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 underrepresented 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
1
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 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.
2
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
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 deepwater masses is derived from the composition of Nd draining into the water masses’
source regions13-17. 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 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
3
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
4
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 nonradiogenic 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 nonradiogenic 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.
5
A switch from southern high-latitude deep-water production to the northern highlatitudes 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 presentday 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.
6
Figure Captions
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
Site 1209
Depth (mbsf) Age (Ma)
114.60
33.88
133.58
38.30
141.61
41.28
146.89
45.02
146.89
45.02
152.62
45.99
163.60
48.07
167.39
48.62
171.60
49.47
177.60
50.35
185.60
53.13
204.60
56.68
210.21
57.68
213.21
58.51
214.10
58.75
214.66
58.91
223.59
61.16
226.60
61.89
230.11
62.92
236.11
64.88
263.71
68.66
265.83
68.96
268.59
69.35
269.20
69.44
270.09
69.57
Nd(t)
-4.52 ± 0.20
-4.60 ± 0.14
-4.00 ± 0.24
-3.61 ± 0.18
-3.57 ± 0.20
-2.93 ± 0.18
-3.48 ± 0.16
-3.13 ± 0.18
-3.38 ± 0.14
-3.03 ± 0.18
-3.39 ± 0.24
-3.05 ± 0.20
-3.07 ± 0.16
-3.14 ± 0.18
-3.10 ± 0.16
-3.06 ± 0.16
-3.37 ± 0.14
-3.65 ± 0.22
-2.84 ± 0.18
-3.20 ± 0.16
-4.82 ± 0.16
-4.69 ± 0.14
-3.09 ± 0.22
-4.32 ± 0.18
-3.37 ± 0.16
143
Nd/144Nd
0.512414 ± 10
0.512400 ± 07
0.512427 ± 12
0.512442 ± 09
0.512444 ± 10
0.512476 ± 09
0.512445 ± 08
0.512462 ± 09
0.512448 ± 07
0.512465 ± 09
0.512443 ± 12
0.512456 ± 10
0.512453 ± 08
0.512449 ± 09
0.512451 ± 08
0.512453 ± 08
0.512434 ± 07
0.512418 ± 11
0.512459 ± 09
0.512438 ± 08
0.512350 ± 08
0.512356 ± 07
0.512438 ± 11
0.512374 ± 09
0.512423 ± 08
7
Site 1211
Depth (mbsf) Age (Ma)
83.70
35.80
86.70
39.48
88.70
40.84
97.70
49.41
101.20
50.30
107.20
51.45
110.70
52.28
116.55
54.81
117.21
55.87
123.20
57.61
128.20
63.05
137.70
66.05
147.20
67.77
156.70
69.49
143Nd/144Nd
0.512372 ± 08
0.512388 ± 06
0.512404 ± 08
0.512475 ± 10
0.512440 ± 08
0.512428 ± 08
0.512472 ± 10
0.512424 ± 09
0.512433 ± 12
0.512422 ± 05
0.512431 ± 09
0.512391 ± 07
0.512388 ± 07
0.512369 ± 08
eNd(t)
-5.22 ± 0.16
-4.81 ± 0.12
-4.46 ± 0.16
-2.86 ± 0.20
-3.52 ± 0.16
-3.72 ± 0.16
-2.85 ± 0.20
-3.71 ± 0.18
-3.52 ± 0.24
-3.69 ± 0.10
-3.38 ± 0.18
-4.08 ± 0.14
-4.10 ± 0.15
-4.42 ± 0.16
Fish debris was handpicked from the >63 m 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
143
Nd/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.
8
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Figure 1
Figure 2
11