Dear Heike Langenburg, Editor of Nature,
I am submitting a manuscript entitled, “Production of North Pacific Deep Waters During the early Cenozoic.” In this submission I present the first unambiguous evidence for significant, long-term deep-water formation in the North Pacific over the time interval of 65 to 40 million years ago. This finding is significant because in the modern setting, deep waters form in the North Atlantic and Southern Ocean, and the “oldest” deep waters are found in the North Pacific. The remarkable aspect of this data set is that the production of North Pacific deep waters was limited to the warmest climatic interval of the Cenozoic. The evidence for North Pacific deep-water production consists of records of Nd isotopes of fossil fish teeth and bones from two Pacific deep-sea sedimentary sections. Fish teeth and debris have been shown to record the Nd isotopic composition of the deep-water mass that bathes a particular location, and are resistant to later diagenetic alteration. Other techniques used to infer deep-water mass composition (oxygen and carbon isotopes of bottom-dwelling foraminifera) suffer from diagenetic alteration and carbonate dissolution in corrosive Pacific deep waters. Even when well preserved, foraminiferal stable isotope values record variations in local conditions that may overprint deep-water mass signals. As consequence, paleoceanographers have not had a clear picture of ancient Pacific deep-water mass composition and circulation patterns.
Recent application of Nd isotopes toward reconstruction of early Cenozoic thermohaline circulation has begun to reveal a significantly different mode of deep-water circulation from the later part of the Cenozoic. However, the Pacific basin is typically under-represented in most paleoceanographic investigations despite its extreme importance as the largest ocean (it was even larger during the early Cenozoic). I was a member of the recent Ocean Drilling Program cruise that targeted Shatsky Rise, an oceanic plateau currently in the northwest Pacific. My goals for participating were to add new Pacific sites to the global reconstruction of early Cenozoic thermohaline circulation. The new data provide an important constraint on Pacific deep-water mass composition during the early Cenozoic, and imply that the evolution of deep-sea circulation patterns may be driven by long-term climate change in addition to long-term tectonic changes.
Thank you for your time and consideration,
Jane Doolittle
Production of North Pacific Deep Waters During the early Cenozoic
Jane Doolittle*
*Department of Oceanography, Puddleby on the Marsh, UK
Circulation of the deep ocean is responsible for a significant component of global heat transport. Deep waters currently form in the North Atlantic and Southern Ocean, with the “oldest” deep waters found in the North Pacific. However, continual tectonic evolution of ocean basin configuration, as well as long-term changes in global climate, must have altered the mode of thermohaline circulation. During the early Cenozoic “greenhouse” (~60 to 40 million years ago), extreme polar warmth may have been a consequence of enhanced oceanic heat transport1. However, understanding the relationship between thermohaline circulation and climate is complicated by differences in the location of oceanic gateways2-4, and hence differences in sites of deep-water formation. Here I report records of neodymium isotopic ratios derived from fossil fish debris of two Pacific Ocean Drilling Program sites. These records indicate a change in deep-water circulation patterns coinciding with the warmest climatic interval of the Cenozoic. Deep-water production shifted from the Southern Ocean to the North Pacific for ~20 million years, then shifted back to the Southern Ocean as global temperatures cooled and precipitation patterns migrated.
Slow deep-water renewal in the modern North Pacific is a consequence of climatic and tectonic factors. Relatively high precipitation in the North Pacific5 results in sea-surface salinities too low to enable downwelling (~33 ppt as compared to ~35 ppt in the North Atlantic6). The Bering Strait sill does not permit dense Arctic bottom waters to enter the North Pacific7. In addition, bottom waters formed in the RossSea (the Pacific sector of the Southern Ocean) are prevented from flowing northward into the Pacific basin by the eastward flow of the Antarctic circumpolar current and the mid-ocean ridge separating the Pacific and Antarctic plates7.
However, a different set of climatic and tectonic boundary conditions prevailed during the early Cenozoic “greenhouse” interval ~65 to 40 million years ago. Global deep-water temperatures were ~8˚C to 12˚C over this time interval8,9, too warm for permanent polar ice to exist. The tectonic configuration of the continents and ocean basins was also different (Figure 1). The Tasman Sea and Drake Passage had not yet opened, and open Tethyan and Caribbean seaways still existed2,3. In addition, the Norwegian and GreenlandSeas began forming ~55 million years ago4, thus North Atlantic deep waters would not have existed during this interval of time.
