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Oxygen and carbon isotope fields for temperate shelf carbonates from Tasmania and New Zealand
Marine Geology, 103 (1992) 273-286
273
Elsevier Science Publishers B.V., Amsterdam
Oxygen and carbon isotope fields for temperate shelf carbonates from Tasmania and New Zealand C. Prasada Rao and Campbell S. Nelson Department of Geology, University of Tasmania. Hobart. Tasmania 7001. Australia Department of Earth Sciences. UniversiO,of Waikato, Private Bag 3105, Hamilton 2001, Nen, Zealand (Received August 16, 1990; revision accepted June 3, 1991)
ABSTRACT Rao, C.P. and Nelson, C.S., 1992. Oxygen and carbon isotope fields for temperate shelf carbonates from Tasmania and New Zealand. Mar. Geol., 103: 273-286. Temperate latitude bryomol carbonates mantle significant areas of the modern Tasmanian and New Zealand shelves and bounding basins. The isotope fields for the temperate skeletons and whole sediments are distinctly separated from their warm shallow marine counterparts because of their heavier 6'80 and narrow range of 6'aC values. However, the temperate isotope fields overlap with the deep-sea carbonate isotope field, and the seafloor diagenesis isotope trend line passes tarough these fields. Tasmanian whole sediments and skeletal isotope fields are similar and are closely clustered along the diagenesis trend line because of intragranular cemanta!i~,~l. The New Zealand skeletal iso:ope field is broader and scattered, possibly due to lack of cementation and some biochen:ical fractionation, but its whole sediment isotope field is tighter and contained within the Tasmanian whole sediment field. In contrast to warm water carbonates, the enrichment of 61aO is small ( < 0.3%o)because of predominantly calcitic min~-r.,Iogy and deposition under essentially normal marine salinities. Ambient water temperatures calculated from the 61sO values are mainly within the range 4-16°C and gradually decre,.;se with increasing sample depth. Variations in ,~13Csuggest the temperate carbonates are in equilibrium more with upwelling oeep waters and not with surface waters as is the case for warm shallow marine carbonates. Positive correlation between 61aO and 613C in the temperate carbonates suggests the isotope paleotemperature of ancient equivalents may be estimated from their ~;~3Ccomposition, which is much less prone to modification during diagenesis than is 6~sO.
Introduction M o d e r n temperate carbonates are widespread on m a n y mid- to high-latitude shelves (Lees, 1975; Nelson, 1988), including those about Tasmania (Davies and Marshall, 1973; Rao, i 98 l) and New Zealand (Nelson et al., 1988). The carbonates from these two localities are forming over a complete spectrum from warm to cool temperate climate conditions, at latitudes ranging from about 34 ° to 48°S (Fig. I). Based on uni,~brmitarian principles, we should expect to find similar widespread temperate carbonates in the rock record, but only a few examples of ancient temperate carbonates have so far been documented (e.g., Nelson, 1978; Brookfield, 1988; Draper, 1988; Rao, 1988a; James and Bone, 1989). This apparent anomaly exists because 0025°3227/92/$05.00
we need both more data on modern temperate carbonates and additional studies o f more ancient carbonates to interpret their temperatures o f formation. The present study aims to delineate for the first time the O and C isotope fields of modern cool to warm temperate carbonates with examples from Tasmanian and New Zealand carbonate sediments. In so doing this will provide much needed baseline isotope data for evaluating paleotemperatures and diagenetic processes affecting ancient calcitic limestones generally which, according to James and Bone (1989), abound in the stratigraphic record and can be setter understood by studying modern calcitic non-tropical carbonates rather than modern aragonitic tropical deposits. Earlier work on O and C isotopes of modern shallow marine temperate carbonates (Rao and
~? 1992 -- Elsevier Science Publishers B.V. All rights reserved
274
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• B582 168OE I Fig. I. Location map (inset) showing sample sites about Tasmania (A) and off northern (B, Three Kings platlbrm) and southern (C, Snares platform) New Zealand. in CA) sample numbers are prefixed by either 19, 20 or 21 depending on the sector locality shown (other sample details in Davies and Marshall (1973)). In (B) and (C) the dots and crosses indicate whole sediment and skeletal analyses, respectively (other sample details in Nelson (1982) and Head (1985)).
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Green, 1983), mainly from the cool temperate western Tasmanian shelf, indicated that the temperate isotope field is distinctly different from the shallow warm water carbonate field and follows the trend line for seafloor diagenesis, an indication of equilibrium conditions in both O and C with bottom waters. To verify whether the cool temperate western Tasmanian isotope field is appropriate to other regions and to a wider range of warm to cool temperate carbonates, we have analysed the O and C isotope composition of modern carbonates from ea~tern and northern Tasmania and from northern and southern New Zealand (Fig. i). The results confirm that the temperate carbonate isotope fields are markedly different from those of their shallow warm water counterparts. Vast volumes of the modern cold seas are under-
!
saturated in C a C O 3 , and the CaCO3 in contact with these waters can be dissolved (Alexandersson, 1978), Occurrences of extensive temperate carbonate~ in southern Australia and New Zealand imply that saturation of CaCO 3 has been maintained in cold seas by oceanographic processes, such as upwelling. The present study makes an assessment or isotope variations in sediments with water depth and records the effect of upwelling waters on the ,;ediments. Modern temperate carbonates are commonly mixed with Last Glacial carbonates, formed when sea level was up to 130 m lower than present. Differentiation of modern from relict carbonate material is essential in understanding modern carbonate sedimentation and oceanic processes operating in their formation. Relict carbonate material
OXYGEN AND CARBON ISOTOPE FIELDS FOR TEMPERATE SHELF CARBONATES. TASMANIA AND NEW ZEALAND
may have been subaerially exposed and affected by meteoric diagenesis during sea-level fluctuations. O and C isotopes aid in differentiation of meteorically altered carbonates from normal marine carbonates. Meteorically affected carbonates are characterised by slight depletion in 61aO relative to marine sediments and marked depletion in 6~3C because of ~2C enrichment by soil CO2 (Lohmann, 1988). Meteorically unaffected marine carbonates provide the temperature range from surface to bottom waters. Winter and summer surface-water temperatures around Tasmania range between about 11 and 16°C. The thermocline (Wass et al., 1970) generally varies from 18 to 45 m, with a gradual drop in temperature with water depth of 5°C. Off northern New Zealand, winter to summer sea-surface temperatures range from 15 to 22°C (Nelson et al. 1982); off southern New Zealand they range from 10 to 14°C (Head, 1985). Shelf depth (<200 m) bottom temperatures are in the range 12 to 15°C and 9 to 13°C off northern and southern New Zealand, respectively. The present study presents for the first time ambient water temperatures in these temperate carbonates, based on t~iaO values, and demonstrates that the C a C O 3 is in equilibrium with ambient rather than with surface-water temperatures. Sedimentology of temperate shelf carbonates The Tasmanian and New Zealand shelf carbonates are skeletal hashes composed mainly of bryozoans, bivalves, molluscs and foraminifera, as well as echinoderms, barnacles, calcareous red algae and brachiopods amongst other minor groups (Marshall and Davies, 1978; Rao, ! 981; Nelson et al., 1988). They are typical bryomol temperate carbonates (Nelson et al., 1988) comprising appreciable amounts of bryozoan material. Textures are mainly sand or, more commonly, mixed sandgravel, occasionaily silty in the case of some of the Tasmanian carbonates. Skeletal grains range from fresh to etched, bored, and encrusted by epibiota, suggesting low rates of sediment accumulation. In most cases calcite skeletons completely dominate over aragonite ones. Off northern New Zealand the calcite includes a spectrum of low-
275
(2-4 mol% MgCO3) to high- (mainly 6-9 mol% MgCO3) Mg varieties (Nelson et al., 1982), while off southern New Zealand and about southern Tasmania low..Mg calcite is usually most important, consistent with the lower ambient water temperatures in these more southern locations (Rao, 1981, 1986; Head, 1985). Rao (1981) reports the occurrence of fibrous spherulitic and rhombohedral calcite cements within the chambers of bryozoan skeletons from the Tasmanian carbonates, but comparable intragranular cementation has not been observed in the New Zealand carbonates. Most recently, Rao (1990a) has shown for the Tasmanian carbonates how a number of geochemical trends involving Mg, Sr, Na, Mn and Fe, and their relation to O and C isotope composition, are indicative of marine diagenesis in cool/ cold waters and quite unlike meteoric diagenetic trends. Methods of study Tasmanian samples (Fig. I A) were collected during 1973 on a l0 n mile grid (Davies and Marshall, 1973); additional samples were collected during 1980 (Rao and Green, 1983). The northern New Zealand samples were obtained during New Zealand Oceanographic Institute (D.S.I.R)cruise 1077 (Nelson, 1982) and by SCUBA diving at Leigh (Fig. I B and inset), while the southern New Zealand samples (Fig. 1C) were collected from the Royal New Zealand Navy hydrographic survey vessel Monowai (Head, 1985). From salt-free and dried samples, splits of whole sediments and p,iekings of individual skeletons of bryozoans and brachiopods (and one gastropod) were crushed and reacted with anhydrous HaPO4 at 25°C. The evolved CO2 gas was analysed on a Micromass 602D at the University of Tasmania for 6lsO and 613C, the values being expressed in conventional permil notation relative to the PDB standard. The precision of data for both O and C is ___0.1%o. Results
Skeletons The 6180 and 613C values of bryozoans, brachiopods and a gastropod from Tasmania and New
276
C.P. RAO AND C.S. NELSON
ske!etons are contained within an isotope field only slightly larger (by + 0.5%e O and C) than, and of similar shape to, the Tasmanian field, and is also bisected by the seafloor diagenesis trend line. Compared to the bryozoans, brachiopods have a similar range of 6~sO values, but tend towards heavier ~3C values.
