Shallow burial diagenesis of skeletal carbonates: selective loss of aragonite shell material (Miocene to Recent, Queensland Plateau and Queensland Trough, NE Australia) — implications for shallow cool-water carbonates

Sedimentary Geology 136 (2000) 169–187 www.elsevier.nl/locate/sedgeo

Shallow burial diagenesis of skeletal carbonates: selective loss of aragonite shell material (Miocene to Recent, Queensland Plateau and Queensland Trough, NE Australia) — implications for shallow cool-water carbonates T.C. Brachert a,*, W.-C. Dullo b a

Institut fu¨r Geowissenschaften, Becherweg 21, D-55099 Mainz, Germany b GEOMAR, Wischhofstraße 1-3, D-24148 Kiel, Germany Received 9 August 1999; accepted 23 May 2000

Abstract In burial environments, carbonate sediments undergo mineralogical stabilization and increasing lithification with depth. As yet, however, little knowledge exists with respect to the corresponding effects on fossil preservation and taphonomic modification of the original sediment composition. Countings of particles (⬎63 mm) in Miocene to Recent periplatform sediments (ODP Leg 133, NE Australia) exhibit a clear trend of reduction of skeletal aragonite downcore. Low- and high-Mg-calcite grains occur in a continuous order of magnitude over the studied interval (⬍600 m sub-bottom depth). Original microtextures are retained in high-Mg-calcite biota, although converted to low-Mg-calcite. Thus, the conversion to low-Mg-calcite appears to occur without introducing a significant quantitative bias. Aragonite skeletons (pelagic gastropods), however, exhibit a successive exposure of deeper crystal layers and a chalky preservation with burial depth, which we interpret to result from dissolution. Hints for the originally more numerous existence of aragonite biota exist in soft sediments and chalks by the presence of internal moulds, shells replaced by microspar, and mouldic porosity in early cemented hardgrounds. In deep sections barren of aragonite, the number of casts and replaced shells remains unchanged and is insignificant as compared to aragonitic biota probably originally present within the sediment (⬍2% in ooze/chalk vs. 30–50% of grains in modern periplatform sediments). Therefore, palaeontological information must be significantly biased through selective removal of aragonite. Rates of preservation and destruction depend on external factors during sediment accretion (sedimentation rates, clay content, total organic carbon content) and rates of fluid flow within the sediment. These observations are relevant with respect to the diagenetic potential and patterns of fossil preservation in little cemented calcitic cool-water carbonates, because they may originally have contained more aragonite biota as important constituents of an ecosystem than is commonly suspected, and calcite/aragonite ratios in ancient carbonate sediments may not necessarily reflect original input signals (climate or sea level). 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: periplatform sediments; burial diagenesis; taphonomic bias; cool-water carbonates

* Corresponding author. Present address: Institut und Museum fu¨r Geologie und Pala¨ontologie, Goldschmidtstraße 3, D-37077 Go¨ttingen, Germany. E-mail addresses: [email protected] (T.C. Brachert), [email protected] (W.-C. Dullo). 0037-0738/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0037-073 8(00)00096-8

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Australia

4000

C or al S ea B

Holmes Reef 824

823

as i n

Queensland Plateau

Qu n ee

825/811 814 812

sla nd

813

T.

818

3000

200

Marion Plateau

0 100 2000

817 TownsvilleT .

Australia

Bathymetric contours in meters

Drowned Miocene Platforms and Shelf

200km

Active reefs on Queensland and Marion Plateau

Fig. 1. Topography off northeastern Australia with position of drowned Queensland Plateau and Marion Plateau carbonate platforms and present continental shelf. Asterisks ( ⴱ) denote position of ODP Sites (Leg 133) referred to in this study.

