Rapid shift and millennial-scale variations in Holocene North Pacific Intermediate Water ventilation.

The Pacific hosts the largest oxygen minimum zones (OMZs) in the world ocean, which are thought to intensify and expand under future climate change, with significant consequences for marine ecosystems, biogeochemical cycles, and fisheries. At present, no deep ventilation occurs in the North Pacific due to a persistent halocline, but relatively better-oxygenated subsurface North Pacific Intermediate Water (NPIW) mitigates OMZ development in lower latitudes. Over the past decades, instrumental data show decreasing oxygenation in NPIW; however, long-term variations in middepth ventilation are potentially large, obscuring anthropogenic influences against millennial-scale natural background shifts. Here, we use paleoceanographic proxy evidence from the Okhotsk Sea, the foremost North Pacific ventilation region, to show that its modern oxygenated pattern is a relatively recent feature, with little to no ventilation before six thousand years ago, constituting an apparent Early-Middle Holocene (EMH) threshold or "tipping point." Complementary paleomodeling results likewise indicate a warmer, saltier EMH NPIW, different from its modern conditions. During the EMH, the Okhotsk Sea switched from a modern oxygenation source to a sink, through a combination of sea ice loss, higher water temperatures, and remineralization rates, inhibiting ventilation. We estimate a strongly decreased EMH NPIW oxygenation of ∼30 to 50%, and increased middepth Pacific nutrient concentrations and carbon storage. Our results (i) imply that under past or future warmer-than-present conditions, oceanic biogeochemical feedback mechanisms may change or even switch direction, and (ii) provide constraints on the high-latitude North Pacific's influence on mesopelagic ventilation dynamics, with consequences for large oceanic regions.

Intermediate waters in the subarctic North Pacific marginal seas play a critical role in supplying oxygen (O2) to the North Pacific oxygen minimum zone (OMZ) (1). Recent studies point to decreasing O2 and increasing temperatures of the mesopelagic North Pacific over the past decades (2, 3). The cause of these changes, however, is hard to unambiguously attribute to either anthropogenic influences or long-term natural variability, as the latter occurs on timescales beyond the reach of instrumental datasets. One decisive factor that today prevents the development of more widespread hypoxia is the supply of dissolved O2 to the mesopelagic water layer via ventilation of subarctic waters (1, 4). In the North Pacific, no ventilation of deep waters occurs (5) due to the existence of a strong halocline, and ventilation of intermediate water is restricted to the marginal seas, as middepth density layers do not outcrop in the open surface ocean (6, 7) (Fig. 1 and ). Today, the Okhotsk Sea constitutes the most important region with active ventilation during the wintertime sea ice season, when polynyas open up on the northeastern shelf areas and dense, O2-enriched water masses form through brine rejection (8). Subsequently exported to the open North Pacific as Okhotsk Sea Intermediate Water (OSIW), they ventilate the middepth North Pacific (9) and prevent mesopelagic hypoxia occurrence (10–12). As the Okhotsk Sea is a unique region in being the world ocean’s lowermost latitude where seasonal sea ice occurs, it responds with high sensitivity to changes in climate forcing, with short response times (13). Previous works have been mostly limited to OSIW and North Pacific Intermediate Water (NPIW) changes during recent glacial periods (14, 15). Few studies have addressed paleoenvironmental changes in the Okhotsk Sea over warm episodes such as the Holocene (16, 17). Here, we present high-resolution proxy records for Holocene changes in OSIW ventilation, using stable carbon isotope ratios of the epibenthic foraminifer Cibicidoides mundulus, a species that reliably records the δ13C of ∑CO2 of surrounding bottom water masses (18). Based on our records, we estimate past changes in ventilation and corresponding oxygenation of middepth Pacific water masses during the last 12 ka, with a particular focus on the warmer-than-present Early–Middle Holocene (EMH).

Fig. 1.

Study area and regional oceanographic characteristics. (A) Bathymetric map with site locations. EKC, East Kamchatka Current; ESC, East Sakhalin Current; NS, Northern Shelf Polynia; Oy, Oyashio; SP, Sakhalin Polynia. (B) Vertical profiles of modern water δ13CDIC for the Okhotsk Sea and North Pacific. Station HS29 (black, open circles) records WSAPW before it enters the Okhotsk Sea (14); Station 112 represents intermediate water before transport to ventilation areas on the Northern Shelf (orange, filled circles, this study); and Station 72 monitors OSIW path “downstream” of ventilation area before export into the North Pacific (blue, filled squares, this study). Station 40N (24) represents final water mix in a southerly position near Kuroshio-Oyashio Extension Region (green, open squares), where new NPIW is formed. Note gradient among WSAPW, OSIW before and after ventilation, and resulting change in NPIW (intermediate water core layer: light blue shaded area). Stations 14/29/84, 110, and 12 (yellow dots in A) indicate additional bottom water and surface sediment sample locations used for validation of modern δ13C characteristics of the epibenthic foraminifer C. mundulus ().