Zealand are listed in Tables I and 2, respectively, along with sample locations and water depths. The isotope values of skeletons are plotted in Fig. 2 along with the field of skeletons from generally warm shallow marine settings (James and Choquette, 1983). The isotope field for the temperate carbonates is distinctly separated from that of the warm shallow marine skeletal field because of the heavier ~ s O values characterising the temperate field. The Tasmanian isotope field is tighter than the New Zealand one and the seafloor diagenesis trend line (Milliman and Muller, 1977) passes through the middle of the Tasmanian field. The seafloor diagenesis trend line passes through the origin of the plot due to the equilibrium conditions of both O and C in carbonate varying with water temperature. The New Zealand isotope field overlaps the Tasmanian one. However, if the most t3Cdepleted sample is excluded, the New Zealand
Whole sediment
The 6xsO and 6t3C composition of whole sediments from Tasmania (Table 3) and New Zealand (Table 4) are plotted on Fig. 3 along with the whole sediment isotope field of warm shallow marine carbonates (Milliman and Muller, 1977). The temperate whole sediment isotope field is distinctly different from warm shallow marine sediments, and is characterised by small positive 6~sO values and by consistently lower ~ 3 C values than the
TABLE 1
6tsO and/itaC values of Tasmanian skeletons Latitude (E)
Longitude (S)
Depth (m)
61s0~ PDB
613C~ PDB
!.5 1.5 0.9
1.7 !.7 !.7 2.0 ! .8 !.5 1.7 !.7 O.4 ! .0 0.9 !.! 0.9 1.3 0.4 0.2
Bryozoa 1982 1983 2002 2020 2030 2051 2076 2082 2088 2093 2097 2098 2100 2124 2129 2143
43"!7.2 43~i7.4 42'21.2 42°20.0 41'39.8 40~50.6 4Y42.2 43°33.5 43~!2.2 43°05.0 42°58.2 42°51.1 42~51.2 41°30.3 41°49.5 41~i9.6
14800.5 14807.2 148'~31.0 148~26.3 148'32.1 148'46.5 145~18.6 145°52.1 14543.3 145~26.0 145~05.0 145'~19.5 145~00.6 144~45.~ 144°46.0 144°26.6.
130 39 115 100 113 399 108 161 62 135 188 91 146 49 86 128
0.9 0.9 0.8 0.6 -O.2 0.5 0.9 0.6 0.6 0.6 0.7 0.8
40°35.5 43°17.2 42°20.0 41"39.8 40°50.6
147~50.5 148~00.5 14826.3 148"32.1 148°46.5
32 130 100 !13 399
1.0
1.8
2.7 2.6 1.5 0.7
1.8 2.0 2.3 1.1
40' 50.6
148~46.5
399
2.0
1.9
1.3
Brachiopods 1929 1982 2020 2030 2051
Gastropod 205 i
OXYGEN AND CARBON ISOTOPE FIELDS FOR TEMPERATESHELF CARBONATES.TASMANIAAND NEW ZEALAND
277
TABLE 2 6aso% o and 6~3C% o values o f New Zealand skeletons Sample number
Latitude (S)
Longitude (E)
Depth (m)
6180% e
6~3C0,~
PDB
PDB
Bryozoans G9 E27 M RL-C P448 E27 P551 P448 P489 M RL-D P55 ! P489 E27 P448 P458 P448 P558 M RL-C P558 P458 P558 P492 P458
Snares Snares Leigh Three Kings Snares Three Kings Three Kings Three Kings Leigh Three Kings Three Kings Snares Three Kings Three Kings Three Kings Three Kings Leigh Three Kings Three Kings Three Kings Three Kings Three Kings
45~00.1" 47040.3 , 36 ° ! 6. !' 34°23.4" 47040.3 , 33068.6 , 340"23.4, 34°18. !" 36 ° 16.1" 33058.6 , 34 ° 18. !" 47040.3 ' 34°23.4' 34 ° 13.8' 34°23.4 . 33058.8 ' 36 ° 16. !" 33 '58.8" 34 ° 13.8" 33°58.8" 34°21.6' 34 ° 13.8"
! 6635.5' 167°38.7 ' 174°47.9 , 172°28. ! 3' 167038.7 ' i 7 i °55.2" 172028.3 . 172049.7 ' ! 74047.9 , 171 °55.2' 172040.7 . ! 67°38.7 ' 172°28.3" ! 7 i °56.4' 172028.3 ' 171°43.4' 174°47.9 ' 171 °43.4' 171 °56.4' 171 °43.4' 172036.5 ` ! 71 °56.4'
! 70 ! 31 8 ! 01 131 920 101 88 6 920 88 13 I 10 ! 200 101 178 8 178 200 178 85 200
I.i 1.1 0.1 1.6 1.0 2.9 1.3 0.4 -0.7 2.5 0.4 1.8 0.1 1.2 0.6 1.0 0.4 1.0 1.3 1.2 0.6 1.4
!.8 !.5 -0.3 1.7 1.6 1.7 2.7 0.4 -2.9 -0.3 -0.7 2.7 0.3 1.4 1.1 1.2 1.1 1.9 1.8 2.1 2.0 2.1
Brachiopods M RL-E G9 P55g P489
Leigh Snares Three Kings Three Kings
36°36.1' 45°00.1' 33058.8 , 34'~18. I'
174047.9 ` 166°35.5 ' ! 71 °43.4' 172°40.7 '
20 170 178 88
0.0 1.1 0.9 0.5
0.7 1.7 2.9 1.3
warm water carbonate field. The seafloor diagem esis trend line again bisects the temperate whole sediment isotope field. In contrast to the skeletal isotope field (Fig. 2), the whole sediment New Zealand field is smaller than, and contained within, the Tasmanian whole sediment field, the latter showing a wider range of both 6'sO and 6'3C than its New Zealand counterpart.
Comparison of isotopefields (Fig. 4) Tasmanian whole sediment and skeletal isotope fields are similar, except for very slightly enriched 6'3C values in the whole sediments. In contrast, the New Zealand whole sediment and skeletal isotope fields are different, with higher ranges in
both 6xsO and 6~3C for skeletons. Warm shallow marine whole sediments are enriched relative to their skeletons in both ~lsO and 6t3C, due to inorganic cementation and higher salinities associated with the whole sediments. Discussion
The 6 ' s o and 6'3C variations in carbonates can be influenced by several factors, including the following: (1) mineralogy, (2) salinity, (3) biochemical fractionation, (4) water depth, (5) ambient water temperature, (6) 6x3C variation in surface to deep water, (7) cementation, and (8) modern versus relict sediment. These factors are considered briefly below.
278
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L_ Fig. 2. 6~80 and 6t3C variation of skeletons from Tasmanian and New Zea!a..-..dcarbonates compared with the field of James ,',-,~d Choquette (1983) for warm water carbonates. Note heavy &:sO and narrow range of 6taC values for the temperate carbonates. The Tasmanian skeleton isotope values a,e more closely clustered along the seafloor diagenesis trend line than the New Zealand data.
Mineralogy The 6~aO composition of carbonates varies with the relative proportions of low-Mg calcite, aragonite, high-Mg calcite and vaterite in the samples. The maximum enrichment of 5~aO relative to lowMg calcite is about 0.6%0 PDB in aragonite, 0.06%0 PDB in high-Mg calcite and 0.50/ooPDB in vaterite (Tarutani et al., 1969). As the main minerals present in temperate carbonates are low- to highMg calcites with some aragonite and possibly vaterite (Rao, 1981, 1986; Nelson, 1988), the enrichment in ~ 8 0 will be small (<0.3%0; Rao and Green, 1983). This is ;n contrast to higher 5180 enrichments in shallow warm water carbonates due to their high content of aragonite and highMg calcite.
SaliniO' The &1so values of carbonates from warm shallow marine settings can increase in response to
evaporation of seawater in (semi-) arid latitudes or in semi-enclosed embayments. The salinity of temperate ocean water is mainly close to normal (ca. 34.7%0). The New Zealand carbonates are forming in waters whose salinity lies between about 34.3 and 35.7%o, the more southern occurrences lying at the lower end of this range (Nelson et al., 1988). The Tasmanian shelf carbonates are also forming under normal salinity conditions but are affected by low-salinity subantarctic waters during winter, which might ha,,e caused a slight depletion in ~180 values.
Biochemicalfractionation The higher or lower than expected values of O and C isotopes compared to known salinity and temperature variations might be due to biochemical fractionation and photosynthesis of blue-green algae (Milliman and Muller, 1977), which are abundant in warm shallow marine environments.