1. Introduction Skeletal sediment composition of deep-water carbonates typically is dominated by remains of open ocean organisms, such as planktonic foraminifers, coccoliths, and pelagic gastropods (pteropods and heteropods, further referred to as “pteropods”). The volumetric contribution by benthic deep-water fauna is minor and dominated by foraminifers. Thus, the mineralogical composition is essentially low-Mg-

calcite. Aragonite content is largely controlled by pteropod input. In environments peripheral to tropical platforms aragonite and high-Mg-calcite correlates with the import of shallow-water sediment grains (both mud and sand-sized). The amount of aragonite is comparatively lower in the surrounding settings of small immature platforms or reefs and non-tropical shelves (cf. Nelson et al., 1988a; Schlager et al., 1994; Rao, 1997). Temporal variation of aragonite contents in periplatform sediments may potentially

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171

Table 1 Position of ODP Sites (Leg 133) used and referred to in this study; m bsl ˆ meters below sea level; m bsf ˆ meters below sea floor Site

Position

812 814 823 824 811/825

17⬚48.841 0 S 17⬚49.985 0 S 16⬚36.981 0 S 16⬚26.703 0 S 16⬚30.977 0 S

49⬚36.313 0 E 149⬚30.831 0 E 149⬚36.045 0 E 147⬚45.737 0 E 148⬚ 9.436 0 E

Depth below sea level (m bsl)

Total penetration (m bsf)

461 520 1649 989 937

189.9 300 1011 430.8 466.3

provide information on small-scale fluctuations of sea level or variations of water temperature (Droxler et al., 1988; Schlager et al., 1994; Eberli et al., 1997). However, modern and ancient carbonate deposits formed in identical settings differ, both qualitatively and quantitatively with respect to their mineralogical and skeletal composition due to diagenetic stabilization (Schlager and James, 1978; Dullo, 1983, 1988; Nelson, 1988; Allison and Briggs, 1991, Brachert et al., 1998). The pathways and effects of meteoric diagenesis are well known (Land et al., 1967; Gavish and Friedman, 1969). Metastable carbonates (aragonite, highMg-calcite) are transformed into calcite by dissolution or neomorphic replacement. Physical preservation of high-Mg-calcite grains, retention of original microtextures, and gradual loss of Mg 2⫹ during stabilization is commonly interpreted in terms of incongruent dissolution (Bathurst, 1976; Manze and Richter, 1979; Richter, 1979, 1984; Bischoff and Mazzullo, 1990; Dullo, 1990). Recent work on marine alteration and burial diagenesis of periplatform sediments has revealed similar patterns of diagenetic stabilization (Milliman, 1974; Schlanger and Douglas, 1974; Schlager and James, 1978; Stoffers and Ross, 1979; Garrison, 1981; Dix and Mullins, 1988; Nelson et al., 1988b; Malone et al., 1993; Macintyre and Reid,

1998) and neomorphic or cement microtextures (Maliva, 1995; Melim et al., 1995; Munnecke et al., 1997). Pore-water profiles in periplatform sediments have been described by Elderfield et al. (1993) and Swart et al. (1993). However, few data exist on the evolution of skeletal microstructures and the preservation potential of skeletal material. This may change the original biotic associations in periplatform sediments significantly due to marine diagenesis and much more due to burial diagenesis. In this paper we discuss the effects of external factors (rates of sedimentation, total organic carbon content (TOC), non-carbonate content) on the nature of the fossil record (shell microstructures and taphonomic alteration) in a periplatform environment not influenced by meteoric diagenesis. 1.1. Setting The present-day sub-sea topography offshore NE Australia exhibits a set of isolated and attached submarine plateaus separated by troughs (Fig. 1, Table 1). The isolated Queensland Plateau (QP) contains only calcareous pelagic and periplatform sediments as it is protected from siliciclastic input. Apart from individual relict reefs (such as Holmes Reef) that survived drowning of the QP carbonate

Table 2 Wet-bulk density, porosity, TOC and sedimentation rates of periplatform sediments at ODP Sites 812, 814, 823, 824 (from Davies et al., 1991; Gartner and Wei, 1993; Gartner et al., 1993) Site