Such different thermal and paleogeographic regimes should have affected deep-sea circulation patterns, and conversely, a different mode of thermohaline circulation may have impacted the evolution of global climate. To investigate the relationship between early Cenozoic climate and thermohaline circulation patterns, I generated records of the neodymium isotopic composition of fossil fish debris from Ocean Drilling Program (ODP) Sites 1209 and 1211 (Shatsky Rise, present-day northwest Pacific).
Neodymium isotopic ratios (143Nd/144Nd, expressed as Nd10) track deep-water mass circulation because Nd has a short oceanic residence time (~1000 years; for example, ref. 11) relative to oceanic mixing (~1500 years12). The Nd composition of individual deep-water masses is derived from the composition of Nd draining into the water masses’ source regions13-17. South Pacific surface watersNd values are highly radiogenic (~0) but are underlain by the nonradiogenic Nd values (~-8) of northward flowing Antarctic bottom waters (AABW)19. North Pacific Nddepth profiles are much less stratified despite the existence of distinct intermediate, deep, and bottom water masses20, a consequence of slow deep-water renewal in this region. Vertical exchange of Nd between radiogenic Pacific Intermediate Water and underlying AABW (~-8) generates the relatively radiogenic Nd signature (~-5) of the deep North Pacific.
To reconstruct ancient Pacific deep-sea circulation I analyzed the Nd isotopic composition of fossil fish teeth and bones. Fish debris record the Nd signal of oceanic deep and bottom waters at the sediment surface21-24, and this signal is resistant to diagenetic alteration24,25. The advantage of fossil fish debris over other benthic sedimentary phases (e.g., Fe-Mn crusts, benthic foraminifera) is their widespread geographic and stratigraphic distribution and resistance to dissolution in corrosive bottom waters.
Fish debris Nd isotope data from ODP Leg 198 (Shatsky Rise, northwest Pacific Ocean) Sites 1209 and 1211 (Table 1) demonstrate the evolution of deep-water mass composition over the period ~70 to 30 million years ago (Ma). The trend in Nd values at ODP Site 1209 (paleodepth ~2300m) begins with an increase from –4.8 to –3.2 over the interval 68.66 Ma to 64.88 Ma (Figure 2). Over the next ~20 million years Nd values fluctuate by up to 0.5 epsilon units about an average value of –3.2. Then Nd values decrease from –2.9 to –4.5 over the interval 45.99 to 33.89 Ma. The parallel Nd record generated from nearby ODP Site 1211 (paleodepth ~2900m) is characterized by an increase in Nd values from –4.4 (69.49 Ma) to –3.4 (63.05 Ma) (Figure 2). From 63.05 to 49.41 Ma, Site 1211 Nd values remain relatively high with a mean of –3.4, although a series of epsilon unit oscillations occurs from 54.81 to 49.41 Ma. After 49.41 Ma, Nd values decrease to –5.2 by 35.80 Ma.
The similarity in the overall Nd trends at both Shatsky Rise sites indicates that these sites were bathed by a common deep-water mass. While the timing and magnitude of short-term Nd oscillations at both sites do not coincide (most likely attributable to variations in sampling resolution and stratigraphic completeness), the long-term trends recorded at both sites indicate a period of major regional oceanographic change during the early Paleogene. Deep waters in the central subtropical Pacific evolved from a more nonradiogenic composition (~-4.5 to –5) from the latest Cretaceous into the Early Paleocene (~70 to 64 Ma), then remained relatively radiogenic (~-3 to –3.5) for the next ~20 million years and subsequently became more nonradiogenic again from ~46 to 33 Ma.
The ~1.5 epsilon unit shifts recorded at both sites must reflect a fundamental change in the source of deep waters bathing Shatsky Rise from ~65 to 45 Ma. The alternative, that tectonic motion of the Pacific Plate positioned Shatsky Rise within a different deep-water mass, is ruled out on the basis of negligible northward plate migration over the interval ~60 to 40 Ma (Figure 1) and the fact that deep-water Nd isotopic values shifted back toward more radiogenic values at ~46Ma. To place the magnitude of this long-term Nd shift in paleoceanographic perspective, glacial to interglacial changes in Atlantic Ocean thermohaline circulation patterns resulted in Nd changes of ~1 to 2 epsilon units29.