OXYGEN AND CARBON ISOTOPE FIELDS FOR TEMPERATESHELF CARBONATES.TASMANIAAND NEW ZEALAND
279
TABLE 3 6~sO and 613C values o f Tasmanian whole sediments. Additional isotope data are listed in R a o a n d Green (1983)
1929 1968 1972 1982 1983 2002 2020 2030 2051 2054 2056 2057 2058 2059 2061 2062 2078 2073 2086 2088 2090 2093 2096 2098 2099 2100 2 ! 02 2116 2117 2122 2123 2124 2125 2132 2 i 36 2139 2141
Latitude (S)
Longitude (E)
Depth (m)
6 ! sO%0 PDB
6 ! aC%° PDB
40035.5 4003 ! .0 40°50.6 43°17.2 43 ° 17.4 42 ° 12.2 42°20.0 41 °39.8 40°50.6 43°28.8 43035.5 43040.5 43°47.0 43°58.0 43°43.5 43°53.2 43024.6 43°57.0 43°22.5 43 ° 12.2 43 ° ! 5.0 43°00.5 42058. ! 42°51.1 42°51.1 42°51.2 42039.5 42000.0 41001.2 41029.5 41'29.5 4 ! ° 30.3 41"39.8 42°00.2 44 ° 10.2 44020.2 41 ° ! ! 2
147o50.5 147o2 ! .0 147°08.4 148°00.5 148°07.2 148°3 ! .0 148°26.3 148°32. l 148°46.5 147°58.0 147°32.3 147040.3 147°48.5 147°30.0 ! 47007. ! 147°08.3 ! 47048.8 146°33.7 145°44.5 145°43.3 145030.6 145026.0 145 ° 15.5 145°19.5 145°09.0 145000.6 145009.6 144°33.7 144021.5 ! 44 ° 24.4 144°36.2 144°45.8 144°47.3 144°51.8 144°57.2 145°00.3 144°35.6
32 55 52 130 39 ! 15 100 I 13 399 161 121 146 212 ! 75 128 148 133 159 144 62 155 135 ! 32 91 124 146 90 55 80 ! 19 91 49 60 132 128 122 80
i.4 1.4 i. 1 2.3 2.4 1.2 1.7 i.6 2.9 1.9 !.8 1.9 !.9 1.0 1.9 ! 07 ! .2 0.5 - O. I O. 1 0.5 1.2 ! .2 1.5 1.2 !.0 1.0 -0. I i.5 O.7 !.5 0.9 - 0.3 0.9 ! .3 0.5 1.8
!.4 ! .9 1.7 2.7 2.4 2.1 2.4 2.7 2.2 1.9 1.9 2.3 2.2 1.9 ! .7 1.8 1.3 0.2 0.5 O. l 0.8 0.0 0.7 1.2 l.i !.i 1.0 0.5 0.9 0.9 !.2 0.6 1.0 1.2 ! .4 0.9 0.8
In temperate carbonates calcareous algae are generally too minor a constituent to affect O and C isotope values appreciably. Biochemical fractionation by fauna in Tasmanian carbonates is minimal because both O and C isotope values are in equilibrium with seawater (Fig. 2) and ambient water temperatures (Fig. 5). The O and C isotope compositions of living bryozoans and micrite cement are also similar in Tasmanian carbonates (Rao and Green, 1983). In the New Zealand
carbonates a degree of biochemical fractionation is evident because different species of bryozoans from the same sample have different isotope values. However, too few skeletons have yet been analysed to gauge the importance of biochemical fractionation processes in delimiting the isotope fields.
Water depth The variations in 6lsO with water depth in the Tasmanian and New Zealand temperate carbon-
280
c.p. RAO AND C.S. NELSON TABLE 4 6~aO and 6~3C values o f New Zealand whole sediments. Additional isotope d a t a are listed in Nelson (1982) Sample number
Latitude (S)
Longitude (E)
Depth (m)
~laO%e PDB
613C% PDB
47 ° i 0.2 47 o 10. ! 47°20.0 47020.0 47040.3 47°50.0 47049.3 48°01.9 48°00.0
167 ° 27. ! 168 o 19. ! 167°25.5 ! 68034.0 167°38.7 166°30.2 167 ° 14.3 166°37.4 167038.0
84 100 130 ! 14 131 178 146 134 143
1.0 1.3 1.9 2.4 !.5 !.3 2.8 0.9 !.5
i. I 1.9 2.0 1.9 1.7 i.4 2.0 !.2 1.9
34°29.6 34"24.0 34 ° 14.8 34 '~19.5 33°55.0 34°0 ! .8 34 ° 17.9 34°23.7 34°07.T 34°08.4 33"48. ! 33°58.8 33°57.0
171°50.3 172 ° 16.8 ! 72°09.5 17 ! °40.2 i 72 ~ 14.2 172 ° ! 2.0 172°34.7 172°50.7 172~'32.6 ' 17 i °42.3 171"56.6 171 ~'43.4 171 ~I ! .0
580 ! 20 292 510 144 508 94 30 98 354 516 178 700
2.5 ! .5 1.8 2.6 2.0 1.9 I..8 2.5 1.8 2.2 2.0 1.2 1.6
1.3 ! .3 1.0 I. I !.2 ! .2 ! .2 !.5 1. I 1.5 - 0.4 1.5 0.7
Snares B!5 B46 C36 E402 E27 F39 F41 G8 B582
Three Kings P436 P441 P454 P463 P473 P476 P488 P496 P532 P548 P555 P558 P562
ates are plotted on Fig. 5, which shows a tendency towards progressive enrichment of 6~aO with increasing shelf depth (up to ca. 200 m). Samples from depths > 200 m are also slightly enriched in 6tad with increasing water depths. The 61sO enrichment with increasing water depth can be explained by the temperature decrease into deeper waters across the shelf and into the bounding basins. The 613C variations with water depth (Fig. 6), like 6tad, tend to become heavier with increasing depth across the shelf. However, in still deeper water (> 200 m) the 61ac values generally decrease with increasing depth. Ambient water temperature
Estimates of ambient water temperatures were calculated from 61aO values of carbonates using the equation of Shackleton and Kennett (1975) with 6w=0%o (normal seawater) and 6w= -1%o
(upwelling water): T (°C)- 16.9- 4.38(6c- 6w)+ 0.10(6c- ~w)z where 6c is the O isotope composition of CO2 produced from the carbonate at 25°C and 6w is the O isotope composition of CO2 in equilibrium with formation water. These calculated temperatures are shown along the bottom of Fig. 5. Taking 6w = 0, the Tasmanian shelf water temperatures range from ! 7 to 5°C and the New Zealand shelf temperatures from 18 to 4°C. The temperatures of the deeper waters are relatively uniform, with a minimum of 2.5°C. Using the upwelling deep water 6w value of - 1%o (Craig and Gordon, 1965) drops the temperatun, calculations by about 4°C, from to 13 to I°C for Tasmania and 14 to 0°C for New Zealand shelf carbonates; the deeper waters range from 3 to -2°C. The temperature variation in the surface water layer (upper ca. 50 m) is considerable due to an appreciable seasonal
OXYGEN AND CARBON ISOTOPE FIELDS FOR TEMPERATE SHELF CARBONATES. TASMANIA AND NEW ZEALAb~t::~
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Fig. 3. Comparison of 61so and 613C values for whole sediments from the Tasmanian and New Zealand temperate carbonates with the warm shallow marine carbonate field of Milliman and Muller (1977). The whole sediment temperate values are clustered along the seafloor diagenesis line and their 613C values are appreciably lighter than for warm shallow marine carbonates.
affect (Fig. 5). The bottom mixed layer shows a progressive drop in temperature with depth. ~ 3 C variation in surface to deep water
In the temperate carbonates the 6~3C value in the basin at the 920 m water depth is -0.3%o PDB, a value which generally becomes heavier towards the shelf edge (Fig. 6). Maximum 5~3C values of ca. 3%o PDB occur at the shelf edge, from where the di~3C values become progressively lighter towards the shallow part of the shelf, the lightest 6t3C value of -2.9%o PDB occurring inshore at a depth of 6 m at Leigh (Fig. I, inset). In the modern mid-latitude Pacific Ocean, the 6~3C of dissolved inorganic carbon (Kroopnick et al., 1977) varies with depth (Fig. 7) from about 2.3°oo PDB at the surface (ca. 50 m) to 0%o PDB in the oxygen minimum layer (ca. 500 m). This decrease in the 6~aC value of seawater with increasing depth is due to variable productivity and mixing of water systems. Upwelling, cold deep
polar waters have depleted 6'3C values due to the oxidation of organic matter (61aC= -25%0 PDB) at depth. As this upwelling water mixes with deep basin water the dissolved 13C increases, resulting in enrichment of &lac with decreasing depth. If the 613C of CO2 (gas) or HCOa- (solution) is held constant, the 6IaC in equilibrium calcite increases with decreasing temperature (Rubinston and Clayton, 1969; Emrich et al., 1970; Kroopnick et al., 1977). The 6~aO of calcite in seawater also increases with decreasing water temperature (Fig. 5). Therefore, positive correlation between 6~sO and tSt3C with varying water temperature occurs in deep water, where equilibrium calcite passes through the origin (6,=0%0; 61ac=0%o PDB). This positive correlation (Fig. 7) also occurs in surface water (where atmospheric CO2 is relatively constant at ca. -7.2°oo ~13C PDB). The temperate carbonate isotope field overlaps the deep-water isotope field (Fig. 7) and equilibriuni calcite passes through both the shallow temperate and deep-marine isotope fields because
282
C.P. RAO AND C.S. NELSON
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Fig. 4. Summary comparison of whole sediment and skeletal temperate water isotope fields with those for warm shallow marine carbonates.
temperate carbonates are in equilibrium with deep water (Fig. 7). In contrast with temperate carbonates, warm shallow marine carbonates are in equilibrium with surface water, largely because of their formation at very shallow water depths (< 10 m), such as those in the Arabian Gulf and Great Bahama Bank (e.g., Scoflin, 1986). Cementation
lntragranular cements are common in the Tasmanian carbonates but are not recorded in the New Zealand carbonates. In deep-sea carbonates the seafloor diagenesis trend line passes through the origin (zero values) because of lithification. The Tasmanian skeletal and whole sediment isotope fields are closely clustered along the seafloor diagenesis trend line because of cementation. In contrast, the New Zealand skeletal isotope values are more scattered and the isotope field partly deviates from the seafloor diagenesis trend line, possibly because of lack of cements. However, the New Zealand whole sediment isotope field falls
within its Tasmanian counterpart (Fig. 3), suggesting •I,,,, , . . . . .,i,. e whole sediments are much more in equilibrium with the seawater than are some of the skeletons. Modern versus relict sediment
When sea level dropped by about 130 m during the Last Glaciation (ca. 18,000 yrs B.P.), any previous shelf carbonates at depths down to 130 m would have been exposed and affected by meteoric diagenesis. Meteoric diagenesis is characterised by slight depletion in ~180 and marked depletion in ~t3C due to CO2 enrichment of 12C in the soil zone because of decomposition of organic matter (Lohmann, 1988). For example, Recent and Pleistocene speleothem deposits from caves in Tasmania have ~ 8 0 values ranging from - 3 . 5 to - 4 . 6 % PDB and ~taC values from - 6 . 7 to - 13.7°~ PDB (Goede et al., 1986). Both ~lsO and ~ n C values of temperate carbonates in depths from 0 to 130 m (Figs. 5 and 6) are much heavier than meteorically affected speleothems, and the
283
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284
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Fig. 7. Lines of equilibrium calcite in surface and bottom waters and the isotope fields of deep sea carbonates (Milliman, 1974) and warm and temperate water shallow marine carbonates. Note that temperate carbonates are in equilibrium with upwelling deep waters whereas warm shallow marine carbonates are in equilibrium with surface waters. See text for details.
inverted "J-trend" characteristic of meteoric diagenesis (Meyers and Lohmann, 1985; Lohmann, 1988) is not present in the temperate carbonates studied. For these reasons the carbonates now occurring in depths from 0 to 130 m are probably mainly of post-Last Glacial age. The deeper water carbonates appear also to have not been affected by meteoric diagenesis.