Results and Discussion

We used a suite of four sediment cores to reconstruct OSIW ventilation (for details, see ). Gravity core 108 was retrieved from the western Kamchatka continental margin (52°01.330′ N, 153°35.006′ E; 627 m water depth) in the pathway of inflowing North Pacific water to the formation region of OSIW on the northeastern Okhotsk Sea continental shelf. This core thus records the precursor water mass—Western Subarctic Pacific Water (WSAPW)—delivered via the East Kamchatka Current (Fig. 1). Three sites record changes in newly formed OSIW on the eastern Sakhalin continental margin, along the export pathway into the North Pacific. Site 78 (52°40.388′ N, 144°42.203′ E; 673 m water depth) and site 4 (51°08.475′ N, 145°18.582′ E, 674 m water depth) are located within the core layer of OSIW (19, 20). Site 79 (52°47.272′ N, 144°57.318′ E; 1,082 m water depth) complements the records with a deeper location at the lower boundary of OSIW. Based on an accelerator mass spectrometry (AMS) 14C-anchored stratigraphic framework (), these cores yield sedimentation rates between 20 and 180 cm/ka, thus allowing for the reconstruction of millennial-scale changes in intermediate water ventilation.

Changes in δ13C for deeper water masses without contact with surface ocean and atmosphere become linearly correlated to regenerated nutrient and O2 concentrations (21, 22). While in some oceanic regions, other factors alter this relationship (e.g., air–sea exchange processes, thermodynamic effects, or water mass mixing) (22, 23), in the Okhotsk Sea and North Pacific, the δ13C–nutrient–O2 relationship has been shown to be relatively simple, and these secondary factors play a less important role (24, 25) under the considerations we follow here ( ). To validate that heavier δ13C signatures represent better-ventilated OSIW compared to WSAPW, we measured water-column δ13CDIC (dissolved inorganic carbon) depth profiles from two stations near our inflow and outflow locations (Fig. 1). Compared with existing North Pacific data (14, 24), we identified about 0.2 to 0.3‰ higher δ13CDIC values in the OSIW outflow (Station LV29-72-4) than WSAPW mean values of about −0.5‰ (Station HS29, Fig. 1), corroborating our use of δ13C as proxy for middepth water mass ventilation (see ).

Before using down-core records for our OSIW reconstructions, we also tested the assumption that epibenthic foraminifera of the genus C. mundulus are reliably recording δ13C of bottom water DIC and do not incorporate unidentified offsets as reported from other oceanic regions and time intervals (18). Our results from eight undisturbed sediment surface sites with corresponding bottom water δ13CDIC measurements within the Okhotsk Sea show that the δ13C of live (rose bengal-stained) C. mundulus mirrors the δ13C of bottom waters within ∼0.1‰, which is within commonly accepted values for intrasample variability in benthic foraminifera ().

A Middle-Holocene Threshold in OSIW Ventilation.

The Sakhalin margin cores that monitor OSIW (sites 4, 78, and 79) are characterized by high-amplitude variations throughout the Holocene, exceeding glacial–interglacial changes in Pacific Deep Water ventilation (14), with values between −0.8‰ and −0.1‰ and a close resemblance between single cores. Sample resolution in the deeper core 79 is slightly worse due to the partial absence of Cibicidoides species from the benthic fauna; however, core 79’s overall δ13C pattern matches that of the other cores. In contrast, the Kamchatka margin core 108, representing WSAPW inflow, shows δ13C values ranging mostly between −0.2‰ and −0.5‰, similar to modern values (25) but with no discernible long-term trend over the Holocene (Fig. 2). For better comparison hereafter, we discuss principal characteristics of OSIW based on a combined record (named C-OSIW) of the three outflow, or “downstream” sites representing the OSIW (sites 4, 78, and 79) (Figs. 2 and 3). We stacked individual records on their own age models into one record and added a 25-point smoothing to the original data to highlight millennial-scale changes (Fig. 3).

Fig. 2.

