Marine sediments from the North Pacific document two episodes of expansion and strengthening of the subsurface oxygen minimum zone (OMZ) accompanied by seafloor hypoxia during the last deglacial transition1,2,3,4. The mechanisms driving this hypoxia remain under debate1,2,3,4,5,6,7,8,9,10,11. We present a new high-resolution alkenone palaeotemperature reconstruction from the Gulf of Alaska that reveals two abrupt warming events of 4–5 degrees Celsius at the onset of the Bølling and Holocene intervals that coincide with sudden shifts to hypoxia at intermediate depths. The presence of diatomaceous laminations and hypoxia-tolerant benthic foraminiferal species, peaks in redox-sensitive trace metals12,13, and enhanced 15N/14N ratio of organic matter13, collectively suggest association with high export production. A decrease in 18O/16O values of benthic foraminifera accompanying the most severe deoxygenation event indicates subsurface warming of up to about 2 degrees Celsius. We infer that abrupt warming triggered expansion of the North Pacific OMZ through reduced oxygen solubility and increased marine productivity via physiological effects; following initiation of hypoxia, remobilization of iron from hypoxic sediments could have provided a positive feedback on ocean deoxygenation through increased nutrient utilization and carbon export. Such a biogeochemical amplification process implies high sensitivity of OMZ expansion to warming.
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We thank J. Padman for assistance with faunal counts, K. Brewster for assistance with alkenone sample preparation and analysis, and A. Guiheneuf for preliminary alkenone measurements and faunal assemblage data. This work was supported by NSF grants AGS-0602395 (Project PALEOVAR, A.C.M.) and OCE-1204204 (A.C.M. and F.G.P.), and an NSF graduate research fellowship for S.K.P.; J.A.A. was supported by the USGS Climate and Land Use Change Research and Development Program and the Volcano Science Center.
The authors declare no competing financial interests.
Extended data figures and tables
Planktonic oxygen isotopes (Nps) from core EW0408-85JC (dark blue)2, benthic oxygen isotopes from core MD02-2503 in the Santa Barbara basin (Uvigerina peregrina; light blue, Bolivina argentea; bright blue)68, benthic Uvigerina peregrina oxygen isotopes from core EW0408-85JC (green)2, ODP Site 1019 ((41° 68′ N, 124° 93′ W, 978 m; light blue)1, cores EW0408-26JC/TC, and core EW0408-87JC (Cibicidoides). Data from the Gulf of Alaska cores (EW0408) are used to make the depth-time map shown in Fig. 4.
Temperatures based on the index show a larger glacial-interglacial change, with colder SSTs during the late glacial period and warmer SST for the Holocene. Temperature estimates for the deglacial period, including the two hypoxic intervals and the Younger Dryas, are virtually identical between methods, giving confidence that diagenetic effects are not influencing the alkenone ratios during the hypoxic warm events. Alkenone concentrations are high during the two hypoxic events (blue), consistent with other proxy evidence for high productivity and/or excellent preservation of organic matter during these events.
The alkenone palaeotemperature record (purple) was interpolated on a 200-yr time step and the average rate of temperature change (°C per century) was calculated over a 400-yr window (black).
Bulimina exilis is the most tolerant of low-oxygen conditions, and is often associated with near-anoxic bottom waters. Bulimina and Bolivina genera are typically found in strongly hypoxic waters, whereas Epistominella pacifica is associated with intermediate hypoxia, and Uvigerina peregrina is associated with more well-oxygenated conditions. Grey shaded bars represent the two laminated intervals, which are almost exclusively comprised of Bolivina and Bulimina genera.