Geological implications from modern temperate stable isotope data The 61sO and 6~3C variations in modern, shallow marine, temperate shelf carbonates suggest that these carbonates are forming in regions influenced by upwelling deep water, which maintains saturation in CaCO3 and provides nutrients for the growth of fauna. In regions lacking upwelling water the cold seas can become undersaturated in CaCO3, in which case submarine dissolution and chemical breakdown (or maceration) is common (Alexandersson, 1978). Continuation of these de-
structive processes would obliterate cold water carbonates from the stratigraphic record. Isotope paleotemperature measurements involve accurate determinations of the 6180 composition of both the original marine calcite and of ancient seawater, which many considered to have varied through time (e.g., James and Choquette, 1983; Rao, 1991). Cathodoluminescence ( P o p p e t al., 1986) and variations in trace element with 6tsO (Veizer et al., 1986), and a combination of these two (Rao, 1990b), provide original marine calcite 6180 values, which should be used along with the appropriate 6~sO of seawater of known geological age to determine paleotemperatures. In ancient carbonates, the ~t3C values tend to be near marine values, despite pronounced phreatic meteoric diagenesis (Lohmann, 1988). As 6180 and 6t 3C values are positively correlated in modern temperate carbonates, the marine 613C values can be used to estimate original 6180 values in ancient carbonates considered to be cold water in origin on geological and chemical evidence. Application
OXYGEN AND CARBON ISOTOPE FIELDS FOR TEMPERATE SHELF CARBONATES. TASMANIA AND NEW ZEALAND
of this 6 t a c approach in paleotemperature measurements for cold water Permian (Rao and Green, 1982: Rao, 1988a, b) and Ordovician (Brookfield, 1988) carbonates have given reasonable ambient water temperatures despite pronounced diagenetic equilibration of 6tsO values. Geochemical differences exist between tropical, temperate and polar carbonates (Rao, 1991). The O and C isotope fields of modern calcitic temperate carbonates provide much needed baseline isotope data for evaluating paleotemperatures and diagenetic processes affecting the calcitic limestones which are so abundant in the stratigraphic record. Many of these ancient limestones were probably non-tropical carbonates and they can be better understood by comparing them with modern calcitic temperate carbonates (Rao and Adabi, 1992) rather than with modern aragonitic tropical carbonates (James and Bone, 1989).
Conclusions Based on modern Tasmanian and New Zealand data, the cool to warm temperate shallow marine carbonate O and C isotope field is distinctly different from that of warm shallow marine carbonates. This clear separation occurs for the following reasons: (1) Temperate skeletons and whole sediments are heavier in ~tso and narrower in their range of 61aC relative to warm shallow marine carbonates. (2) The temperate isotope field overlaps with that of deep-sea carbonates and is bisected by the seafloor diagenesis trend line because the temperate carbonates are in equilibrium with upwelling deep water. (3) In the case of the Tasmanian carbonates, intragranular cementation is the main reason for the whole sediment and skeletal isotope field clustering closely along the seafloor diagenesis trend line. Lack of cements and the occurrence of some biochemical fractionation, as in New Zealand skeletons, results in a broader and more scattered isotope field. (4) Predominance of a calcitic mineralogy and more or less normal marine salinities are responsible for minimal enrichment of 6~sO in temperate carbonates, which contrasts with the appreciable enrichment
285
in warm water carbonates due to their abundant aragonite and high-Mg calcite mineralogy. The calculated 61sO ambient temperature in shallow temperate seas ranges from 4 to 18°C (assuming 8w = 0%o), whereas for deeper carbonates the temperatures are low and more uniform, from 2 to 5°C. Under the influence of upwelling waters (6w = -1%o) these temperatures drop by up to 4°C. There is appreciable temperature variation in surface waters ( < 50 m) due to strong seasonal effects. The 613C variation indicates that the temperate carbonates studied are i~a equilibrium with upwelling deep water in contrast to equilibrium with surface waters in warm shallow marine carbonates. Because 6tsO and 6~3C are positively correlated in temperate carbonates, the isotope paleotemperatures of ancient carbonates may be estimated from the 6~aC values of these carbonates, which do not change significantly compared to 6t80 modifications during diagenesis.
Acknowledgements We thank Michael Power (University of Tasmania) for the isotope analyses, the BMR for providing Tasmanian samples, Abigail Smith (University of Waikato) for selecting and identifying the New Zealand skeletons used in this isotope study, and Frank Bailey (University of Waikato) for drafting the figures.
References Alexandersson, E.T., 1978.Destructivediagenesisof carbonate sediments in the eastern Skagerrak, North Sea. Geology,6: 324-327. Brookfield, M.E., 1988.A mid-Ordoviciantemperatecarbonate shelf--the Black River and Trenton LimestoneGroups of southern Ontario, Canada. In: C.S. Nelson (Editor), Nontropical ShelfCarbonates--Modern and Ancient. Sediment. Geol., 60: 137-153. Craig, H. and Gordon, L.I., 1965. Deuterium and oxygen-18 variations in the ocean and marine atmosphere. In: Stable Isotopes in Oceanographic Studies and Paleotemperatures. Spoleto, July 26-27, 1965. CNR, Lab. Geol. Nucl., Pisda, pp. 1-22. Davies, P.J. and Marshall, J.F., 1973. BMR marine geology cruise in Bass Strait and Tasmanian waters--February to May, !973. Bur. Miner. Resour., Australia, Rec. 134, 19 pp. D r a p e r , J.J., 1988. Permian limestone in the southeastern Bowen Basin, Queensland:an exampleof temperate carbonate deposition. In: C.S. Nelson (Editor), Non-tropical Shelf
286 Carbonates--Modern and Ancient. Sediment. Geol., 60: 155-162. Emrich, K., Erhalt, D.H. and Vogel, J.C., 1970. Carbon isotope fractionation during precipitation of calcium carbonate. Earth Planet. Sci. Lett., 8: 363-371. Goede, A., Green, D.C. and Harmon, R.S., 1986. Late Pleistocene palaeotemperature record from a Tasmanian speleothem. Aust. J. Earth Sci., 33: 333-342. Head, P.S., 1985. Surficial sediments on the Snares Platform, southern New Zealand: a cold-water, carbonate-dominated shelf. M.Sc. Thesis, Univ. Waikato, Hamilton, 235 pp. (Unpubl.). James, N.P. and Bone, Y., 1989. Petrogenesis of Cenozoic temperate water caicarenites, South Australia: a model for meteoric/shallow burial diagenesis of shallow water calcite cements. J. Sediment. Petrol., 59: 191-203. James, N.P. and Choquette, P.W., 1983. Diagenesis, 6. Limestones--the sea floor diagenetic environment. Geosci. Can., 10: 162-179. Kroopnick, P.M., Margolis, S.V. and Wong, C.S., 1977. t3C variations in marine carbonate sediments as indicators of the CO2 balance between the atmosphere and the oceans. In: N.R. Anderson and A. Mlahoff (Editors), The Fate of Fossil Fuel CO2 in the Oceans. Plenum, New York, pp. 305-321. Lees, A., 1975. Possible influence of salinity and temperature on modern shelf carbonate sedimentation. Mar. Geol., 19: 159-198. Lohmann, K.C., 1988. Geochemical patterns of meteoric diagenetic systems and their application to studies of paleokarst. In: N.P. James and P.W. Choquette (Editors), Paleokarst. Springer, New York, pp. 58-80. Marshall, J.F. and Davies, P.J., 1978. Skeletal carbonate variation on the continental shelf of eastern Australia. BMR J. Aust. Geol. Geophys., 3: 85-92. Meyers, W.J. and Lohmann, K.C., 1985. Isotope geochemistry of regionally extensive calcite cement zones and marine components in Mississippian limestone, New Mexico. In: N. Sehneiderman and P.M. Harris (Editors), Carbonate Cements. Soc. Econ. Paleontol. Mineral. Spec. Publ., 36: 223-240. Milliman, J.D., 1974, Marine Carbonates, Recent Sedimentary Carbonates, Part !. Springer, New York, 375 pp. Milliman, J.D. and Muller, J., 1977. Characteristics and genesis of shallow-water and deep-sea limestones. In: N.R. Anderson and A. Malahoff (Editors), The Fate of Fossil Fuel CO 2 in the Oceans. Plenum, New York, pp. 655-672. Nelson, C.S., 1978. Temperate shelf carbonate sediments in the Cenozoic of New Zealand. Sedimentology, 25: 737-771. Nelson, C.S., 1982. Compendium of sample data for temperate car!'3nate sediments, Three Kings Plateau, northern New F=:::!an4 ~,~iv. Waikato Dep. Earth Scl. Occas. Rep. 7, 95 pp. Nelson,. C.S. ' ?~;8. A~"introductory perspective on non-tropical shzi(c~ .: ~nates. in: C.S. Nelson (Editor), Non-tropical Shelf Carbonates--Modern and Ancient. Sediment. Geol., 60: 3-12. Nelson, C.S., Hancock, G.E. and Kamp, P.J.J., 1982. Shelf to basin temperate skeletal carbonate sediments, Three Kings Plateau, New Zealand. J. Sediment. Petrol., 52: 717-732.