Wet-bulk density (g/cm 3) near surface/lower core

Porosity (%) near surface/lower core

TOC (%)

Sedimentation rates (cm/ky)

812 814 823 824

1.7–1.8/1.9 1.7–1.8/1.9 1.6/2.1 1.7/1.8

70/55–60 70/55–60 55–70/40 65/55

⬍0.15 ⬍0.3 ⬍0.55 ⬍0.25

⬍2.5 ⬍2.5 16 10 (Quat.), 2–3 (Neogene)

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Carbonate Mineralogy Alkalinity [mM] 0 0

5

10

Calcium [mM] 15

2

3

4

5

6

7

8

Calcite [%]

9 10

0

Sulfate 100

Ca2+

100

20 30 40 50 60 70 80 90 100

[%] 0

0

0

100

100

10

20

30

40

50

Mg-Calcite

Depth [mbsf]

200

200

200

200

300

300

300

300

Aragonite

Alkalinity 400

400

500

500

600 28

29

30

31

600 0

Sr2+

500

500

1000

400

400

1500

600 0

Calcite

500

Dolomite

600 10 20 30 40 50 60 70

Sulfate [mM]

Strontium [µM]

Aragonite [%]

A

B

C

D

Fig. 2. Interstitial water geochemistry and X-ray diffraction data (bulk sediment) for Site 823 (from Davies et al., 1991).

platform during the Miocene (Katz and Miller, 1993; Betzler et al., 1995), the present mean water depth is 1100 m bsl (meters below sea level). The sediments of the narrow Queensland Trough (QT) and Townsville Trough (TT) to the west and to the south, respectively, have moderate amounts of organic matter and high content of non-carbonates as they are directly connected to the Australian hinterland. The lithological successions and palaeoceanography are well known based on dredging (Davies et al., 1987, 1989; Feary et al., 1991), and drilling during ODP Leg 133 (Davies et al., 1991). Detailed lithological, petrophysical and stratigraphical information was provided by Davies et al. (1991, 1993). A summary of wetbulk densities, porosity, TOC and sedimentation rates is listed in Table 2.

1.2. Materials and methods A total of five ODP sites (812, 814, 823, 824; offshore NE Australia) were chosen to study the effects of sedimentation rates and the amount of non-carbonate residue on the preservation potential of paleontological information (Fig. 1; Table 1). Particular attention was placed on the influence of organic matter and the rates of mineralogical stabilization (dissolution, recrystallization, cementation, lithification) within the process of fossilization. ODP shipboard measurements considered in this study include bulk carbonate mineralogy, carbonate content (complemented by our own carbonate bomb data), interstitial water geochemistry (Ca 2⫹, Sr 2⫹, Alkalinity), and wet-bulk density (grape) and porosity (Davies et al., 1991). A total of 101 sediment samples

T.C. Brachert, W.-C. Dullo / Sedimentary Geology 136 (2000) 169–187

Alkalinity [g/kg] 2, 8

3

3, 2

3, 4

3, 6

Calcium [mM] 10

3, 8

10, 2

10, 4

10, 6

Carbonate mineralogy [%] 10, 8

0

0

Alkalinity

50

50

173

0

20

40

60

80

100

0

Ca2+

50

Depth [mbsf]

100

100

100

150

150

150

Aragonite 200

Sulfate

200

200

2+

Calcite

Sr 250

250

250 28

29

30

Sulfate (mM)

31

150

200

250

300

350

Strontium (µM)

A

B

C

Fig. 3. Interstitial water geochemistry and X-ray diffraction data (bulk sediment) for Site 824 (from Davies et al., 1991). Background shading denotes major calciturbidite intervals.