The Site 1209 and 1211 Nd data contain evidence for a radiogenic and a non-radiogenic end-member source of deep waters. The source of radiogenic Nd to the modern Pacific is weathering and drainage of the circum-Pacific arc volcanics13. The tectonic process that generated the arc volcanics, subduction of the Kula and Farallon Plates around the entire perimeter of the ancient northern Pacific, had been underway at least since ~110 Ma30. Thus the North Pacific was likely the source of radiogenic Nd to the Late Cretaceous and early Paleogene oceans. The shift of Pacific deep-water Nd isotopic composition toward more radiogenic values implies a more focused transfer of highly radiogenic surface waters to the deep ocean. This transfer is best explained by deep-water production in the North Pacific.
The non-radiogenic Nd end-member signature was produced in the Southern Ocean during the early Paleogene31. Insufficient data currently exist to determine in which sector of the Southern Ocean these deep waters formed in the past, however this non-radiogenic end-member composition is still clearly distinct from deep waters with a North Pacific signature. Formation of deep waters in the North Pacific represents a mode of thermohaline circulation considerably different from the modern, a mode that may have been dictated by the tectonic and climatic boundary conditions of the early Cenozoic.
Comparison of the Shatsky Rise Nd records to the Cenozoic record of global deep-sea temperatures derived from oxygen isotope analyses of bottom-dwelling foraminifera8 (Figure 2) indicates that the major shifts in deep-water Nd composition are bracketed by the warmest deep-sea conditions of the Cenozoic. The change from non-radiogenic, Southern Ocean Nd values to radiogenic, North Pacific Nd values coincides approximately with warming following the Cretaceous/Tertiary event. Shatsky Rise deep-waters carried a North Pacific deep-water Nd signal throughout the “greenhouse” portion of the Cenozoic record, when global deep-water temperatures were above ~7°C (Figure 2). As global deep-waters continued to cool during the Eocene, the source of deep waters shifted back to the Southern Ocean.
A switch from southern high-latitude deep-water production to the northern high-latitudes during a period of protracted global warmth and diminished equator-to-pole thermal gradients contradicts the prevalent belief in subtropical warm, saline deep-water production during times of extreme warmth. Thermodynamic and modeling constraints suggest that significant, prolonged production of warm and saline deep waters (a “halothermal” mode of deep-water circulation) during greenhouse climates might not have been feasible given the diminished role of salinity in the overall density of seawater at higher temperatures and the redistribution of oceanic salinity necessary to achieve downwelling of warmer waters32,33. Thus, despite overall warmer temperatures, the densest deep-waters likely still formed in the high-latitudes. Model simulations that account for continental runoff and precipitation-evaporation distributions34 predict downwelling in the North Pacific under early Cenozoic tectonic and climatic boundary conditions. The transition to extreme global warmth during the early Cenozoic may have altered the distribution of precipitation patterns, rendering Southern Ocean deep-water formation regions slightly fresher and/or North Pacific deep-water formation regions slightly more saline. Such a change may have been sufficient to alter the density regime of deep-waters, permitting significant downwelling of North Pacific waters.
The Nd isotopic composition of fossil fish debris reflects the source of deep-water masses in the ancient oceans. Two long-term paleo-Nd isotopic records from the present-day northwestern Pacific Ocean reveal a fundamental switch in deep-water production from the Southern Ocean to the North Pacific. The data indicate that production of North Pacific deep waters lasted ~20 million years, and coincided with the warmest climatic interval of the Cenozoic Era.
Figure Captions
- Paleogeographic reconstructions for 65 and 40 Ma (Ocean Drilling Stratigraphic Network, showing the location of Ocean Drilling Program (ODP) Sites 1209 and 1211. Note that the location of Sites 1209 and 1211 did not change significantly over the time interval investigated.
- Nd isotope data from ODP Site 1209 (red diamonds) and Site 1211 (blue triangles). Benthic foraminiferal oxygen isotope values (green circles)9 provide an estimate of deep-sea temperature. The orange line represents a deep-sea temperature of ~7°C calculated from the oxygen isotope values9.