C.P. RAO AND C.S. NELSON
Nelson, C.S., Keane, S.L. and Head, P.S., 1988. Non-tropical carbonate deposits on the modern New Zealand shelf. Sediment. Geol., 60: 71-94. Popp, B.N., Anderson, T.F. and Sandberg, P.A., 1986. Brachiopods as indicators of original composition in some Paleozoie limestones. Geol. Soc. Am. Bull., 97: ! 262-1269. Ran, C.P., 1981. Cementation in cold-water bryozoan sand, Tasmania, Australia. Mar. Geol., 40: M23-M33. Ran, C.P., 1986. Geochemistry of temperate-water carbonates, Tasmania, Australia. Mar. Geol., 71: 363-370. Ran, C.P., 1988a. Paleoclimate of some Permo-Triassic carbonates of Malaysia. In: C.S. Nelson (Editor), Non-tropical Shelf Carbonates--Modern and Ancient. Sediment. Geol., 60:117-129. Ran, C.P., 1988b. Oxygen and carbon isotope composition of cold-water Berriedale Limestone (Lower Permian), Tasmania, Australia. In: C.S. Nelson (Editor), Non-tropical Shelf Carbonates--Modern and Ancient. Sediment. Geol., 60: 221-231. Ran, C.P., 1990a. Geochemical characteristics of cool-temperate carbonates, Tasmania, Australia. Carbonates Evaporites, 5: 209-221. Ran, C.P., 1990b. Petrography, trace elements and oxygen and carbon isotopes of Gordon Group carbonates (Ordovician), Florentine Valley, Tasmania, Australia. Sediment. Geol., 66: 83-97. Ran, C.P,, 1991. Geochemical differences between tropical (Ordovician), temperate (Recent and Pleistocene) and subpolar (Permian) carbonates, Tasmania, Australia. Carbonates Evaporites, 6: 83-106. Ran, C.P. and Adabi, M.H., 1992. Carbonate minerals, major and minor elements and oxygen and carbon isotopes and their variation with water depth in cool, temperate carbonates, western Tasmania, Australia. Mar. Geol., 103: 249-272. Ran, C.P. and Green, D.C., 1982. Oxygen and carbon isotopes of Early Permian cold-water carbonates, Tasmania, Australia. J. Sediment. Petrol., 52: ! ! I !-1125. Ran, C.P. and Green, D.C., 1983. Oxygen- and carbon-isotope composition of cold shallow- marine carbonates of Tasmania, Australia. Mar. Geol., 53: !17-129. Rubinston, H. and Clayton, R.N., 1969. Carbon-13 fractionation between aragonite and calcite. Geochim. Cosmochim. Acta, 33: 997-1004. Scoflin, T.P., 1986. An Introduction to Carbonate Sediments and Rocks. Blackie, Glasgow, 280 pp. Shackleton, N.J. and Kennctt, J.P., 1975. Paleotemperature history of the Cenozoic and the initiation of Antarctic glaciation: oxygen and carbon isotope analyses in DSDP sites 277, 279 and 28 !. lnit. Rep. DSDP, XXIX: 743-755. Tarutani, T., Clayton, R.N. and Mayeda, T.K., 1969. The effect of polymorphism and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochim. Cosmochim. Acta, 33: 987-996. Veizer, J., Fritz, P. and Jones, B., 1986. Geochemistry of brachiopods: oxygen and carbon isotopic records of Paleozoic oceans. Geochi:n. Cosmochim. Acta, 50: 1679-1696. Wass, R.E., Connolly, R.J. and Maclntyre, R.J., 1970. Bryozoan carbonate sand continuous along southern Australia. Mar. Geol., 9: 63-73.
273
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Oxygen and carbon isotope fields for temperate shelf carbonates from Tasmania and New Zealand C. Prasada Rao and Campbell S. Nelson Department of Geology, University of Tasmania. Hobart. Tasmania 7001. Australia Department of Earth Sciences. UniversiO,of Waikato, Private Bag 3105, Hamilton 2001, Nen, Zealand (Received August 16, 1990; revision accepted June 3, 1991)
ABSTRACT Rao, C.P. and Nelson, C.S., 1992. Oxygen and carbon isotope fields for temperate shelf carbonates from Tasmania and New Zealand. Mar. Geol., 103: 273-286. Temperate latitude bryomol carbonates mantle significant areas of the modern Tasmanian and New Zealand shelves and bounding basins. The isotope fields for the temperate skeletons and whole sediments are distinctly separated from their warm shallow marine counterparts because of their heavier 6'80 and narrow range of 6'aC values. However, the temperate isotope fields overlap with the deep-sea carbonate isotope field, and the seafloor diagenesis isotope trend line passes tarough these fields. Tasmanian whole sediments and skeletal isotope fields are similar and are closely clustered along the diagenesis trend line because of intragranular cemanta!i~,~l. The New Zealand skeletal iso:ope field is broader and scattered, possibly due to lack of cementation and some biochen:ical fractionation, but its whole sediment isotope field is tighter and contained within the Tasmanian whole sediment field. In contrast to warm water carbonates, the enrichment of 61aO is small ( < 0.3%o)because of predominantly calcitic min~-r.,Iogy and deposition under essentially normal marine salinities. Ambient water temperatures calculated from the 61sO values are mainly within the range 4-16°C and gradually decre,.;se with increasing sample depth. Variations in ,~13Csuggest the temperate carbonates are in equilibrium more with upwelling oeep waters and not with surface waters as is the case for warm shallow marine carbonates. Positive correlation between 61aO and 613C in the temperate carbonates suggests the isotope paleotemperature of ancient equivalents may be estimated from their ~;~3Ccomposition, which is much less prone to modification during diagenesis than is 6~sO.
Introduction M o d e r n temperate carbonates are widespread on m a n y mid- to high-latitude shelves (Lees, 1975; Nelson, 1988), including those about Tasmania (Davies and Marshall, 1973; Rao, i 98 l) and New Zealand (Nelson et al., 1988). The carbonates from these two localities are forming over a complete spectrum from warm to cool temperate climate conditions, at latitudes ranging from about 34 ° to 48°S (Fig. I). Based on uni,~brmitarian principles, we should expect to find similar widespread temperate carbonates in the rock record, but only a few examples of ancient temperate carbonates have so far been documented (e.g., Nelson, 1978; Brookfield, 1988; Draper, 1988; Rao, 1988a; James and Bone, 1989). This apparent anomaly exists because 0025°3227/92/$05.00
we need both more data on modern temperate carbonates and additional studies o f more ancient carbonates to interpret their temperatures o f formation. The present study aims to delineate for the first time the O and C isotope fields of modern cool to warm temperate carbonates with examples from Tasmanian and New Zealand carbonate sediments. In so doing this will provide much needed baseline isotope data for evaluating paleotemperatures and diagenetic processes affecting ancient calcitic limestones generally which, according to James and Bone (1989), abound in the stratigraphic record and can be setter understood by studying modern calcitic non-tropical carbonates rather than modern aragonitic tropical deposits. Earlier work on O and C isotopes of modern shallow marine temperate carbonates (Rao and
~? 1992 -- Elsevier Science Publishers B.V. All rights reserved
274
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• B582 168OE I Fig. I. Location map (inset) showing sample sites about Tasmania (A) and off northern (B, Three Kings platlbrm) and southern (C, Snares platform) New Zealand. in CA) sample numbers are prefixed by either 19, 20 or 21 depending on the sector locality shown (other sample details in Davies and Marshall (1973)). In (B) and (C) the dots and crosses indicate whole sediment and skeletal analyses, respectively (other sample details in Nelson (1982) and Head (1985)).
l~~,
Green, 1983), mainly from the cool temperate western Tasmanian shelf, indicated that the temperate isotope field is distinctly different from the shallow warm water carbonate field and follows the trend line for seafloor diagenesis, an indication of equilibrium conditions in both O and C with bottom waters. To verify whether the cool temperate western Tasmanian isotope field is appropriate to other regions and to a wider range of warm to cool temperate carbonates, we have analysed the O and C isotope composition of modern carbonates from ea~tern and northern Tasmania and from northern and southern New Zealand (Fig. i). The results confirm that the temperate carbonate isotope fields are markedly different from those of their shallow warm water counterparts. Vast volumes of the modern cold seas are under-
!