Stable isotope records of studied cores indicating changes in OSIW ventilation in relation to reference records. (A) NGRIP ice core δ18O time series (63). (B) Kamchatka margin core 108 epibenthic δ13C, indicating WSAP water inflow. (C–E) Sakhalin margin cores epibenthic δ13C, indicating changes in OSIW ventilation: (C) core 78 from shallow N Sakhalin margin, (D) core 4 from shallow middle Sakhalin margin, and (E) core 79 from lower N Sakhalin margin, as labeled. (F) Dongge Cave speleothem δ18O time series, representing changes in Southeast Asian summer monsoon and links to North Atlantic climate (29). Colored background highlights the two different main ventilation phases from 0 to 6 ka (LH) and from 6 to 11 ka (EMH).

Fig. 3.

OSIW ventilation dynamics and gradients compared with long-term changes in regional and global climate records. (A) δ13COSIW combined record of sites 4, 78, and 79, indicating OSIW ventilation dynamics. Gray line: original data; black thick line: resampled 25-point moving average. (B) Proxy record for intensity of EASM changes in Huguangyan Maar Lake, southern China, as expressed in total organic carbon (TOC) content (30). Note inverted scale for better comparison with OSIW ventilation record. (C) Reconstruction of intermediate water temperature (IWT) (inverted) in the equatorial West Pacific at 500 m water depth, indicative of northern-sourced NPIW reaching the low-latitude thermocline. Calculated changes over the Holocene with reference to the last 100 y, based on Mg/Ca ratios of benthic foraminifera (40). (D) Composite alkenone-based high-resolution SST record from Japan margin (cores MD01-2421, KR02-06_MC, and KR02-06_GC) (38). (E) NE Pacific Santa Barbara Basin (SBB) δ15Nbulk record, indicating denitrification and nutrient utilization changes in waters under NPIW influence (55). (F) Ventilation difference between OSIW and WSAPW, expressed as Δδ13C between the stacked C-OISW record and site 108 (this study). Positive values (above dashed “zero” line) denote active ventilation in the Okhotsk Sea; negative values indicate ventilation loss in the Okhotsk Sea due to remineralization processes and lack of new water mass formation. (G) Atmospheric CO2 record from EPICA Dome C ice core record (64).

Our C-OSIW δ13C record shows a subdivision of the Holocene into two periods that have differing ventilation patterns (Figs. 2 and 3). The EMH (11 to 6 ka) exhibits significantly lower than modern δ13C values (Figs. 2 and 3), suggesting persistent ventilation minima in OSIW. In contrast, Late Holocene (LH) (0–6 ka) ventilation increases after a sudden shift around 6 ka (Fig. 2 ) toward modern conditions, featuring a principal Middle-Holocene threshold (or “tipping point”) in NPIW ventilation. In addition, the LH (Fig. 3) shows a slight but more gradual increase in the C-OSIW stack toward modern values of δ13C over the last 1 to 2 ka of about 0.1‰ (Figs. 2 and 3). Therefore, a critical initial observation of this C-OSIW ventilation reconstruction is that the subarctic North Pacific middepth water layer rapidly changed its characteristics during the Middle Holocene around 6 ka toward the present-day OSIW and the connected mesopelagic North Pacific ventilation prevalence. For a better comparison between the OSIW outflow site (cores 4, 78, and 79) and the WSAPW inflow site (core 108), we subtracted the latter from our C-OSIW record to create a Δδ13C time series (Fig. 3), after resampling our data in evenly spaced 50-y intervals. Our Δδ13C data (Fig. 3) reveal that OSIW ventilation decreased below values of its precursor WSAPW before the 6-ka shift. This negative OSIW–WSAPW gradient (Fig. 3) implies that OSIW export into the North Pacific was, in effect, not actively ventilated in the Okhotsk Sea before 6 ka.

We assume that under Holocene boundary conditions, the principles of OSIW formation through seasonal sea ice, polynya-induced brine rejection, and mixing between surface waters and WSAPW remained essentially similar. However, apparently, the intensity of mixing well-ventilated waters into middepth levels must have increased after the Middle Holocene toward the modern setting. Other processes like changed air–sea exchange or simple source water mass variations (e.g., through higher contributions of Pacific Deep Water or mixed layer water, respectively) cannot fully explain the large observed changes in absolute δ13C values on millennial timescales, and particularly the EMH negative Δδ13C gradient (). Earlier qualitative multiproxy studies of nearby sites in the Okhotsk Sea (17) showed oceanographic reorganizations around 6 ka. In those results, independent evidence from microfossil assemblages suggested higher mesopelagic productivity with increased microbial biomass and higher organic matter respiration in OSIW before 6 ka (see ), while support for reduced formation of OSIW and intensified OMZ conditions was also found in nearby sites (17, 26, 27). As a result, we presume that during the EMH, physical processes likely inhibited the modern ventilation scheme, while increased remineralization of organic matter through respiration within the mixed layer and in OSIW decreased the ventilation of OSIW relative to WSAPW entering the Okhotsk Sea, leading to higher inventories of nutrients and carbon in NPIW.