Table 1 Shatsky Rise Nd isotope data
Site 1209Depth (mbsf) / Age (Ma) / 143Nd/144Nd / Nd(t)
114.60 / 33.88 / 0.512414 ± 10 / -4.52 ± 0.20
133.58 / 38.30 / 0.512400 ± 07 / -4.60 ± 0.14
141.61 / 41.28 / 0.512427 ± 12 / -4.00 ± 0.24
146.89 / 45.02 / 0.512442 ± 09 / -3.61 ± 0.18
146.89 / 45.02 / 0.512444 ± 10 / -3.57 ± 0.20
152.62 / 45.99 / 0.512476 ± 09 / -2.93 ± 0.18
163.60 / 48.07 / 0.512445 ± 08 / -3.48 ± 0.16
167.39 / 48.62 / 0.512462 ± 09 / -3.13 ± 0.18
171.60 / 49.47 / 0.512448 ± 07 / -3.38 ± 0.14
177.60 / 50.35 / 0.512465 ± 09 / -3.03 ± 0.18
185.60 / 53.13 / 0.512443 ± 12 / -3.39 ± 0.24
204.60 / 56.68 / 0.512456 ± 10 / -3.05 ± 0.20
210.21 / 57.68 / 0.512453 ± 08 / -3.07 ± 0.16
213.21 / 58.51 / 0.512449 ± 09 / -3.14 ± 0.18
214.10 / 58.75 / 0.512451 ± 08 / -3.10 ± 0.16
214.66 / 58.91 / 0.512453 ± 08 / -3.06 ± 0.16
223.59 / 61.16 / 0.512434 ± 07 / -3.37 ± 0.14
226.60 / 61.89 / 0.512418 ± 11 / -3.65 ± 0.22
230.11 / 62.92 / 0.512459 ± 09 / -2.84 ± 0.18
236.11 / 64.88 / 0.512438 ± 08 / -3.20 ± 0.16
263.71 / 68.66 / 0.512350 ± 08 / -4.82 ± 0.16
265.83 / 68.96 / 0.512356 ± 07 / -4.69 ± 0.14
268.59 / 69.35 / 0.512438 ± 11 / -3.09 ± 0.22
269.20 / 69.44 / 0.512374 ± 09 / -4.32 ± 0.18
270.09 / 69.57 / 0.512423 ± 08 / -3.37 ± 0.16
Site 1211
Depth (mbsf) / Age (Ma) / 143Nd/144Nd / eNd(t)
83.70 / 35.80 / 0.512372 ± 08 / -5.22 ± 0.16
86.70 / 39.48 / 0.512388 ± 06 / -4.81 ± 0.12
88.70 / 40.84 / 0.512404 ± 08 / -4.46 ± 0.16
97.70 / 49.41 / 0.512475 ± 10 / -2.86 ± 0.20
101.20 / 50.30 / 0.512440 ± 08 / -3.52 ± 0.16
107.20 / 51.45 / 0.512428 ± 08 / -3.72 ± 0.16
110.70 / 52.28 / 0.512472 ± 10 / -2.85 ± 0.20
116.55 / 54.81 / 0.512424 ± 09 / -3.71 ± 0.18
117.21 / 55.87 / 0.512433 ± 12 / -3.52 ± 0.24
123.20 / 57.61 / 0.512422 ± 05 / -3.69 ± 0.10
128.20 / 63.05 / 0.512431 ± 09 / -3.38 ± 0.18
137.70 / 66.05 / 0.512391 ± 07 / -4.08 ± 0.14
147.20 / 67.77 / 0.512388 ± 07 / -4.10 ± 0.15
156.70 / 69.49 / 0.512369 ± 08 / -4.42 ± 0.16
Fish debris was handpicked from the >63m size-fraction of washed samples then cleaned using an established reductive/oxidative cleaning protocol26,27. Samples were analyzed as NdO+ using a multi-collector Micromass Sector 54 at UNC-CH. Monitor peak (144Nd16O) beams of ~1 volt were achieved by introducing pure oxygen into the source via a leak valve. External analytical precision based upon replicate analysis of the UNC Ames Nd standard (as NdO+) was 0.512140 0.000014 (2) and analysis of the international standard JNdi28 yielded 0.512111 0.000022 which is calibrated relative to the La Jolla standard (0.511858) as 0.512116. Reported errors are within-run 2 values, which corresponds to a minimum uncertainty of 0.28 epsilon units when combined with the external precision. The procedural blank is ~50pg and is considered negligible. Nd isotope values are reported using the epsilon notation, Nd, which normalizes the analyzed 143Nd/144Nd ratio to the present-day bulk earth value of CHUR (chondritic uniform reservoir), 0.5126389. The Sm isotopic composition of five samples yielded a mean 147Sm /144Nd ratio of 0.131 from which Nd(t) values were calculated.
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