The sedimentary δ15N record (violet) and δ15N corrected for terrestrial organic matter (light violet)13 show elevated values during the two hypoxic intervals (grey bars), which also coincide with enhanced organic matter deposition, including total organic carbon (light green)13 and an increase in the alkenone K37 abundance (dark blue). High planktonic δ13C values are observed during these intervals (bright blue)2, consistent with an increase in surface productivity rather than upwelling of deep waters exported from low-latitudes. The progressive increase in planktonic δ13C through the Holocene is accompanied by an increase in the relative abundance of U. peregrina (purple), likely indicating a better ventilated water column in the Holocene.
a, δ13C of planktonic foraminifera, Nps (blue), Gb (red), average of the two species (black). b, Comparison of the average planktonic foraminiferal δ13C (black) with changes in δ13C of atmospheric CO2 (blue)57 and estimated surface ocean δ13C of CO3=. (red, calculated from the smooth atmospheric values using the temperature relationship of Zhang et al.60 c, Relationship between δ15N in organic matter and δ13C in planktonic foraminifera (average of Gb and Nps). The δ15N and δ13C measurements were in most cases made in adjacent samples, typically separated by 1 cm. To prevent directional bias in the scatter plot, the two variables were first interpolated linearly onto the depth of the other variable. In no cases were interpolations allowed over an interval >5 cm or 200 years. The positive correlation between organic δ15N and planktonic foraminiferal δ13C during the deglacial interval (17–9 ka, red points, r2 = 0.51) supports an interpretation of increased nutrient utilization and carbon export from near-surface waters associated with the high δ15N events. In contrast, within Holocene time (9–0 ka) the relationship between δ15N and δ13C reverses, suggesting no systematic variations in nutrient utilization. d, Time series of organic δ15N (organic) and δ13C (average of the planktonic foraminifera Gb and Nps) as a time series; these data form the comparison in c.
Extended Data Figure 7 Expanded view of proxy data (plotted as depth in core) for EW0408-85JC during the two hypoxic intervals.
CT grey scale (black) reflects changes in the biogenic:lithogenic fraction of sediment, with low values indicating times of high biogenic input (primarily diatoms)2. The laminated intervals (pink shading) coincide with high diatom abundance and SSTs near or exceeding 10 °C, whereas evidence for low-oxygen conditions appears to extend both before (blue shading) and after (grey shading) the BA laminated zone based on trace metal concentrations12,13, benthic faunal assemblages, and preservation of TOC13, coinciding with the initial increase in SST and decrease in benthic δ18O (dark blue)2.
Extended Data Figure 8 Surface climate proxies compared with changes in benthic δ18O and fauna from different depth sites in the Gulf of Alaska.
The alkenone palaeotemperature record from core EW0408-85JC (purple), a composite record of planktonic δ18O from cores EW0408-26JC and EW0408-66JC (black)36, benthic δ18O (dark blue) and biogenic silica (bright blue) from core EW0408-85JC2, the combined abundance of low-oxygen tolerant Bolivina and Bulimina species from cores EW0408-85JC (682 m; green) and EW0408-26JC (1,620 m; violet). An increase in low-oxygen benthic fauna is apparent in the deeper site (EW0408-26JC) commencing at 16 ka, which coincides with the pre-Bølling warming in the SST record and an increase in the planktonic and benthic δ18O records. This initial decrease (blue shading) in sedimentary oxygen content at the base of the OMZ clearly precedes the large increase in biogenic silica and the shift to hypoxic conditions in core EW0408-85JC near the onset of sedimentary laminations (pink shading). Sediment laminations in core EW0408-26JC occur from 15–14 ka (shaded bar on x-axis).
Extended Data Figure 9 Northeast Pacific SSTs, productivity indices, and atmospheric greenhouse gases.
Data from top: CO2 record from EDC (dark blue)66 and WAIS (light blue)67, a record of N2O from TALOS Dome (bright blue)65, the Gulf of Alaska SST record (purple), a SST record from the California margin (black)47, records of total organic carbon (TOC: light green), total nitrogen (TN: dark green), and δ15N records on bulk organic matter from core EW0408-85JC13. Grey shaded bars indicate the two intervals in which the deglacial rise in CO2 plateaus/reverses, which generally correspond to the episodes of widespread North Pacific hypoxia, high SSTs, enhanced nitrate utilization, and increased export productivity.
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Praetorius, S., Mix, A., Walczak, M. et al. North Pacific deglacial hypoxic events linked to abrupt ocean warming. Nature 527, 362–366 (2015). https://doi.org/10.1038/nature15753
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