This is. 20 probably because Na2O is easily leached during surface weathering and vaporized by ignition. 21 ..... change. Geol. Magazine 135, 735-753 (1998).
1
1
Supplementary Note 1. Major element contents
2
Major elements compositions were analyzed for ninety-one samples to estimate the BSi
3
and terrigenous contents. The analyzed samples include two rock types, chert and shale. In all
4
samples analyzed, SiO2 is a major component (60.4 % to 97.9 %), followed by Al2O3 (1.5 % to
5
18.1 %) (Fig. 2). The SiO2 contents in chert beds (80.5 % to 97.9 %) were higher than in shale
6
beds (60.4 % to 80.1%) (Fig. 2).
7
Correlations between elements were examined for all the elements analyzed in order to
8
estimate BSi content from the major element composition of samples. SiO2 content is negatively
9
correlated with contents of other elements, especially Al2O3, TiO2, Fe2O3, MgO, and K2O (r >
10
-0.90). Al2O3, TiO2, Fe2O3, MgO, and K2O contents are positively correlated with each other (r
11
> 0.90). Because Al is the major component of terrigenous materials that are dominantly
12
contained in aluminosilicates, and is considered as one of the most immobile elements in the
13
surface environment1, it is reasonable to regard Al as a representative element of terrigenous
14
materials. Consequently, elements that show high positive correlation with Al are also regarded
15
as being held mostly in terrigenous materials. Thus, these elements are named the terrigenous
16
elements. Lower but still positive correlations are observed between Al2O3 and MnO, P2O5, and
2
17
CaO (0.70 < r < 0.90). Relatively low correlation coefficients may reflect association of parts of
18
these elements with authigenic phases, such as Mn-oxides and apatite, in addition to their
19
association with terrigenous materials2. There is no clear correlation between Na2O and Al2O3 (r
20
= 0.25), although Na is also considered to be held dominantly in aluminosilicates1. This is
21
probably because Na2O is easily leached during surface weathering and vaporized by ignition
22
during sample preparation1. Because contents of MnO, P2O5, CaO, and Na2O are less than 1% in
23
samples analysed, these elements will not be discussed in this study.
24 25
Supplementary Note 2. Estimation of BSi and terrigenous material contents
26
To estimate the BSi content from the major element contents of bedded chert, it is
27
necessary to know SiO2 and Al2O3 contents of the terrigenous material in bedded chert.
28
Previous studies on bedded chert assumed that SiO2 and Al2O3 contents of terrigenous material
29
as 52.8% and 26.1%, respectively, with SiO2/Al2O3 ratio of 2.02, which based on the chemical
30
composition of illite3,
31
modern pelagic ocean contains not only illite, but also quartz and feldspars with their contents
32
being 20 to 40% and 15 to 30%, respectively, based on the mineral compositional analysis of
4, 5
(Fig. 2A). However, the terrigenous material accumulated in the
3
33
pelagic red clay6, 7. Therefore, SiO2/Al2O3 ratio of terrigenous material accumulated in the
34
modern pelagic ocean is 3 to 4.5 based on the major element chemical analysis of pelagic red
35
clay6, 7 (Fig. 2A). Thus, the terrigenous material in bedded chert is probably more SiO2 rich, and
36
its SiO2/Al2O3 ratio should be larger than previously estimated.
37
Assuming the smallest SiO2 content among all the analyzed shale samples should be close
38
to the SiO2 content of terrigenous material in bedded chert, the calculated BSi contents in
39
individual chert and shale beds range from 75 to 93% and 0 to 75% with average values of 81%
40
and 20%, respectively. The standard deviations of calculated BSi contents of the individual
41
chert and shale beds are 7.8% and 14% with a relative standard deviation of 9.6% and 70%,
42
respectively (Fig. 2A). The accumulation amounts of BSi and terrigenous material per one
43
chert-shale couplet per unit area range from 2.5 to 14 g cm-2 and from 0.5 to 5.1 g cm-2 with
44
average values of 6.8 g cm-2 and 3.0 g cm-2, respectively (Fig. 2B).
