Spero and Lea , 2002] changes. The best available estimates of export productivity, from 230 Thorium ( 230 Th) normalized accumulation rate measurements, present a coherent pattern of change throughout the EEP since the last ice age.…”

Section: Introductionmentioning

confidence: 99%

Nitrogen isotopic evidence for deglacial changes in nutrient supply in the eastern equatorial Pacific

et al. 2009

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[1] The Eastern Equatorial Pacific (EEP) is a high nutrient-low chlorophyll region of the ocean. Downcore nitrogen isotope records from the EEP have been previously interpreted as a direct reflection of changes in nutrient consumption. However, the observed changes in sedimentary d 15 N since the last glacial maximum have no coherent relationship with export productivity or an inferred variation in the iron-to-nitrate ratio of the surface waters. Rather, downcore N isotope records in the EEP strongly resemble changes in the extent of water column denitrification as recorded in nearby sedimentary d 15 N records along the western margin of the Americas. This similarity is attributed to the overprinting of the N isotopic composition of nitrate in the EEP through the advection of nitrate westward from the margins in the subsurface. A local nitrogen isotope record of changes in the degree of nutrient consumption is extracted from the bulk sedimentary record by subtracting two different sedimentary d15 N records of denitrification changes from two new EEP d 15N records (TR163-22 and ODP Site 1240). The denitrification records used are from 1) the Central American margin (ODP Site 1242) and 2) the South American margin (GeoB7139-2). The degree of consumption in the surface waters declines rapidly from elevated values during the last glacial maximum to a pair of minima around 15 and 11-13 ka, and finally it increases into the Holocene. The derived EEP nitrogen isotope record indicates that the regional peak in export productivity occurred when the supply of nutrients exceeded the apparently high demand. The influx of nutrients during the deglaciation is attributed to the resumption of intense overturning in the Southern Ocean and the release of sequestered CO 2 and nutrient-rich, O 2 poor waters from the deep ocean. This has important implications for understanding the glacial-interglacial scale variation in intermediate water suboxia and water column denitrification.

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“…The affected regions include waters south of the Antarctic Polar Front (Francois et al, 1997;Jaccard et al, 2013), the North Pacific (Crusius et al, 2004;Jaccard et al, 2005;Kohfeld and Chase, 2011;Ortiz et al, 2004), the tropical Indian Ocean (Singh et al, 2011) and the equatorial Pacific (Costa et al, 2016;Herguera, 2000;Loubere et al, 2007). A weaker export production in these regions would have offset a strengthened biological pump in the subantarctic, thereby weakening the ability of the ocean to store carbon during glacial conditions.…”

Section: Introductionmentioning

confidence: 99%

The simulated climate of the Last Glacial Maximum and insights into the global marine carbon cycle

et al. 2016

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Abstract. The ocean's ability to store large quantities of carbon, combined with the millennial longevity over which this reservoir is overturned, has implicated the ocean as a key driver of glacial-interglacial climates. However, the combination of processes that cause an accumulation of carbon within the ocean during glacial periods is still under debate. Here we present simulations of the Last Glacial Maximum (LGM) using the CSIRO Mk3L-COAL (Carbon-OceanAtmosphere-Land) earth system model to test the contribution of physical and biogeochemical processes to ocean carbon storage. For the LGM simulation, we find a significant global cooling of the surface ocean (3.2 • C) and the expansion of both minimum and maximum sea ice cover broadly consistent with proxy reconstructions. The glacial ocean stores an additional 267 Pg C in the deep ocean relative to the pre-industrial (PI) simulation due to stronger Antarctic Bottom Water formation. However, 889 Pg C is lost from the upper ocean via equilibration with a lower atmospheric CO 2 concentration and a global decrease in export production, causing a net loss of carbon relative to the PI ocean. The LGM deep ocean also experiences an oxygenation (> 100 mmol O 2 m −3 ) and deepening of the calcite saturation horizon (exceeds the ocean bottom) at odds with proxy reconstructions. With modifications to key biogeochemical processes, which include an increased export of organic matter due to a simulated release from iron limitation, a deepening of remineralisation and decreased inorganic carbon export driven by cooler temperatures, we find that the carbon content of the glacial ocean can be sufficiently increased (317 Pg C) to explain the reduction in atmospheric and terrestrial carbon at the LGM (194 ± 2 and 330 ± 400 Pg C, respectively). Assuming an LGM-PI difference of 95 ppm pCO 2 , we find that 55 ppm can be attributed to the biological pump, 28 ppm to circulation changes and the remaining 12 ppm to solubility. The biogeochemical modifications also improve model-proxy agreement in export production, carbonate chemistry and dissolved oxygen fields. Thus, we find strong evidence that variations in the oceanic biological pump exert a primary control on the climate.

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Variability in tropical thermocline nutrient chemistry on the glacial/interglacial timescale

Cited by 18 publications

References 62 publications

Alternating Influence of Northern Versus Southern‐Sourced Water Masses on the Equatorial Pacific Subthermocline During the Past 240 ka

Alternating Influence of Northern Versus Southern‐Sourced Water Masses on the Equatorial Pacific Subthermocline During the Past 240 ka

Nitrogen isotopic evidence for deglacial changes in nutrient supply in the eastern equatorial Pacific

The simulated climate of the Last Glacial Maximum and insights into the global marine carbon cycle

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