was taken as close as possible to where shipboard samples were collected and analysed for X-ray diffraction. All samples were washed with warm water over a 63 mm sieve. The sediment grains of the fraction ⬎63 mm were picked and counted (⬎500/sample) under a stereomicroscope. Quantitative documentation of individuals relies on the number of intact or nearly intact shells (e.g. number of gastropod apices preserved). Counting differentiated for the following categories: • Low-Mg-calcite plankton — planktonic foraminifers; • Aragonite plankton — pteropods and heteropods ( ˆ “pteropods“);

• Low-Mg-calcite benthos — benthic foraminifers, ostracods, brachiopods, calcareous worm tubes, octocoral spicules, problematica; • High-Mg-calcite benthos — echinodermate ossicles, coralline algae, miliolid foraminifers; • Aragonite benthos — bivalves, benthic gastropods, chlorophycean algae (Halimeda), coral, fish otolites; • Mixed low-/high-Mg-calcite/aragonite benthos — bryozoans. Microstructures of skeletal grains were documented and described using a Cambrige SEM. Microfacies analysis was performed on limestones in thin sections (12) according to Flu¨gel (1983). Point-counting is based on a mean of 300 points.

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Particle counts [%] 0 20

Depth [mbsf]

30

20

40

60

80

20

lithifiedsection

40

50

50

1

1,5

2

2,5

3

planktonic.Forams

0

3,5

20

40

60

80

100

26,5

27

27,5

Pteropoda

60

60

80

0,5

0

30

40

70

Point counts [%]

Particle counts [%]

100

28

70 28,5

benthicForam.

Echinodermata

80

90

90

100

100

1 23

29

0

0,1

0,2

0,3

0,4

0,5

4

29,5

Pteropoda[%]

Alkalinity [mM]

Carbonate mineralogy [wt% ] 0 0

20

40

60

80

lithifiedsection

Depth [mbsf]

50

C

B

A 100

2,6 0

2,8

3

3,2

Calcium [mM] 3,4

20

10 0

10,4

10,8

11,2

11,6

20

lithifiedsection

Aragonite 40

40

60

60

Sr 2+ 100

Calcite

Sulfate

80

150

80

Ca 2+

Alkalinity 100

100

neriticdolostone 200

neriticdolostone

120 28

29

30

31

120 95

neriticdolostone 100

105

110

115

120

Sulfate [mM]

Strontium [µM]

E

F

D

125

Fig. 4. Particle counts (fraction ⬎63 mm), point counts (thin-section), interstitial water geochemistry, and X-ray diffraction data for Site 812. 1 ˆ serpulids, 2 ˆ benthic foraminifers, 3 ˆ planktonic foraminifers, 4 ˆ bioclasts. Background shading denotes lithified sections. D–F from Davies et al. (1991).

1.3. Carbonate mineralogy The mineralogical composition of the carbonate fraction within the cores studied exhibits significant variation with burial depth (shipboard measurements by Davies et al., 1991). The general pattern of this variation compares well in all cores studied, and relates with a decrease and full disappearance of high-Mgcalcite (documented for Site 823 only) and aragonite in favour of low-Mg-calcite and some dolomite (Figs. 2c, 3c, 4 and 5). Maximum aragonite contents are found in the upper parts of the cores (50–60%), and

drops downcore to zero. The maximum sub-bottom depth of aragonite occurrences varies considerably. At Sites 812 and 814 it is located around 25 m bsf; however, isolated occurrences are found deeper at Site 814 at 80 and 257 m bsf. At Site 824 aragonite extends until 238 m bsf with an isolated record at 325 m bsf, whereas at Site 823 aragonite is present up to 256 m bsf (Fig. 2c). At Site 823 high-Mg-calcite disappears below 195 m bsf, a shallower depth than aragonite (Fig. 2c). Within the core intervals studied, dolomite is irregularly scattered and commonly much less than 10% (Figs. 2d, 4d and 5f).