saturated in C a C O 3 , and the CaCO3 in contact with these waters can be dissolved (Alexandersson, 1978), Occurrences of extensive temperate carbonate~ in southern Australia and New Zealand imply that saturation of CaCO 3 has been maintained in cold seas by oceanographic processes, such as upwelling. The present study makes an assessment or isotope variations in sediments with water depth and records the effect of upwelling waters on the ,;ediments. Modern temperate carbonates are commonly mixed with Last Glacial carbonates, formed when sea level was up to 130 m lower than present. Differentiation of modern from relict carbonate material is essential in understanding modern carbonate sedimentation and oceanic processes operating in their formation. Relict carbonate material
OXYGEN AND CARBON ISOTOPE FIELDS FOR TEMPERATE SHELF CARBONATES. TASMANIA AND NEW ZEALAND
may have been subaerially exposed and affected by meteoric diagenesis during sea-level fluctuations. O and C isotopes aid in differentiation of meteorically altered carbonates from normal marine carbonates. Meteorically affected carbonates are characterised by slight depletion in 61aO relative to marine sediments and marked depletion in 6~3C because of ~2C enrichment by soil CO2 (Lohmann, 1988). Meteorically unaffected marine carbonates provide the temperature range from surface to bottom waters. Winter and summer surface-water temperatures around Tasmania range between about 11 and 16°C. The thermocline (Wass et al., 1970) generally varies from 18 to 45 m, with a gradual drop in temperature with water depth of 5°C. Off northern New Zealand, winter to summer sea-surface temperatures range from 15 to 22°C (Nelson et al. 1982); off southern New Zealand they range from 10 to 14°C (Head, 1985). Shelf depth (<200 m) bottom temperatures are in the range 12 to 15°C and 9 to 13°C off northern and southern New Zealand, respectively. The present study presents for the first time ambient water temperatures in these temperate carbonates, based on t~iaO values, and demonstrates that the C a C O 3 is in equilibrium with ambient rather than with surface-water temperatures. Sedimentology of temperate shelf carbonates The Tasmanian and New Zealand shelf carbonates are skeletal hashes composed mainly of bryozoans, bivalves, molluscs and foraminifera, as well as echinoderms, barnacles, calcareous red algae and brachiopods amongst other minor groups (Marshall and Davies, 1978; Rao, ! 981; Nelson et al., 1988). They are typical bryomol temperate carbonates (Nelson et al., 1988) comprising appreciable amounts of bryozoan material. Textures are mainly sand or, more commonly, mixed sandgravel, occasionaily silty in the case of some of the Tasmanian carbonates. Skeletal grains range from fresh to etched, bored, and encrusted by epibiota, suggesting low rates of sediment accumulation. In most cases calcite skeletons completely dominate over aragonite ones. Off northern New Zealand the calcite includes a spectrum of low-
275
(2-4 mol% MgCO3) to high- (mainly 6-9 mol% MgCO3) Mg varieties (Nelson et al., 1982), while off southern New Zealand and about southern Tasmania low..Mg calcite is usually most important, consistent with the lower ambient water temperatures in these more southern locations (Rao, 1981, 1986; Head, 1985). Rao (1981) reports the occurrence of fibrous spherulitic and rhombohedral calcite cements within the chambers of bryozoan skeletons from the Tasmanian carbonates, but comparable intragranular cementation has not been observed in the New Zealand carbonates. Most recently, Rao (1990a) has shown for the Tasmanian carbonates how a number of geochemical trends involving Mg, Sr, Na, Mn and Fe, and their relation to O and C isotope composition, are indicative of marine diagenesis in cool/ cold waters and quite unlike meteoric diagenetic trends. Methods of study Tasmanian samples (Fig. I A) were collected during 1973 on a l0 n mile grid (Davies and Marshall, 1973); additional samples were collected during 1980 (Rao and Green, 1983). The northern New Zealand samples were obtained during New Zealand Oceanographic Institute (D.S.I.R)cruise 1077 (Nelson, 1982) and by SCUBA diving at Leigh (Fig. I B and inset), while the southern New Zealand samples (Fig. 1C) were collected from the Royal New Zealand Navy hydrographic survey vessel Monowai (Head, 1985). From salt-free and dried samples, splits of whole sediments and p,iekings of individual skeletons of bryozoans and brachiopods (and one gastropod) were crushed and reacted with anhydrous HaPO4 at 25°C. The evolved CO2 gas was analysed on a Micromass 602D at the University of Tasmania for 6lsO and 613C, the values being expressed in conventional permil notation relative to the PDB standard. The precision of data for both O and C is ___0.1%o. Results
Skeletons The 6180 and 613C values of bryozoans, brachiopods and a gastropod from Tasmania and New
276
C.P. RAO AND C.S. NELSON
ske!etons are contained within an isotope field only slightly larger (by + 0.5%e O and C) than, and of similar shape to, the Tasmanian field, and is also bisected by the seafloor diagenesis trend line. Compared to the bryozoans, brachiopods have a similar range of 6~sO values, but tend towards heavier ~3C values.
Zealand are listed in Tables I and 2, respectively, along with sample locations and water depths. The isotope values of skeletons are plotted in Fig. 2 along with the field of skeletons from generally warm shallow marine settings (James and Choquette, 1983). The isotope field for the temperate carbonates is distinctly separated from that of the warm shallow marine skeletal field because of the heavier ~ s O values characterising the temperate field. The Tasmanian isotope field is tighter than the New Zealand one and the seafloor diagenesis trend line (Milliman and Muller, 1977) passes through the middle of the Tasmanian field. The seafloor diagenesis trend line passes through the origin of the plot due to the equilibrium conditions of both O and C in carbonate varying with water temperature. The New Zealand isotope field overlaps the Tasmanian one. However, if the most t3Cdepleted sample is excluded, the New Zealand
Whole sediment
The 6xsO and 6t3C composition of whole sediments from Tasmania (Table 3) and New Zealand (Table 4) are plotted on Fig. 3 along with the whole sediment isotope field of warm shallow marine carbonates (Milliman and Muller, 1977). The temperate whole sediment isotope field is distinctly different from warm shallow marine sediments, and is characterised by small positive 6~sO values and by consistently lower ~ 3 C values than the
TABLE 1
6tsO and/itaC values of Tasmanian skeletons Latitude (E)
Longitude (S)
Depth (m)
61s0~ PDB
613C~ PDB
!.5 1.5 0.9
1.7 !.7 !.7 2.0 ! .8 !.5 1.7 !.7 O.4 ! .0 0.9 !.! 0.9 1.3 0.4 0.2
Bryozoa 1982 1983 2002 2020 2030 2051 2076 2082 2088 2093 2097 2098 2100 2124 2129 2143
43"!7.2 43~i7.4 42'21.2 42°20.0 41'39.8 40~50.6 4Y42.2 43°33.5 43~!2.2 43°05.0 42°58.2 42°51.1 42~51.2 41°30.3 41°49.5 41~i9.6
14800.5 14807.2 148'~31.0 148~26.3 148'32.1 148'46.5 145~18.6 145°52.1 14543.3 145~26.0 145~05.0 145'~19.5 145~00.6 144~45.~ 144°46.0 144°26.6.
130 39 115 100 113 399 108 161 62 135 188 91 146 49 86 128
0.9 0.9 0.8 0.6 -O.2 0.5 0.9 0.6 0.6 0.6 0.7 0.8
40°35.5 43°17.2 42°20.0 41"39.8 40°50.6
147~50.5 148~00.5 14826.3 148"32.1 148°46.5
32 130 100 !13 399
1.0
1.8
2.7 2.6 1.5 0.7
1.8 2.0 2.3 1.1
40' 50.6
148~46.5
399
2.0
1.9
1.3
Brachiopods 1929 1982 2020 2030 2051
Gastropod 205 i
OXYGEN AND CARBON ISOTOPE FIELDS FOR TEMPERATESHELF CARBONATES.TASMANIAAND NEW ZEALAND
277
TABLE 2 6aso% o and 6~3C% o values o f New Zealand skeletons Sample number
Latitude (S)
Longitude (E)
Depth (m)
6180% e
6~3C0,~
PDB
PDB
Bryozoans G9 E27 M RL-C P448 E27 P551 P448 P489 M RL-D P55 ! P489 E27 P448 P458 P448 P558 M RL-C P558 P458 P558 P492 P458
Snares Snares Leigh Three Kings Snares Three Kings Three Kings Three Kings Leigh Three Kings Three Kings Snares Three Kings Three Kings Three Kings Three Kings Leigh Three Kings Three Kings Three Kings Three Kings Three Kings
45~00.1" 47040.3 , 36 ° ! 6. !' 34°23.4" 47040.3 , 33068.6 , 340"23.4, 34°18. !" 36 ° 16.1" 33058.6 , 34 ° 18. !" 47040.3 ' 34°23.4' 34 ° 13.8' 34°23.4 . 33058.8 ' 36 ° 16. !" 33 '58.8" 34 ° 13.8" 33°58.8" 34°21.6' 34 ° 13.8"
! 6635.5' 167°38.7 ' 174°47.9 , 172°28. ! 3' 167038.7 ' i 7 i °55.2" 172028.3 . 172049.7 ' ! 74047.9 , 171 °55.2' 172040.7 . ! 67°38.7 ' 172°28.3" ! 7 i °56.4' 172028.3 ' 171°43.4' 174°47.9 ' 171 °43.4' 171 °56.4' 171 °43.4' 172036.5 ` ! 71 °56.4'
! 70 ! 31 8 ! 01 131 920 101 88 6 920 88 13 I 10 ! 200 101 178 8 178 200 178 85 200
I.i 1.1 0.1 1.6 1.0 2.9 1.3 0.4 -0.7 2.5 0.4 1.8 0.1 1.2 0.6 1.0 0.4 1.0 1.3 1.2 0.6 1.4
!.8 !.5 -0.3 1.7 1.6 1.7 2.7 0.4 -2.9 -0.3 -0.7 2.7 0.3 1.4 1.1 1.2 1.1 1.9 1.8 2.1 2.0 2.1
Brachiopods M RL-E G9 P55g P489
Leigh Snares Three Kings Three Kings
36°36.1' 45°00.1' 33058.8 , 34'~18. I'
174047.9 ` 166°35.5 ' ! 71 °43.4' 172°40.7 '
20 170 178 88
0.0 1.1 0.9 0.5
0.7 1.7 2.9 1.3
warm water carbonate field. The seafloor diagem esis trend line again bisects the temperate whole sediment isotope field. In contrast to the skeletal isotope field (Fig. 2), the whole sediment New Zealand field is smaller than, and contained within, the Tasmanian whole sediment field, the latter showing a wider range of both 6'sO and 6'3C than its New Zealand counterpart.
Comparison of isotopefields (Fig. 4) Tasmanian whole sediment and skeletal isotope fields are similar, except for very slightly enriched 6'3C values in the whole sediments. In contrast, the New Zealand whole sediment and skeletal isotope fields are different, with higher ranges in
both 6xsO and 6~3C for skeletons. Warm shallow marine whole sediments are enriched relative to their skeletons in both ~lsO and 6t3C, due to inorganic cementation and higher salinities associated with the whole sediments. Discussion
The 6 ' s o and 6'3C variations in carbonates can be influenced by several factors, including the following: (1) mineralogy, (2) salinity, (3) biochemical fractionation, (4) water depth, (5) ambient water temperature, (6) 6x3C variation in surface to deep water, (7) cementation, and (8) modern versus relict sediment. These factors are considered briefly below.
