Glacial deep-water properties in the west-equatorial Pacific: bathyal thermocline near a depth of 2000 m (original) (raw)
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Quaternary Science Reviews, 2010
Intermediate ocean circulation changes during the last Glacial Maximum (LGM) in the North Pacific have been linked with Northern Hemisphere climate through airesea interactions, although the extent and the source of the variability of the processes forcing these changes are still not well resolved. The ventilated volumes and ages in the upper wind driven layer are related to the wind stress curl and surface buoyancy fluxes at mid to high latitudes in the North Pacific. In contrast, the deeper thermohaline layers are more effectively ventilated by direct atmosphere-sea exchange during convective formation of Subantarctic Mode Waters (SAMW) and Antarctic Intermediate Waters (AAIW) in the Southern Ocean, the precursors of Pacific Intermediate Waters (PIW) in the North Pacific. Results reported here show a fundamental change in the carbon isotopic gradient between intermediate and deep waters during the LGM in the eastern North Pacific indicating a deepening of nutrient and carbon rich waters. These observations suggest changes in the source and nature of intermediate waters of Southern Ocean origin that feed PIW and enhanced ventilation processes in the North Pacific, further affecting paleoproductivity and export patters in this basin. Furthermore, oxygen isotopic results indicate these changes may have been accomplished in part by changes in circulation affecting the intermediate depths during the LGM.
2007
a b s t r a c t Intermediate ocean circulation changes during the last Glacial Maximum (LGM) in the North Pacific have been linked with Northern Hemisphere climate through airesea interactions, although the extent and the source of the variability of the processes forcing these changes are still not well resolved. The ventilated volumes and ages in the upper wind driven layer are related to the wind stress curl and surface buoyancy fluxes at mid to high latitudes in the North Pacific. In contrast, the deeper thermohaline layers are more effectively ventilated by direct atmosphere-sea exchange during convective formation of Subantarctic Mode Waters (SAMW) and Antarctic Intermediate Waters (AAIW) in the Southern Ocean, the precursors of Pacific Intermediate Waters (PIW) in the North Pacific. Results reported here show a fundamental change in the carbon isotopic gradient between intermediate and deep waters during the LGM in the eastern North Pacific indicating a deepening of nutrient and carbon rich waters. These observations suggest changes in the source and nature of intermediate waters of Southern Ocean origin that feed PIW and enhanced ventilation processes in the North Pacific, further affecting paleoproductivity and export patters in this basin. Furthermore, oxygen isotopic results indicate these changes may have been accomplished in part by changes in circulation affecting the intermediate depths during the LGM.
Reduced oxygenation at intermediate depths of the southwest Pacific during the last glacial maximum
Earth and Planetary Science Letters
To investigate changes in oxygenation at intermediate depths in the southwest Pacific between the Last Glacial Maximum (LGM) and the Holocene, redox sensitive elements uranium and rhenium were measured in 12 sediment cores located on the Campbell and Challenger plateaux offshore from New Zealand. The core sites are currently bathed by Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW) and Upper Circumpolar Deep Water (UCDW). The sedimentary distributions of authigenic uranium and rhenium reveal reduced oxygen content at intermediate depths (800-1500 m) during the LGM compared to the Holocene. In contrast, data from deeper waters (≥ 1500 m) indicate higher oxygen content during the LGM compared to the Holocene. These data, together with variations in benthic foraminiferal δ 13 C, are consistent with a shallower AAIW-UCDW boundary over the Campbell Plateau during the LGM. Whilst AAIW continued to bathe the intermediate depths (≤ 1500 m) of the Challenger Plateau during the LGM, the data suggest that the AAIW at these core sites contained less oxygen compared to the Holocene. These results are at odds with the general notion that AAIW
Paleoceanography, 2004
1] Deep-sea sediment core FR1/97 GC-12 is located 990 mbsl in the northern Tasman Sea, southwest Pacific, where Antarctic Intermediate Water (AAIW) presently impinges the continental slope of the southern Great Barrier Reef. Analysis of carbon (d 13 C) and oxygen (d 18 O) isotope ratios on a suite of planktonic and benthic foraminifera reveals rapid changes in surface and intermediate water circulation over the last 30 kyr. During the Last Glacial Maximum, there was a large d 13 C offset (1.1%) between the surface-dwelling planktonic foraminifera and benthic species living within the AAIW. In contrast, during the last deglaciation (Termination 1), the d 13 C planktonic-benthic offset reduced to 0.4% prior to an intermediate offset (0.7%) during the Holocene. We suggest that variations in the dominance and direction of AAIW circulation in the Tasman Sea, and increased oceanic ventilation, can account for the rapid change in the water column d 13 C planktonic-benthic offset during the glacial-interglacial transition. Our results support the hypothesis that intermediate water plays an important role in propagating climatic changes from the polar regions to the tropics. In this case, climatic variations in the Southern Hemisphere may have led to the rapid ventilation of deep water and AAIW during Termination 1, which contributed to the postglacial rise in atmospheric CO 2 .
