The reductions in nutrient upwelling (high Δδ15N and lower opal accumulation; blue bars in Fig. 3) between 930–1020 ka are predominantly restricted to glacial intervals between MIS 24–28, with nutrient upwelling and productivity increasing after each deglacial (dashed black lines in Fig. 3). Parts of this interval are characterised by covariation of utilization and productivity, which could be explained by variations in iron fertilization, perhaps from terrestrial sources to the region. Conversely, when productivity decreases and nutrient utilization increases (blue bars in Fig. 3), enhanced nutrient utilization cannot be accounted for by, for example, iron fertilization as this would increase productivity. Enhanced halocline stratification is unlikely to have caused these productivity minima along the Bering Slope (unlike at Bowers Ridge21,47) as the modern Pacific halocline is relatively shallow (upper 300 m) and eddies at the slope can reach down to 1 km water depth20. These strong eddies are also likely to rule out a change in the depth of the shallower mixed layer as the primary cause of reduced productivity. We propose the cause of the reduced glacial nutrient upwelling from 930–1020 ka was due to enhanced NPIW (Fig. 4a). NPIW today produces a temperature and salinity low centred at ~500 m in the North Pacific, which is sourced partly from winter Okhotsk Sea pack ice brine rejection, and exhibits low nutrients including nitrate50. Evidence for expanded NPIW during the last glacial maximum is extensive26,27,36,51,52, with ɛ Nd records identifying the Bering Sea as a probable source region for sea ice brine rejection during glacials26,52. The central Bering Sea appears to have been bathed in a relatively nutrient-poor (likely younger than deep Pacific) NPIW to at least ~0.8 km water depth during glacials over the past 1.2 Ma, evidenced by Site U1342 (Bowers Ridge) negative offsets in benthic foraminiferal δ18O from deep Pacific end members27. These records indicate that the lower boundary of NPIW deepened to >0.8 km water depth during glacials27, offsetting shallow benthic δ18O from U1343 values at ~2 km water depth (Fig. 3b). The proposed mechanism for producing relatively low Bering Sea deep water δ18O is sea ice brine rejection, which can increase salinity without significantly altering δ18O, and is able to transport highly depleted δ18O surface water to depth27,36.

Fig. 4 Schematic Bering Sea cross section (N-S) showing palaeoceanographic interpretations. a Glacials between MIS 24–28 (before 930 ka). b ‘Failed’ interglacial MIS 23. c Glacial MIS 22 (the ‘900 ka event’). Interpretations of North Pacific Intermediate Water (NPIW) and North Pacific Deep Water (NPDW) are based on benthic δ18O from Sites U1343 (this study) and U134227. Arrows show the inferred flow of warmer (red) and colder (blue) currents Full size image

The shift away from precession (~23–28 ka) frequency in opal accumulation cycles at ~930 ka (Fig. 2c, Supplementary Fig. 8), suggests a change in the causal mechanisms driving export production during the MPT, possibly from a combination of NPIW and terrigenous nutrient influences before 930 ka, towards predominant NPIW control thereafter. During MIS 24–22, reduced nutrient upwelling (increased nutrient utilization and lower productivity) occurred for an extended period of time (>30 ka). This striking feature of the dataset is similar to δ15N and opal accumulation records during the last glacial from Bowers Ridge (low glacial opal accumulation and higher δ15N)21, and the last glacial cycle from the Bering Slope (Fig. 5). This provides evidence for both a more limited upwelling nutrient supply, and more complete nitrate utilization from continued iron delivery, during severe glacials21. During the ‘failed’ interglacial MIS 23, which directly contributed to the first quasi 100-ka cycle, nutrient upwelling remained low (blue bars in Fig. 3) likely due to the sustained expansion of NPIW, whilst insolation at 65°N was increasing (Fig. 4b). Subsequently, MIS 22 exhibited a further deepening of the lower boundary of NPIW to >2 km water depth, as evidenced by U1343 offsets in benthic δ18O towards shallower water values (Fig. 3b) and potentially δ13C (Supplementary Fig. 3) from global and Pacific records respectively, further removing the NPDW source of nutrients from the photic zone, and stifling both productivity and CO 2 outgassing (Fig. 4c). A lower resolution record from Site U1339 at 1.8 km water depth is consistent with this hypothesis, with benthic δ18O values similar to Site U1343 (Fig. 3b). When considering the long-term record of nutrient utilization and primary productivity at Site U1343 (Figs. 1c, d, 5c), the data available show that severe glacials from MIS 22–24 onwards (MIS 2, 6, 8, 12 and 14) were coincident with longer productivity crashes and reduced nutrient upwelling (blue bars in Fig. 5). There is also evidence for deep expansion of NPIW to greater that 2 km from the lower resolution benthic δ18O during these latter glacials (Fig. 5b). However, before ~930 ka productivity and nutrient utilization proxies indicate shorter-lived times of reduced nutrient upwelling, which we propose was due to less influence of NPIW in the early part of the MPT and before.