278
C.P. RAOAND C.S. NELSON
--"-d:'. ,' J
/ "
/
~
/
It
5~
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,
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~
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o/'-~_ / • ~
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SKELETONS Newx BryozoaZealand + Brachiopods
L_ Fig. 2. 6~80 and 6t3C variation of skeletons from Tasmanian and New Zea!a..-..dcarbonates compared with the field of James ,',-,~d Choquette (1983) for warm water carbonates. Note heavy &:sO and narrow range of 6taC values for the temperate carbonates. The Tasmanian skeleton isotope values a,e more closely clustered along the seafloor diagenesis trend line than the New Zealand data.
Mineralogy The 6~aO composition of carbonates varies with the relative proportions of low-Mg calcite, aragonite, high-Mg calcite and vaterite in the samples. The maximum enrichment of 5~aO relative to lowMg calcite is about 0.6%0 PDB in aragonite, 0.06%0 PDB in high-Mg calcite and 0.50/ooPDB in vaterite (Tarutani et al., 1969). As the main minerals present in temperate carbonates are low- to highMg calcites with some aragonite and possibly vaterite (Rao, 1981, 1986; Nelson, 1988), the enrichment in ~ 8 0 will be small (<0.3%0; Rao and Green, 1983). This is ;n contrast to higher 5180 enrichments in shallow warm water carbonates due to their high content of aragonite and highMg calcite.
SaliniO' The &1so values of carbonates from warm shallow marine settings can increase in response to
evaporation of seawater in (semi-) arid latitudes or in semi-enclosed embayments. The salinity of temperate ocean water is mainly close to normal (ca. 34.7%0). The New Zealand carbonates are forming in waters whose salinity lies between about 34.3 and 35.7%o, the more southern occurrences lying at the lower end of this range (Nelson et al., 1988). The Tasmanian shelf carbonates are also forming under normal salinity conditions but are affected by low-salinity subantarctic waters during winter, which might ha,,e caused a slight depletion in ~180 values.
Biochemicalfractionation The higher or lower than expected values of O and C isotopes compared to known salinity and temperature variations might be due to biochemical fractionation and photosynthesis of blue-green algae (Milliman and Muller, 1977), which are abundant in warm shallow marine environments.
OXYGEN AND CARBON ISOTOPE FIELDS FOR TEMPERATESHELF CARBONATES.TASMANIAAND NEW ZEALAND
279
TABLE 3 6~sO and 613C values o f Tasmanian whole sediments. Additional isotope data are listed in R a o a n d Green (1983)
1929 1968 1972 1982 1983 2002 2020 2030 2051 2054 2056 2057 2058 2059 2061 2062 2078 2073 2086 2088 2090 2093 2096 2098 2099 2100 2 ! 02 2116 2117 2122 2123 2124 2125 2132 2 i 36 2139 2141
Latitude (S)
Longitude (E)
Depth (m)
6 ! sO%0 PDB
6 ! aC%° PDB
40035.5 4003 ! .0 40°50.6 43°17.2 43 ° 17.4 42 ° 12.2 42°20.0 41 °39.8 40°50.6 43°28.8 43035.5 43040.5 43°47.0 43°58.0 43°43.5 43°53.2 43024.6 43°57.0 43°22.5 43 ° 12.2 43 ° ! 5.0 43°00.5 42058. ! 42°51.1 42°51.1 42°51.2 42039.5 42000.0 41001.2 41029.5 41'29.5 4 ! ° 30.3 41"39.8 42°00.2 44 ° 10.2 44020.2 41 ° ! ! 2
147o50.5 147o2 ! .0 147°08.4 148°00.5 148°07.2 148°3 ! .0 148°26.3 148°32. l 148°46.5 147°58.0 147°32.3 147040.3 147°48.5 147°30.0 ! 47007. ! 147°08.3 ! 47048.8 146°33.7 145°44.5 145°43.3 145030.6 145026.0 145 ° 15.5 145°19.5 145°09.0 145000.6 145009.6 144°33.7 144021.5 ! 44 ° 24.4 144°36.2 144°45.8 144°47.3 144°51.8 144°57.2 145°00.3 144°35.6
32 55 52 130 39 ! 15 100 I 13 399 161 121 146 212 ! 75 128 148 133 159 144 62 155 135 ! 32 91 124 146 90 55 80 ! 19 91 49 60 132 128 122 80
i.4 1.4 i. 1 2.3 2.4 1.2 1.7 i.6 2.9 1.9 !.8 1.9 !.9 1.0 1.9 ! 07 ! .2 0.5 - O. I O. 1 0.5 1.2 ! .2 1.5 1.2 !.0 1.0 -0. I i.5 O.7 !.5 0.9 - 0.3 0.9 ! .3 0.5 1.8
!.4 ! .9 1.7 2.7 2.4 2.1 2.4 2.7 2.2 1.9 1.9 2.3 2.2 1.9 ! .7 1.8 1.3 0.2 0.5 O. l 0.8 0.0 0.7 1.2 l.i !.i 1.0 0.5 0.9 0.9 !.2 0.6 1.0 1.2 ! .4 0.9 0.8
In temperate carbonates calcareous algae are generally too minor a constituent to affect O and C isotope values appreciably. Biochemical fractionation by fauna in Tasmanian carbonates is minimal because both O and C isotope values are in equilibrium with seawater (Fig. 2) and ambient water temperatures (Fig. 5). The O and C isotope compositions of living bryozoans and micrite cement are also similar in Tasmanian carbonates (Rao and Green, 1983). In the New Zealand
carbonates a degree of biochemical fractionation is evident because different species of bryozoans from the same sample have different isotope values. However, too few skeletons have yet been analysed to gauge the importance of biochemical fractionation processes in delimiting the isotope fields.
Water depth The variations in 6lsO with water depth in the Tasmanian and New Zealand temperate carbon-
280
c.p. RAO AND C.S. NELSON TABLE 4 6~aO and 6~3C values o f New Zealand whole sediments. Additional isotope d a t a are listed in Nelson (1982) Sample number
Latitude (S)
Longitude (E)
Depth (m)
~laO%e PDB
613C% PDB
47 ° i 0.2 47 o 10. ! 47°20.0 47020.0 47040.3 47°50.0 47049.3 48°01.9 48°00.0
167 ° 27. ! 168 o 19. ! 167°25.5 ! 68034.0 167°38.7 166°30.2 167 ° 14.3 166°37.4 167038.0
84 100 130 ! 14 131 178 146 134 143
1.0 1.3 1.9 2.4 !.5 !.3 2.8 0.9 !.5
i. I 1.9 2.0 1.9 1.7 i.4 2.0 !.2 1.9
34°29.6 34"24.0 34 ° 14.8 34 '~19.5 33°55.0 34°0 ! .8 34 ° 17.9 34°23.7 34°07.T 34°08.4 33"48. ! 33°58.8 33°57.0
171°50.3 172 ° 16.8 ! 72°09.5 17 ! °40.2 i 72 ~ 14.2 172 ° ! 2.0 172°34.7 172°50.7 172~'32.6 ' 17 i °42.3 171"56.6 171 ~'43.4 171 ~I ! .0
580 ! 20 292 510 144 508 94 30 98 354 516 178 700
2.5 ! .5 1.8 2.6 2.0 1.9 I..8 2.5 1.8 2.2 2.0 1.2 1.6
1.3 ! .3 1.0 I. I !.2 ! .2 ! .2 !.5 1. I 1.5 - 0.4 1.5 0.7
Snares B!5 B46 C36 E402 E27 F39 F41 G8 B582
Three Kings P436 P441 P454 P463 P473 P476 P488 P496 P532 P548 P555 P558 P562
ates are plotted on Fig. 5, which shows a tendency towards progressive enrichment of 6~aO with increasing shelf depth (up to ca. 200 m). Samples from depths > 200 m are also slightly enriched in 6tad with increasing water depths. The 61sO enrichment with increasing water depth can be explained by the temperature decrease into deeper waters across the shelf and into the bounding basins. The 613C variations with water depth (Fig. 6), like 6tad, tend to become heavier with increasing depth across the shelf. However, in still deeper water (> 200 m) the 61ac values generally decrease with increasing depth. Ambient water temperature
Estimates of ambient water temperatures were calculated from 61aO values of carbonates using the equation of Shackleton and Kennett (1975) with 6w=0%o (normal seawater) and 6w= -1%o
(upwelling water): T (°C)- 16.9- 4.38(6c- 6w)+ 0.10(6c- ~w)z where 6c is the O isotope composition of CO2 produced from the carbonate at 25°C and 6w is the O isotope composition of CO2 in equilibrium with formation water. These calculated temperatures are shown along the bottom of Fig. 5. Taking 6w = 0, the Tasmanian shelf water temperatures range from ! 7 to 5°C and the New Zealand shelf temperatures from 18 to 4°C. The temperatures of the deeper waters are relatively uniform, with a minimum of 2.5°C. Using the upwelling deep water 6w value of - 1%o (Craig and Gordon, 1965) drops the temperatun, calculations by about 4°C, from to 13 to I°C for Tasmania and 14 to 0°C for New Zealand shelf carbonates; the deeper waters range from 3 to -2°C. The temperature variation in the surface water layer (upper ca. 50 m) is considerable due to an appreciable seasonal
OXYGEN AND CARBON ISOTOPE FIELDS FOR TEMPERATE SHELF CARBONATES. TASMANIA AND NEW ZEALAb~t::~
j
281
%
•,
:
i
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-"e
=
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Fig. 3. Comparison of 61so and 613C values for whole sediments from the Tasmanian and New Zealand temperate carbonates with the warm shallow marine carbonate field of Milliman and Muller (1977). The whole sediment temperate values are clustered along the seafloor diagenesis line and their 613C values are appreciably lighter than for warm shallow marine carbonates.