Changes in Eastern Pacific ocean ventilation at intermediate depth over the last 150 kyr BP
Earth and Planetary Science Letters, 2010
The circulation patterns of the deep glacial Pacific Ocean are still debated. Difficulties arise due to the scarcity of reliable paleoceanographic records that can document the past movements and properties of Pacific Ocean water masses. Here, we jointly use δ 13 C and δ 18 O measured on the epibenthic foraminifer Cibicidoides wuellerstorfi, from the MD02-2529 sediment core collected at 1619 m water depth in the eastern equatorial Pacific, to monitor changes in water mass circulation spanning the past 150 kyr BP. After the extraction of short-term (centennial to millennial-scale) δ 13 C and δ 18 O changes, which were~1.0 and 0.5‰, respectively, we observed that these rapid δ 13 C and δ 18 O shifts were closely interrelated during the last 150 kyr BP. A comparison of MD02-2529 with other benthic δ 13 C records localized to the north and south of the core location revealed that MD02-2529 was alternately bathed by a northern nutrient-rich and a southern nutrient-poor water mass. The comparison provided a diagnostic for the latitudinal movements of a sharp water mass front that was particularly evident during marine isotope stages 4 and 3 on the millennial timescale. By considering that δ 13 C is an indicator of the northern vs. southern origin of the water that bathed the MD02-2529 coring site in the past, we found that a North Pacific water mass, that occasionally spreads to the eastern Pacific Ocean as deep as 1600 m and as far south as 8°N, was responsible for shifts toward the positive δ 18 O we observed in the past. We then used the δ 13 C/δ 18 O relationship to reconstruct latitudinal temperature and/or salinity gradients of the water mass that were linked to changes in the northern and/or the southern water mass end-members. Evolution of the δ 13 C/δ 18 O relationship spanning the past 150 kyr BP has shed light on how hydrological processes occurring at northern and southern high latitudes are transmitted to the ocean's interior through water mass advection.
Quaternary Science Reviews, 2010
In piston cores from the open subarctic Pacific and the Okhotsk Sea, diatom-bound d 15 N (d 15 N db ), biogenic opal, calcium carbonate, and barium were measured from coretop to the previous glacial maximum (MIS 6). Glacial intervals are generally characterized by high d 15 N db (w8&) and low productivity, whereas interglacial intervals have a lower d 15 N db (5.7e6.3&) and indicate high biogenic productivity. These data extend the regional swath of evidence for nearly complete surface nutrient utilization during glacial maxima, consistent with stronger upper water column stratification throughout the subarctic region during colder intervals. An early deglacial decline in d 15 N db of 2& at w17.5 ka, previously observed in the Bering Sea, is found here in the open subarctic Pacific record and arguably also in the Okhotsk, and a case can be made that a similar decrease in d 15 N db occurred in both regions at the previous deglaciation as well. The early deglacial d 15 N db decrease, best explained by a decrease in surface nutrient utilization, appears synchronous with southern hemisphere-associated deglacial changes and with the Heinrich 1 event in the North Atlantic. This d 15 N db decrease may signal the initial deglacial weakening in subarctic North Pacific stratification and/or a deglacial increase in shallow subsurface nitrate concentration. If the former, it would be the North Pacific analogue to the increase in vertical exchange inferred for the Southern Ocean at the time of Heinrich Event 1. In either case, the lack of any clear change in paleoproductivity proxies during this interval would seem to require an early deglacial decrease in the iron-to-nitrate ratio of subsurface nutrient supply or the predominance of light limitation of phytoplankton growth during the deglaciation prior to Bølling-Allerød warming.
Earth and Planetary Science Letters, 2016
Marine radiocarbon (14 C) is widely used to trace ocean circulation and the 14 C levels of interior ocean water masses can provide insight into atmosphere-ocean exchange of CO 2 the since the last glaciation. Using tephras as stratigraphic tie points with which to estimate past atmospheric 14 C, we reconstructed a series of deep radiocarbon ages for several time slices from the last glaciation through the deglaciation and Holocene in the Southwestern Pacific. Glacial ventilation ages were much greater in magnitude than modern and had a strong mid-depth 14 C minimum centered on ~2500 m. Glacial radiocarbon ages of intermediate depth waters (600-1200 m) were ~800 to 1600 14 C years, about twice modern and persisted through the early deglaciation. Notably, in the glaciation shallower depths were significantly more enriched in 14 C than waters between 1600-3800m, which were ~4000 to 6200 14 C years, or about 3-5 times older than modern. Abyssal waters deeper than 4000m were also more 14 C rich than the overlying deep water. With radiocarbon ages of 1800-2300 14 C years, this was similar to modern values. In the early deglaciation, 14 C depleted waters were flushed from shallower depths first and replaced with progressively younger waters such that by ~18 ka, the deep to intermediate age difference was reduced by half, and by ~14 ka a modern-type 14 C profile for deep ocean water masses was in place. Our results 1) confirm a deep 14 C depleted water mass during the LGM and early deglaciation, and 2) constrain the extent of this "old" water in the Southern Pacific as between 1600m and 3800m. The availability of atmospheric ages from tephras reveals that the presence of older surface reservoir ages in the glaciation caused the estimation of ventilation ages from simple benthic-planktonic offsets to significantly underestimate the depletion of 14 C in deep waters. This may have had a role in masking the large change in reservoir ages since the glaciation when using benthic-planktonic reservoir age estimates.