Fig. 5 Proxy data from Site U1343 compared with sea level, for three time periods over the past 1.5 Ma. Vertical blue bars are times of reduced productivity and elevated nutrient utilization indicating reduced deep water nutrient upwelling along the Bering Slope. a Global sea level estimates from Site 1123 (blue)2 and the Mediterranean (orange)3. The current Bering Strait sill depth is indicated with a horizontal dashed line. b Benthic δ18O data from U1343 (pink) and LR04 (grey)30. U1343 δ18O data from 850–1020 ka are from this study, other data are published elsewhere33. c Bulk sediment δ15N for Site 101248 subtracted from U1343 (blue) (Δδ15N). U1343 δ15N data from 850–1020 ka are from this study, other are data are published elsewhere39,68. Opal mass accumulation rate (MAR) as a proxy for productivity (green) for Site U134325 Full size image

A cause for expanded NPIW has been suggested as enhanced sea ice and the production of brines, perhaps due to shifts in wind stress promoting polynya formation near the shelf27. Modelling of wind patterns over the last glacial does show significantly increased windiness over the Bering Sea18. A sea-ice controlled expansion of NPIW during glacials over the MPT is consistent with increased proportions of diatoms at the Bering Slope indicative of pack ice (Fig. 2e), although these records are of low resolution. Higher resolution but discontinuous IP 25 and HBI III proxy records from U1343 were interpreted as glacial expansion of seasonal sea ice from 1150 ka, caused by high latitude atmospheric cooling1, and then a more persistent seasonal/extended sea ice cover during glacials from MIS 22, possibly from reduced Pacific surface water influence8. Our dinoflagellate cyst assemblage record is consistent with an MIS 22 expansion of sea ice (Fig. 4c). Although a continuous and high-resolution sea ice proxy record from the Bering Sea is yet to be produced, further evidence for a sea ice control on NPIW and Bering Slope nutrient upwelling comes from sedimentation rate reductions along the slope during glacials particularly from MIS 22–24 onwards (Fig. 2f). Enhanced glaciation around the Bering Sea would have expanded shorefast and pack ice, known to inhibit modern day winter sedimentation53.

Glacial expansions of sea ice influence may have been controlled by reduced air temperatures and/or increased windiness, and another factor promoting these conditions could have been closure of the Bering Strait27. Modern polynya brines formed in the Gulf of Anadyr and Anadyr Strait region of the Bering Sea flow into the Arctic54. A closed Bering Strait would have retained glacial brines within the Bering Sea. Modelling studies of a closed Bering Strait55, although failing to predict expanded glacial NPIW perhaps due to limited modelling of Bering Sea shelf processes27, do show entrained colder water within the Bering Sea. The first marked long-term period of reduced nutrient upwelling in our records was at MIS 22–24, which coincides with the first time sea level was below 50 m of the present2,3 (Fig. 2b). The modern Bering Strait sill depth is 50 m. This region is thought to have undergone subsidence in the Pliocene56 and first opened at about 4.8–5.5 Ma57. After an initial period of southward flow, the strait has been exchanging water with the Arctic in a (modern) northward flow since 3.3 Ma58,59. There is no direct evidence for how the water depth of the Bering Strait may have changed since, and it is possible that the strong currents did not allow significant sediment deposition and therefore any significant change from its current depth of ~50 m. We hypothesise that the failed MIS 23 deglacial may have directly coincided with the first closure of the Bering Strait (and subsequent closure during long glacials thereafter; Fig. 5a), retaining brines and NPIW in the Bering Sea, and cutting off the source of deep nutrients and CO 2 from the photic zone (Fig. 4b, c). This would have acted as a positive feedback on a process of global cooling already in motion. In addition to lower glacial temperatures and increased windiness, the idea of a closed Bering Strait promoting further enhanced sea ice, brines, NPIW and reduced nutrient upwelling along the Bering Slope, can only be supported by a comparison of upwelling with sea level records. Over the last 1.5 Ma, nutrient upwelling (offset between opal MAR and Δδ15N) has been lowest when sea level2 was below 50 m of present (Table 1), compared to when global SST was below –3 °C of present, or deep ocean δ18O (combined deep ocean temperature and ice volume) was above 4‰ (severe glacials). This provides some support for a closed Bering Strait having a stronger influence on NPIW and nutrient upwelling in the Bering Sea than either the average global surface or bottom water temperatures alone.