Connection to Pacific and Northern Hemisphere Holocene Climate.

To explain the forcing behind the observed variations in OSIW ventilation, and in particular the Middle-Holocene climatic shift toward the modern well-ventilated regime, we compared our data to other regional records. Globally, and in regions influenced by the East Asian summer monsoon (EASM) (Figs. 2 and 4), a Middle-Holocene shift is documented (28). Before 6 ka, a stronger EASM (29, 30) propagated further northeastward than at present, yielding higher precipitation and temperatures in the Okhotsk Sea hinterland (31), increasing runoff from the Amur River catchment into the Okhotsk Sea (32). Higher Amur freshwater discharge also transports significant amounts of heat to the Okhotsk Sea (32, 33), which suppresses sea ice formation and, in concert with stronger thermal stratification of the upper mixed layer (34), decreases OSIW ventilation. After 6 ka, strengthening and expansion of the winter Siberian High in concert with a southward retreating EASM front was likely responsible for strengthening cold northeasterly winds over the continental shelf region (35). In addition, the maximum sea ice extent in the Okhotsk Sea is dependent on mean sea surface temperature (SST) conditions in summer (36). We thus infer a longer and more extensive sea ice cover, more polynyas, and more vigorous mixing of the mixed layer after 6 ka, which together enhanced the ventilation of new OSIW.

Fig. 4.

Model results from the earth system model COSMOS. Results are plotted in all panels as anomalies, calculated as differences resulting from 9 ka minus PI runs. (A) Annual mean temperature (T) anomaly, shown on a meridional transect along 150° E from the equator to 60° N. The transect crosses directly into the Okhotsk Sea north of 42° N. (B) Same as in A but for annual mean salinity (S) anomaly. (C) Sea ice volume anomaly 9 ka minus PI, calculated from sea ice concentration multiplied by sea ice thickness in the North Pacific. (D) Same as C, but for sea ice thickness.

Alkenone-derived SSTs for the Okhotsk Sea and the Japan continental margin consistently show maxima in Holocene SSTs between ∼8 and 6 ka, decreasing SSTs between 6 and 4 ka, and a last shift toward persistent colder SSTs around 2 ka (37, 38) (Fig. 3). Our observed decrease in OSIW ventilation during the EMH is thus in close agreement with increased SSTs of about 1.5 to 2 °C derived from both regional and global reconstructions (38, 39). Importantly, recent low-latitude middepth thermocline temperature changes (at around 500 m water depth) in the equatorial West Pacific show concomitant higher temperatures in the EMH of similar magnitude, with northern NPIW-sourced thermocline or intermediate water temperatures (Fig. 3) being warmer by 2.1 ± 0.4 °C than during the last century (40), in strong accord with our assumed decrease in ventilation in OSIW and NPIW (Fig. 3 ).

Results from Earth System Model COSMOS (Community Earth System Model) simulations corroborate our proxy-based assumptions. This model captures the modern physical characteristics of NPIW well under preindustrial (PI) conditions (). We analyzed Pacific data from PI and 9-ka time-slice runs (for details, see and ref. 41). On a meridional transect along 150° E through the Okhotsk Sea toward the equatorial Pacific, our model results show a consistently warmer (around 1 to 2 °C) and saltier intermediate water layer at 9 ka than at PI conditions (Fig. 4 and ), matching the proxy-based temperature changes surprisingly well (40). Model-derived anomalies (9 ka minus PI) in sea ice dynamics show a localized, significant decrease of both sea ice volume (Fig. 4) and thickness (Fig. 4) in the Okhotsk Sea in the EMH, in agreement with our hypothesized reductions in sea ice and polynya formation in the OSIW source region that inhibited the modern ventilation regime.

Millennial-Scale Variations in Middepth Ventilation.