Paleoceanography and Paleoclimatology, 2019
Most conceptual models of ocean circulation during past glacial periods invoke a shallowed North Atlantic-sourced water mass overlying an expanded, poorly ventilated Southern Ocean (SO)-sourced deep water mass (Southern Component Water or SCW), rich in remineralized carbon, within the Atlantic basin. However, the ventilation state, carbon inventory, and circulation pathway of SCW sourced in the Pacific sector of the SO (Pacific SO) during glacial periods are less well understood. Here we present multiproxy data-including δ 18 O and δ 13 C measured on the benthic and planktic foraminifera Cibicidoides wuellerstorfi, and Neogloboquadrina pachyderma, and productivity proxies including percent CaCO 3 , total organic carbon, and Ba/Ti-from a sediment core located in the high-latitude (71°S) Pacific SO spanning the last 800 kyr. Typical glacial δ 13 C values of SCW at this core site are~0‰. We find no evidence for SCW with extremely low δ 13 C values during glacials in the high-latitude Pacific SO. This leads to a spatial gradient in the stable carbon isotope composition of SCW from the high-latitude SO, suggesting that there are different processes of deep-and bottom-water formation around Antarctica. A reduced imprint of air-sea gas exchange is evident in the SCW formed in the Atlantic SO compared with the Pacific SO. A spatial δ 13 C gradient in SCW is apparent throughout much of the last 800,000 years, including interglacials. A SO-wide depletion in benthic δ 13 C is observed in early MIS 16, coinciding with the lowest atmospheric pCO 2 recorded in Antarctic ice cores.
Paleoceanography and Paleoclimatology, 2018
Published estimates of the radiocarbon content of middepth waters suggest a decrease in ventilation in multiple locations during the last glacial maximum (LGM; 24.0-18.1 ka). Reduced glacial ventilation would have allowed respired carbon to accumulate in those waters. A subsequent deglacial release of this respired carbon reservoir to the atmosphere could then account for the observed increases in atmospheric CO 2 and decline in atmospheric radiocarbon content. However, age model error and a release of 14 C-depleted mantle carbon have also been cited as possible explanations for the observed middepth radiocarbon depletions, calling into question the deep ocean's role in storing respired carbon during the LGM. Joint measurements of benthic foraminiferal carbon isotope values (δ 13 C) and cadmium/calcium (Cd/Ca) ratios provide a method for isolating the air-sea component of a water mass from changes in remineralization. Here we use benthic foraminiferal δ 13 C and Cd/Ca records from the eastern equatorial Pacific to constrain changes in remineralization and water-mass mixing over the last glacial-interglacial transition. These records are complemented with elemental measurements of the authigenic coatings of foraminifera to monitor postdepositional changes in bottom water properties. Our results suggest an increase of deep waters at midwater depths consistent with a shoaling of the boundary between the upper and lower branches of Southern Ocean overturning circulation. Additionally, our records demonstrate increased organic matter remineralization in middepth waters during the LGM, suggesting that respired carbon did accumulate in middepth waters under periods of reduced ventilation. Plain Language Summary Records of gas bubbles trapped in ice suggest that the concentration of CO2 in the atmosphere was lower during cold, glacial periods when ice covered more of Earth. These records show that both atmospheric CO2 and temperature increased as Earth's climate transitioned from last glacial period, about 20,000 years ago, to the warmer climate Earth has experienced over the last 10,000 years. Data suggest that the oceans are a significant source of this CO2. During ocean circulation, a water mass sinks from the surface of the ocean where it is no longer exchanging CO2 with the atmosphere, allowing the CO2 to accumulate in those waters. To investigate whether the oceans stored CO2 during the last glacial period, we developed records of past ocean chemistry using the shells of small marine organisms that record the chemistry of the water they are living in as they build their shells. Our results from the eastern equatorial Pacific indicate that CO2 accumulated in these waters during the last glacial, likely as a result of changes in ocean circulation. By understanding how the oceans and atmosphere have exchanged CO2 in the past, we better understand what role the oceans may play under modern CO2 increases. 14 C-depleted reservoir.