Table 1 Average nutrient upwelling for glacial and interglacial phases Full size table

The regional significance of reduced MPT nutrient upwelling along the Bering Slope, caused by NPIW expansion (Fig. 5), is that NPIW would have inhibited CO 2 atmospheric exchange not only in the Bering Sea, but also across the subarctic North Pacific13,16, creating a wide regional barrier for the upwelling of CO 2 -rich deep water and associated primary productivity. Recent studies have highlighted the importance of North Pacific deep overturning60 for increased outgassing of deep ocean CO 2 during the last deglacial17,18, and the role that expanded NPIW may have played in suppressing North Pacific CO 2 ventilation to the atmosphere during the last glacial maximum18. A closed Bering Strait, therefore, could have contributed to MPT cooling by promoting local sea ice8 and regionally-extensive NPIW and reduced nutrient upwelling (this study). NPIW would have created an effective barrier to deep North Pacific overturning, and subsequent atmospheric ventilation of CO 2 . In this scenario, overturning of the North Pacific from the MPT onwards was only instigated at the termination of severe glacial conditions and associated global ocean circulation changes such as a shutdown of Atlantic Meridional Overturning Circulation60.

The initial cause of MPT cooling remains enigmatic, but our new records show the North Pacific and the Bering Strait were important components within this global climate shift. Atmospheric cooling in high latitudes began at ~1.2 Ma1 and may have been associated with a reduction in atmospheric CO 2 from enhanced dust fertilization in the Southern Ocean5 (Fig. 2g). There is evidence for increases in Bering Sea sea ice from at least 1.2 Ma8, and possible associated expansion of NPIW from reduced primary productivity at 1.2 Ma (Fig. 2c). These pieces of evidence suggest both the North Pacific and the Southern Ocean were likely ventilating less CO 2 to the atmosphere during glacials from the early phase of the MPT, although further records are needed to test this and ascertain timings. The second step in the development of the MPT, the severe ice sheet expansion3 and glacial lengthening at ~900 ka (MIS 22–24; Fig. 2a), has been suggested to be associated with further CO 2 decline from deep ocean reorganisation and sequestration of CO 2 (ref. 12). In agreement with this, we find that MIS 22–24 was associated with a severe and extended period of reduced Bering Slope nutrient upwelling and NPIW expansion to >2 km water depth, similar to subsequent severe glacials (Fig. 5), which would have further inhibited any CO 2 ventilation across the subarctic North Pacific. We suggest closure of the Bering Strait was contributory to this expanded NPIW, either as a threshold response crossed due to an as yet undefined boundary condition change, or as a boundary condition change in itself such as a hypothetical shoaling of the strait through sediment accumulation. This could be tested with future coring of the Bering Strait to ascertain its sedimentation rate history. Southern Ocean productivity proxies are currently of low resolution15, and although they show consistently increased glacial water column stratification, thought critical for sequestration of glacial CO 2 (ref. 14), they do not show a clear increase in stratification over the MPT15 highlighting the potential importance of NPIW in increased oceanic sequestration of CO 2 during the MPT. Although a further enhancement of dust deposition across the Southern Ocean may have contributed to MIS 22–24 cooling and CO 2 decline (Fig. 2g), we note that dust deposition was actually decreasing into the MIS 22 glacial maximum (Fig. 3d) whilst nutrient upwelling on the Bering Slope remained inhibited and NPIW expanded, thus supporting changes in upwelling of the North Pacific as an important mechanism of CO 2 reduction compared with Southern Ocean dust fertilization.