Production of North Pacific Deep Waters During the early Cenozoic

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 gateways^2-4, 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 Ross Sea (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 interval^8,9, 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 existed^2,3. In addition, the Norwegian and Greenland Seas 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 regions^13-17. South Pacific surface waters _Nd values are highly radiogenic (~0) but are underlain by the nonradiogenic _Nd values (~-8) of northward flowing Antarctic bottom waters (AABW)¹⁹. North Pacific _Nd depth 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 surface^21-24, and this signal is resistant to diagenetic alteration^24,25. 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 waters^32,33. 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.

1. 
   Paleogeographic reconstructions for 65 and 40 Ma (Ocean Drilling Stratigraphic Network, www.odsn.de) 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.
   
2. 
   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

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