45 46
Supplementary Note 3.Estimation of BSi flux in cherts as oceanic Si sink in the early
47
Mesozoic Panthalassa superocean
48
To support the significant contribution of bedded chert for the biogeochemical silica cycle
4
49
in the Mesozoic ocean, we examined the worldwide distribution of the early Mesozoic bedded
50
cherts. The paleolatitude estimation at the time of deposition of the Middle and Upper Triassic
51
bedded chert in the Inuyama area suggests low latitudes (5.6° ± 2.2°) and northern low to
52
middle latitudes (29.5°N ± 17.4°) of western Panthalassa8, 9, 10 (Fig. 1). The Middle Triassic to
53
Middle Jurassic bedded chert in the low latitude (2.1◦ ± 5.2◦S) of the eastern Panthalassa was
54
found in Tsukumi section, Shakumasan Group of the Chichibu Terrane, Kyushu, southwestern
55
Japan11 (Fig. 1 and supplementary Fig. 1). The Upper Triassic to Lower Cretaceous bedded
56
chert of the western Panthalassa with no paleolatitude data was found in the Pisenaizawa section,
57
Kamuikotan Terrane, Hokkaido, northern Japan12, 13 (Fig. 1 and Supplementary Fig. 1). The
58
paleolatitude of the Pisenaizawa section can be inferred as low latitude on the basis of the
59
nearby Upper Cretaceous fore-arc basin sequence in the low latitude14 (16.7◦ +11.0/-9.8◦N) and
60
plate motion direction of the Izanagi Plate15. The distance that these terranes traveled prior to
61
accretion is not known with certainty, but a rough estimate of 2100–4200, 2400–4800, 3000–
62
6000 km can be made on the basis of approximately ~70-Myr (Early Triassic– Early Jurassic),
63
~80-Myr (Early Triassic–Middle Jurassic), and ~100-Myr (Late Triassic– Early Cretaceous)
64
travel history, respectively, and a rough convergence rate of 3–6 cm yr−1 15. The Lower Triassic
5
65
bedded chert deposited at the southern middle latitude (34°S ± 8°) of western Panthalassa was
66
found in Waiheke island of the Waipapa composite Terrane, New Zealand16 (Fig. 1). The Lower
67
Jurassic to Lower Cretaceous bedded chert deposited at low latitude (0 ± 2°, 1 ± 2°, 2 ± 4°)17 to
68
the middle latitude (32°N ± 8°)18 of western Panthalassa was found in the Franciscan Terrane,
69
North America (Fig. 1). Thus, it can be concluded that bedded chert was widely deposited at
70
least in the low to middle latitude in the both hemispheres of the eastern and western
71
Panthalassa during the Early Triassic to Early Jurassic (Fig. 1).
72
The superocean Panthalassa comprised an area of 80–90% in area of the global ocean
73
during the Early Triassic to Early Jurassic (Fig. 1)19. Although its distribution could have been
74
extended to higher latitudes, there are no high latitude pelagic records available at this moment.
75
Assuming that bedded chert covered the area of the low latitude Panthalassa between 30°±10°
76
S and N, the depositional area of bedded chert would have occupied at least approximately 40 to
77
60 % (~1.2-2.1 x 108 km2) of the global ocean during the Early Triassic to Early Jurassic (Fig.
78
1).
79
We also compiled the BSi burial fluxes for bedded chert sequences of the area other than
80
the Inuyama area to support the claim that the BSi records of the Inuyama bedded chert is
6
81
representative of the low-mid latitudes of Panthalassa (Fig. 3). The average BSi burial fluxes for
82
the Middle Triassic bedded chert of the equatorial western Panthalassa in the Tsukumi section
83
are 0.25 to 0.34 g cm−2 kyr−1 with average value of 0.29 g cm−2 kyr−1
84
burial flux for the Upper Triassic to Lower Jurassic bedded chert of the central Panthalassa in
85
the Pisenaizawa section ranges from 0.12 to 0.19 g cm−2 kyr−1 with an average value of 0.15 g
86
cm−2 kyr−1
87
the low latitude of eastern Panthalassa was >0.40 g cm−2 kyr−1 (at Franciscan Terrane, western
88
North America; Murchey, 1984, Hagstrum et al., 1993). These Myr-scale BSi burial fluxes are
89
the same order as those for the Inuyama bedded chert (Fig. 2; 0.18 to 0.39 g cm−2 kyr−1 with
90
average value of 0.29 g cm−2 kyr−1). Phase differences of Myr-scale cycles between the Inuyama
91
and Pisenaizawa sections might have resulted from latitudinal difference with probably lower
92
upwelling intensity of Pisenaizawa section on the outside of main equatorial upwelling region
93
during periods of higher equatorial upwelling (Fig. 1). The intensified equatorial upwelling
94
could have enhanced spatial variations of BSi burial, which makes anti-phase relationship
95
between the inside and outside of upwelling regions. This interpretation is consistent with the
96
lower BSi burial flux and the relative amplitudes of orbital cycles in Pisenaizawa section than
20, 21, 22
. The average BSi
12, 13
. The average BSi burial flux for the Lower Jurassic bedded chert deposited in
7
97
those of Inuyama section (Fig. 3).
98
The BSi burial fluxes for the Lower Triassic bedded chert in the southeastern Panthalassa
99
range from 0.61 to 0.76 g cm−2 kyr−1 with an average value of 0.66 g cm−2 kyr−1 (Waiheke
100
section, Waipapa Terrane, New Zealand23. This estimate is much larger than that of Inuyama
101
area. However, in the Inuyama area, Sakuma et al.24 reported the Smithian-Spathian siliceous
102
claystone sequence with a linear sedimentation rate several times higher than that of the
103
overlain bedded chert sequence, suggesting a much higher BSi burial flux during the
104
Smithian-Spathian, although the BSi contents and BSi burial flux of Smithian/Spathian
105
sequence has never been estimated. Thus, it can be concluded that the BSi burial flux of the
106
Lower Triassic sequences in the Inuyama and Waiheke sections would be same order.
107 108
Supplementary Note 4. Simple weathering model for orbital-scale variations in chemical
109
weathering
110
Regional chemical weathering rates can be approximated as a function of runoff and
111
Arrhenius temperature functions based on the modern observations25, 26, 27, 28 (see Methods). The
112
results show small amplitudes of