Superimposed on the described stepwise background changes, we detected distinct cyclic millennial-scale variations in OSIW ventilation, visible in the δ13C C-OSIW ventilation record (Fig. 3) and in wavelet analyses of both the combined and individual records (). These variations occur in the range of 1,500 to 1,800 y, which have been reported from a number of locations and Holocene proxy records (42, 43). Notably, this cyclicity is most prominent before the Middle-Holocene tipping point (28). While other records yielded comparable changes in terrestrial and surface ocean records, which were attributed to either internal (43) or external [e.g., solar (44)] forcing, evidence for a direct connection to middepth and deep water formation or ventilation patterns, such as we observe here, has remained relatively scarce to date (28, 42). Thus, our record provides evidence for the close link between high- to midlatitude surface ocean processes (SST and stratification changes) and intermediate water ventilation dynamics on Holocene millennial (and potentially shorter) timescales.

In the subarctic Pacific, other evidence for Holocene millennial-scale cycles in marine sequences is scarce; however, a southerly located Japan margin SST record (38) also showed 1,500-y cycles in SST anomalies (see original data of ref. 38 in Fig. 3). A northward propagation of low-latitude warm Kuroshio waters would plausibly explain the strong correspondence between Japan offshore SST records and Okhotsk Sea decreases in ventilation due to warming and salinification of upper ocean water masses (38), as well as the closer coupling between the midlatitude North Pacific and the subarctic Okhotsk Sea region. For future warming scenarios, such a pattern has been observed in recent modeling results, which indicated an intensification and northward shift of most Western Boundary Currents (45). As the 1,500-y variations are most prominent during the EMH in our records, we presume that during periods of suppressed OSIW ventilation, this modulation via oceanic forcing and the global Meridional Overturning Circulation gains importance.

High-Latitude Mesopelagic Oxygenation and Remineralization Changes: Effects and Feedbacks.

In principle, benthic δ13C may, in addition, provide qualitative information on past baseline variations of bottom water oxygenation (46, 47) (see ). However, because O2 in seawater equilibrates with the atmosphere almost instantaneously (i.e., on timescales of a year or less), whereas DIC and its carbon isotopes react on timescales of a decade [i.e., with about an order of magnitude difference (48)], only long-term, millennial-scale changes can be inferred. Correlation between dissolved water column O2 and δ13CDIC based on water column data from the Okhotsk Sea () can thus provide some insight into potential ranges of past Holocene middepth oxygenation levels. When applying this modern relationship between O2 and δ13C to assess long-term oxygenation changes for OSIW, the last 0 to 6 ka would broadly range within modern instrumental data, indicating a moderately oxic environment (i.e., 100 to 200 µmol/L ± 50 µmol/L) (2, 3). In contrast, O2 estimates for the preceding EMH interval, before the 6-ka threshold, show oxygenation levels that did not reach even the lower range of modern O2 concentrations (). EMH O2 values would have mostly yielded just 30 to 50% of modern ranges (50 to 120 µmol/L ± 50 µmol/L) and thus imply a substantial reduction in oxygenation of middepth waters. Taken at face value, OSIW oxygen supply would have been significantly reduced in a warmer-than-present EMH, changing the Okhotsk Sea from a modern mesopelagic O2 source into an episodically hypoxic sink before 6 ka (), with likely limitations for marine life, OMZ strengthening (49, 50), and altered biogeochemical nutrient cycles (51, 52). This largely qualitative assessment, while not without methodological caveats like most proxy-based assessments of paleooxygenation levels (53, 54), is in agreement with synthesis results indicating an expansion of Early-Holocene OMZs on both a global scale and in the middepth Indo-Pacific (54). Also, independent evidence based on benthic foraminiferal assemblage analyses and multiproxy records recently indicated intensifications of O2-limited conditions in both the subarctic Northwest (27) and the Northeast Pacific, with more dysoxic conditions during the EMH (55, 56) (Fig. 3).

Modern NPIW provides nutrients, in particular silicate and phosphate, to lower latitudes via the “ocean tunnel” mechanism—that is, the meridional export of entrained nutrients in intermediate waters to subtropical and equatorial Pacific thermocline waters that are nutrient limited (57). Currently, nutrient limitation in these low-latitude regions hampers their efficiency of new and export biological production and the resulting sequestration of carbon to the deep ocean via the “biological carbon pump.” Thus, the modern low-latitude Pacific is one of the largest CO2 source regions to the atmosphere. Today, while only about one-third of water reaching the equatorial thermocline is estimated to stem from NPIW, it provides about two-thirds of silicate and other nutrients needed to sustain biosiliceous primary production in the low-latitude Pacific (58, 59). On glacial–interglacial timescales, this nutrient transport might have increased during cold time intervals (60, 61). We suggest that, in the Holocene as well, NPIW export acted as a feedback mechanism on the biological carbon pump and thus atmospheric CO2 (i) through intensification and deepening of remineralization processes, thereby storing higher amounts of organic carbon in North Pacific middepth waters; and (ii) by exporting larger amounts of nutrients, in particular silicate, through OSIW and NPIW to lower-latitude thermocline waters, thus attenuating silicate limitation in the northern subtropical-to-equatorial Pacific during warmer-than-present Holocene phases (Fig. 3 ). This latter process is supported by evidence for higher nutrient concentrations and utilization in the western equatorial Pacific and its subtropical marginal seas during the EMH that are currently under the influence of NPIW inflow (61) and concurrent minima in atmospheric CO2 concentrations (Fig. 3).

