1
Text S1
2
In an effort to understand our observations regarding the PETM at Mead and Dee
3
Streams, we have built a simple box model for dissolved O2 concentrations in the ocean.
4
It is comprised of four oceanic reservoirs with a cycling time of 2 kyrs between them.
5
Reservoirs include a low-latitude shallow ocean (0-500 mbsl), a high-latitude shallow
6
ocean (0-500 mbsl), a global intermediate water ocean (500-2000 mbsl), and a global
7
deep water ocean (2000-5000 mbsl). We assume that the high-latitude surface ocean
8
served as the source of dissolved O2 entering intermediate and deep ocean water masses,
9
similar to the modern ocean. Dissolved O2 in intermediate ocean are thus impacted by
10
water flow from the high-latitude shallow ocean, by oxidation of sinking organic carbon
11
and, during carbon injection of the PETM, potentially by aerobic oxidation of methane.
12
Deep ocean dissolved O2 is treated as a mixture of high-latitude surface ocean and
13
intermediate dissolved O2, and the low-latitude surface ocean is calculated independent of
14
the other reservoirs. Here we focus on variations in dissolved O2 in the intermediate
15
ocean because these results relate to the bioturbation records we have generated for Mead
16
and Dee Streams.
17
We calculate dissolved O2 in the surface ocean reservoirs as a function of
18
temperature based on solubility relationships presented by Garcia and Gordon [1992]. To
19
understand changes in dissolved O2 during the PETM (Figure S1), we first set the high-
20
latitude surface ocean temperature to 9C. This value comes from benthic foraminifera
21
18O records [Zachos et al., 2001], which should be representative of both deep ocean
22
temperatures and the temperature at the location those waters formed. We then increase
23
surface ocean temperatures by 5.5 C over the duration of the PETM (Figure S1). This is
24
generally consistent with temperature variations records from both surface and deep
25
ocean reservoirs [e.g., Kennett and Stott, 1991; Zachos et al., 2001, 2003, Sluijs et al.,
26
2006]. The temperature increase relates to a decrease in dissolved O2 of ~0.03 mols/m3 in
27
the high-latitude surface ocean during the warmest point of the PETM (Figure S1).
28
The drop in dissolved O2 that creates the modern OMZ within intermediate waters
29
is driven by oxidation of organic carbon sinking from the surface ocean. We assume in
30
our model that a similar mass of organic carbon was oxidized in the early Paleogene
31
OMZ. Therefore, we model the effect of organic carbon oxidation on the OMZ dissolved
32
O2 content by decreasing the number of O2 moles in our intermediate water reservoir by a
33
value necessary to reduce the modern dissolved O2 concentration of sinking surface
34
waters to the modern mean OMZ dissolved O2 concentration, a drop of ~0.23 mols/m3.
35
Our model suggests that the rise in temperature would have driven average dissolved O2
36
concentrations of intermediate waters to a minimum of ~0.016 mols/m3 during the
37
warmest portion of the PETM (Figure S1).
38
The amount and location of carbon added during the PETM remain the source of
39
debate. Nonetheless, several lines of evidence support the idea that it was methane
40
released from the seafloor when gas hydrate dissociated in marine sediment [e.g.,
41
Dickens et al., 1995, 1997; Sluijs et al., 2007; Zeebe et al., 2009]. This is an especially
42
appealing mechanism to drop dissolved O2 concentrations in intermediate waters because
43
the methane would necessarily enter these waters directly [Dickens et al., 1995; Dickens,
44
2000]. To get a theoretical methane input, we assume a 13C composition of –60 ‰ and
45
then estimate the injection rate and duration by taking the first derivative of an
46
interpolated curve fit to the bulk carbonate 13C record at Site 690 [Bains et al., 1999], an
47
approach similar to that taken by Dickens [2001]. The methane carbon input can be
48
modeled as a ~2.4 x 1018 g (~2.0 x 1017 mole) injection of reduced carbon into the upper
49
2000 m of the ocean within ~65 kyrs (Figure S1). This overall carbon input is somewhat
50
similar to that suggested by Zeebe et al. [2009], which was based on global records of
51
deep-sea carbonate dissolution.
52
Aerobic methane oxidation removes 2 moles of dissolved O2 for every one mole
53
of C. If all methane in the above postulated release oxidized to CO2 within intermediate
54
waters, average dissolved O2 concentrations within the OMZ would have fallen to 0
55
mols/m3 for ~10 kyrs at the onset of the PETM, and below ~0.01 mols/m3 for another
56
~50 kyrs (Figure S1). This may be consistent with the observed bioturbation records at
57
Mead and Dee Stream, if they are representative of large regions of intermediate water
58
depth.