affect (Fig. 5). The bottom mixed layer shows a progressive drop in temperature with depth. ~ 3 C variation in surface to deep water
In the temperate carbonates the 6~3C value in the basin at the 920 m water depth is -0.3%o PDB, a value which generally becomes heavier towards the shelf edge (Fig. 6). Maximum 5~3C values of ca. 3%o PDB occur at the shelf edge, from where the di~3C values become progressively lighter towards the shallow part of the shelf, the lightest 6t3C value of -2.9%o PDB occurring inshore at a depth of 6 m at Leigh (Fig. I, inset). In the modern mid-latitude Pacific Ocean, the 6~3C of dissolved inorganic carbon (Kroopnick et al., 1977) varies with depth (Fig. 7) from about 2.3°oo PDB at the surface (ca. 50 m) to 0%o PDB in the oxygen minimum layer (ca. 500 m). This decrease in the 6~aC value of seawater with increasing depth is due to variable productivity and mixing of water systems. Upwelling, cold deep
polar waters have depleted 6'3C values due to the oxidation of organic matter (61aC= -25%0 PDB) at depth. As this upwelling water mixes with deep basin water the dissolved 13C increases, resulting in enrichment of &lac with decreasing depth. If the 613C of CO2 (gas) or HCOa- (solution) is held constant, the 6IaC in equilibrium calcite increases with decreasing temperature (Rubinston and Clayton, 1969; Emrich et al., 1970; Kroopnick et al., 1977). The 6~aO of calcite in seawater also increases with decreasing water temperature (Fig. 5). Therefore, positive correlation between 6~sO and tSt3C with varying water temperature occurs in deep water, where equilibrium calcite passes through the origin (6,=0%0; 61ac=0%o PDB). This positive correlation (Fig. 7) also occurs in surface water (where atmospheric CO2 is relatively constant at ca. -7.2°oo ~13C PDB). The temperate carbonate isotope field overlaps the deep-water isotope field (Fig. 7) and equilibriuni calcite passes through both the shallow temperate and deep-marine isotope fields because
282
C.P. RAO AND C.S. NELSON
t I
6 rJ
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\
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WHOLE SEDIMENT New Zealand
/
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SKEt ETONS New Zealand
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%. - 3 - "
Fig. 4. Summary comparison of whole sediment and skeletal temperate water isotope fields with those for warm shallow marine carbonates.
temperate carbonates are in equilibrium with deep water (Fig. 7). In contrast with temperate carbonates, warm shallow marine carbonates are in equilibrium with surface water, largely because of their formation at very shallow water depths (< 10 m), such as those in the Arabian Gulf and Great Bahama Bank (e.g., Scoflin, 1986). Cementation
lntragranular cements are common in the Tasmanian carbonates but are not recorded in the New Zealand carbonates. In deep-sea carbonates the seafloor diagenesis trend line passes through the origin (zero values) because of lithification. The Tasmanian skeletal and whole sediment isotope fields are closely clustered along the seafloor diagenesis trend line because of cementation. In contrast, the New Zealand skeletal isotope values are more scattered and the isotope field partly deviates from the seafloor diagenesis trend line, possibly because of lack of cements. However, the New Zealand whole sediment isotope field falls
within its Tasmanian counterpart (Fig. 3), suggesting •I,,,, , . . . . .,i,. e whole sediments are much more in equilibrium with the seawater than are some of the skeletons. Modern versus relict sediment
When sea level dropped by about 130 m during the Last Glaciation (ca. 18,000 yrs B.P.), any previous shelf carbonates at depths down to 130 m would have been exposed and affected by meteoric diagenesis. Meteoric diagenesis is characterised by slight depletion in ~180 and marked depletion in ~t3C due to CO2 enrichment of 12C in the soil zone because of decomposition of organic matter (Lohmann, 1988). For example, Recent and Pleistocene speleothem deposits from caves in Tasmania have ~ 8 0 values ranging from - 3 . 5 to - 4 . 6 % PDB and ~taC values from - 6 . 7 to - 13.7°~ PDB (Goede et al., 1986). Both ~lsO and ~ n C values of temperate carbonates in depths from 0 to 130 m (Figs. 5 and 6) are much heavier than meteorically affected speleothems, and the
283
OXYGEN AND CARBON ISOTOPE FIELDS FOR TEMPERATE SHELF CARBONATES, TASMANIA AND NEW ZEALAND
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Fig. 5. Plot of t~t80 values of skeletons and whole sediments versus their water depths. The 6'sO values show gradual enrichment from the surface to the shelf edge, whereas deeper ( > 2 0 0 m) samples are within a narrow iso~.epe range. Calculated isotope temperatures are shown along the base of the diagram for 6,, = 0 (normal seawater) and/i,, = - ! (upwelling water)•
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284
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Fig. 7. Lines of equilibrium calcite in surface and bottom waters and the isotope fields of deep sea carbonates (Milliman, 1974) and warm and temperate water shallow marine carbonates. Note that temperate carbonates are in equilibrium with upwelling deep waters whereas warm shallow marine carbonates are in equilibrium with surface waters. See text for details.
inverted "J-trend" characteristic of meteoric diagenesis (Meyers and Lohmann, 1985; Lohmann, 1988) is not present in the temperate carbonates studied. For these reasons the carbonates now occurring in depths from 0 to 130 m are probably mainly of post-Last Glacial age. The deeper water carbonates appear also to have not been affected by meteoric diagenesis.
Geological implications from modern temperate stable isotope data The 61sO and 6~3C variations in modern, shallow marine, temperate shelf carbonates suggest that these carbonates are forming in regions influenced by upwelling deep water, which maintains saturation in CaCO3 and provides nutrients for the growth of fauna. In regions lacking upwelling water the cold seas can become undersaturated in CaCO3, in which case submarine dissolution and chemical breakdown (or maceration) is common (Alexandersson, 1978). Continuation of these de-
structive processes would obliterate cold water carbonates from the stratigraphic record. Isotope paleotemperature measurements involve accurate determinations of the 6180 composition of both the original marine calcite and of ancient seawater, which many considered to have varied through time (e.g., James and Choquette, 1983; Rao, 1991). Cathodoluminescence ( P o p p e t al., 1986) and variations in trace element with 6tsO (Veizer et al., 1986), and a combination of these two (Rao, 1990b), provide original marine calcite 6180 values, which should be used along with the appropriate 6~sO of seawater of known geological age to determine paleotemperatures. In ancient carbonates, the ~t3C values tend to be near marine values, despite pronounced phreatic meteoric diagenesis (Lohmann, 1988). As 6180 and 6t 3C values are positively correlated in modern temperate carbonates, the marine 613C values can be used to estimate original 6180 values in ancient carbonates considered to be cold water in origin on geological and chemical evidence. Application
OXYGEN AND CARBON ISOTOPE FIELDS FOR TEMPERATE SHELF CARBONATES. TASMANIA AND NEW ZEALAND
of this 6 t a c approach in paleotemperature measurements for cold water Permian (Rao and Green, 1982: Rao, 1988a, b) and Ordovician (Brookfield, 1988) carbonates have given reasonable ambient water temperatures despite pronounced diagenetic equilibration of 6tsO values. Geochemical differences exist between tropical, temperate and polar carbonates (Rao, 1991). The O and C isotope fields of modern calcitic temperate carbonates provide much needed baseline isotope data for evaluating paleotemperatures and diagenetic processes affecting the calcitic limestones which are so abundant in the stratigraphic record. Many of these ancient limestones were probably non-tropical carbonates and they can be better understood by comparing them with modern calcitic temperate carbonates (Rao and Adabi, 1992) rather than with modern aragonitic tropical carbonates (James and Bone, 1989).
Conclusions Based on modern Tasmanian and New Zealand data, the cool to warm temperate shallow marine carbonate O and C isotope field is distinctly different from that of warm shallow marine carbonates. This clear separation occurs for the following reasons: (1) Temperate skeletons and whole sediments are heavier in ~tso and narrower in their range of 61aC relative to warm shallow marine carbonates. (2) The temperate isotope field overlaps with that of deep-sea carbonates and is bisected by the seafloor diagenesis trend line because the temperate carbonates are in equilibrium with upwelling deep water. (3) In the case of the Tasmanian carbonates, intragranular cementation is the main reason for the whole sediment and skeletal isotope field clustering closely along the seafloor diagenesis trend line. Lack of cements and the occurrence of some biochemical fractionation, as in New Zealand skeletons, results in a broader and more scattered isotope field. (4) Predominance of a calcitic mineralogy and more or less normal marine salinities are responsible for minimal enrichment of 6~sO in temperate carbonates, which contrasts with the appreciable enrichment
285
in warm water carbonates due to their abundant aragonite and high-Mg calcite mineralogy. The calculated 61sO ambient temperature in shallow temperate seas ranges from 4 to 18°C (assuming 8w = 0%o), whereas for deeper carbonates the temperatures are low and more uniform, from 2 to 5°C. Under the influence of upwelling waters (6w = -1%o) these temperatures drop by up to 4°C. There is appreciable temperature variation in surface waters ( < 50 m) due to strong seasonal effects. The 613C variation indicates that the temperate carbonates studied are i~a equilibrium with upwelling deep water in contrast to equilibrium with surface waters in warm shallow marine carbonates. Because 6tsO and 6~3C are positively correlated in temperate carbonates, the isotope paleotemperatures of ancient carbonates may be estimated from the 6~aC values of these carbonates, which do not change significantly compared to 6t80 modifications during diagenesis.
Acknowledgements We thank Michael Power (University of Tasmania) for the isotope analyses, the BMR for providing Tasmanian samples, Abigail Smith (University of Waikato) for selecting and identifying the New Zealand skeletons used in this isotope study, and Frank Bailey (University of Waikato) for drafting the figures.
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