In summary, our combined proxy- and modeling-based reconstructions show a crucial influence of Okhotsk Sea-sourced NPIW on Pacific Holocene oceanography. In particular, export of warmer, oxygen-poorer, and nutrient-richer NPIW from the subarctic domain likely reached and influenced the low-latitude Pacific thermocline during the warmer-than-present EMH, thus not only leading to increases in oceanic heat content (40), but also to significant changes in the biogeochemical signatures of thermocline waters, including equatorial Pacific nutrient and oxygen inventories (12, 52).

Conclusions

Our study highlights the nonlinear behavior of North Pacific middepth waters on millennial timescales under past, warmer-than-present climate conditions. We show that the modern NPIW ventilation pattern, as reconstructed from high-resolution δ13C records from the Okhotsk Sea, has only been prevalent since about 1 to 2 ka, while little to no ventilation prevailed for most of the warmer EMH. Millennial-scale variations in OSIW ventilation occur superimposed on these long-term baseline ventilation adjustments, with a main cyclicity of 1,500 to 1,800 y, indicating a relation of our dominant forcing factors to globally observed internal oscillation dynamics of ocean overturning.

A comparison between subarctic Pacific Gyre source waters and intermediate water exported from the Okhotsk Sea over the last 11 ka implies that a threshold occurred at 6 ka and rapidly changed NPIW characteristics from a remineralization-dominated high-nutrient inventory to the modern regime of relatively better-ventilated middepth waters. Accordingly, we hypothesize that before 6 ka, NPIW was exporting higher amounts of nutrients, especially silicate, to nutrient-limited subtropical-to-equatorial Pacific thermocline waters via ocean tunneling, potentially providing a feedback mechanism that enhanced new biosiliceous production and thus the efficiency of the biological carbon pump during the EMH.

Based on the composite δ13C record of OSIW we also estimate that average middepth NPIW oxygen concentrations during the EMH were likely lower by 25 to 50% than they are today, which would have led to near-hypoxic conditions in the modern source region of North Pacific ventilation. Starting around 6 ka, the Okhotsk Sea switched from an EMH ventilation deficiency to the modern oxygenated regime, seemingly in lockstep with thermocline (500 m) temperature declines in the low-latitude equatorial Pacific and decreases in high-latitude Northwest Pacific SSTs.

As future global SSTs are thought to resemble or surpass Early-Holocene SST values (39), significant changes toward lower ventilation of middepth waters and thus O2 supply from subarctic latitudes, coupled with changed ventilation feedback mechanisms, may be expected for the middepth Pacific. These will likely affect oxygen- and nutrient-dependent biogeochemical cycles in the lower latitudes. Given the significant importance of intermediate water masses in the Pacific Ocean and their short response time within years or decades to changing environmental forcing (2), further studies should try to elucidate the role of intermediate waters in influencing global marine nutrient, oxygen, and carbon budgets during warmer-than-modern times like the Early Holocene, which at present remain inadequately understood.

Methods

To reconstruct changes in ventilation patterns of middepth water masses, we used the stable carbon isotopic composition (δ13C) of benthic foraminiferal C. mundulus tests. Samples were measured on a Thermo Finnigan MAT 252 mass spectrometer coupled online to a Kiel CARBO II unit, and δ13C of seawater DIC values were measured on a Thermo Finnigan Delta E with an automated DICI-II preparation unit (see for details). All values are reported in δ notation vs. Vienna Pee Dee Belemnite (‰ V-PDB).

Age control for cores 4 and 79 has been previously reported in detail (62). Age control for cores 78 and 108 was achieved through six and nine AMS 14C dates, respectively, of planktic foraminifera (). Methodology for construction of age models for all cores and age-depth relationships for cores 78 and 108 are provided in detail in .