59
To assess the relationship between our regional South Pacific records and the modeled
60
global mean results, we have fit modern global OMZ dissolved O2 data [Boyer et al.,
61
2006] to a normal cumulative distribution function. We then shifted this function to our
62
modeled mean OMZ values with a standard deviation that reduces as a function of the
63
mean. This approach maintains the shape of the modern dissolved O2 distribution at any
64
given mean level and can be used to calculate a percent of intermediate waters exposed to
65
hypoxic conditions. Results indicate that ~6% of the latest Paleocene OMZ was
66
characterized by hypoxic or lower dissolved O2 levels, a value roughly double the amount
67
of the modern ocean because of warmer surface ocean temperatures (Figure S2).
68
Additional warming during the PETM would have risen the amount of hypoxic waters at
69
intermediate depths to ~23%. The combined effect of warming and methane oxidation, as
70
modeled above, would further this to ~59% (Figure S2), a result which certainly warrants
71
a more sophisticated modeling approach but nonetheless offers an intriguing link to our
72
NZ records. The 13C and bioturbation records generated at Mead and Dee Streams
73
suggest a direct relationship between carbon injection and oxygen depletion. If the
74
dissolved oxygen distributions we have used are in fact applicable, these records indicate
75
that the New Zealand margin would have been in the ~36% of the intermediate ocean our
76
model suggests would have only been exposed to hypoxic waters as a result of both
77
warming and methane oxidation.
78
79
Auxiliary Figure Captions
80
Figure S1. Dissolved O2 Box Model Constraints and Results. (a) Idealized PETM high-
81
latitude surface ocean temperature curve (see Appendix 1 for details) and the resultant high-
82
latitude surface ocean dissolved O2 content. (b) The ODP Site 690 bulk carbonate 13C record
83
(purple diamonds and line) [Bains et al., 1999], an interpolated curve fit to this 13C data (blue
84
circles and line), and the injection rate of –60 ‰ C necessary to drive the interpolated 13C curve
85
(green circles and line). (c) The modeled oxygen minimum zone (OMZ) mean dissolved O2
86
response to the PETM increase in high-latitude surface temperature (grey circles and line) and
87
the combined effect of this warming and the oxidation of the methane injection illustrated in
88
pane “b” (orange diamonds). Distribution points (i.e. D1, D2, D3) represent the mean dissolved
89
O2 values used to constrain distribution functions illustrated in Figure S2.
90
91
Figure S2. OMZ Dissolved O2 Cumulative Distributions. The OMZ normal dissolved
92
O2 cumulative distributions for the modern ocean (grey circles and line; data from Boyer
93
et al., 2006), the latest Paleocene (blue diamonds and line; representing point D1 in
94
Figure S1c), and two PETM realizations, one produced from PETM warming alone
95
(orange circles and line; representing point D2 in Figure S1c), and one produced by the
96
combined effect of PETM warming and methane oxidation (red diamonds and line;
97
representing point D3 in Figure S1c).
98
99
Auxiliary Table Captions
100
Table S1. Mead Stream and Dee Stream Geochemical and B% Results. *Sample depths
101
reported in previous studies have been shifted slightly to account for our updated
102
lithostratigraphic logs. See noted references for original heights by Field Number.
103
104
Table S2. Sample-sets used for B% error evaluation.
105
106
Table S3. Mead Stream and Dee Stream Age Models. *The Dee Stream outcrop
107
section between the PETM and H1/ELMO/ETM2 is complicated by a combination of
108
faults, heavily vegetated rock face, and a broad swing in the river that has created
109
separate but internally continuous intervals of obviously usable outcrop; one containing
110
the PETM and one the early Eocene H1-I2 CIEs (Nicolo et al., 2007). Therefore, because
111
this section has not yet been logged in sufficient detail to fully resolve the true
112
stratigraphic height of the HI CIE onset relative to the PETM, here we report heights
113
consist with prior work [Hancock et al., 2003; Nicolo et al., 2007] but rely on the pre-
114
PETM sedimentation rate to calculate Dee Stream sample ages for the sections both
115
below and above the PETM CIE (Figure 7 and Figure 8). Age references relate to the
116
following superscript lettering: A = Westerhold et al., 2007; B = Farley and Eltgroth,
117
2003; C = Röhl et al., 2000; D = the mean age of B and C; E = Hancock et al., 2003.