T.C. Brachert, W.-C. Dullo / Sedimentary Geology 136 (2000) 169–187

40

20

60

80

Point counts [%] Benthics (sine foraminifera)

Particle counts [%]

Particle counts [%] 0

0

100

1

2

3

4

5

6

Planktonicforams

Depth [mbsf]

lithified section 65

30

50

40

Planktonicf.

56,8

57

60

60

20

Benthicforams

Echinodermata 55

55

10

0 56,6

50

50

175

lithified section

65

57,2

70

57,4

3 70

2 1

Bryozoa

Benthicforams 75

57,6

75

0

4 20

40

60

80

100

Foraminifera [%]

B

A Calcium [mM] 9

10

11

12

C Mineralogy [%]

Alkalinity [mM] 2

13

2,5

3

3,5

4

4,5

0

0

Alkalinity 50

50

100

100

0 10 20 30 40 50 60 70 80 90 100 0

50

100

Depth [mbsf]

Sulfate 150

Aragonite

150

150

200

200

Calcite

Sr 2+ 200

Ca2+

250

250

250

neritic dolostone

neritic dolostone

neritic dolostone 300

300

300 80

100 120 140 160 180 200

25

Strontium [µM]

D

26

27

28

29

30

Sulfate [µM]

E

F

Fig. 5. Particle counts (fraction ⬎63 mm), point counts (thin-section), interstitial water geochemistry, and X-ray diffraction data for Site 814. 1 ˆ Echinoderm remains, 2 ˆ serpulids, 3 ˆ mouldic porosity (interpreted as former aragonite grains), 4 ˆ unidentified bioclast. Background shading denotes lithified sections. D–F from Davies et al. (1991).

2. Results 2.1. Lithologies 2.1.1. Site 824: high carbonate content, high sedimentation rates Site 824 lithofacies comprise periplatform oozes and chalks with intervening skeletal calciturbidites, and shallow-platform carbonates (Fig. 6; Brachert et al., 1993; Betzler et al., 1995; Brachert, 1996). The degree of lithification of the periplatform sediments

increases downcore, grading from uncemented sediments into chalks. The shallow-platform carbonates at the bottom of the core are well lithified. Carbonate content is extremely high (⬎95%), and tends to increase downcore. Stratigraphic resolution is poor at this site; however, sedimentation rates were high during the late Quaternary (10 cm/ky) and significantly lower during the Neogene (2–3 cm/ky; Gartner and Wei, 1993). The decrease of aragonite with depth (Fig. 3) is reflected in the abundance of skeletal grains present

176

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Bulk carbonate [%]

Particle counts > 63µm [%] 0

20

40

60

80

0

100

20

40

60

0

30

20

10

94

0

0

50

50

95

96

97

98

99

100

100

100

Pteropoda Benthic Foraminifera

Planktonic Foraminifera

150

150

200

200

250

250

Shallow Platform

Shallow Platform

Shallow Platform

Shallow Platform

300

300

B

A

C

D

Particle counts > 63µm [%] 0

2

4

6

8

10

12

0

10

20

30

40

0

10

20

30

40

50

60

0

5

10

15

20

25

30

0

50

100

Bryozoa

150

200

HighMg-Calcite Benthics

Aragonite Algae Aragonite Benthics (total)

Low-MgCalcite Benthics

250

Shallow Platform

Shallow Platform

Shallow Platform

Shallow Platform

E

F

G

H

300

Fig. 6. Particle counts (fraction ⬎63 mm) and bulk carbonate data for Site 824. Light-grey background shading marks two major calciturbidite intervals. Dark-grey shading denotes lithified sections of shallow-platform carbonates. Carbonate data from Davies et al. (1991).

(Fig. 6). There is a gradual reduction in the concentration of the molluscs and green algae (including coral fragments), and a corresponding increase of the foraminifers (Fig. 6). Within the upper 52 m of the core, the frequency of skeletal grains is highly consistent with the predominating lithologies: pelagic carbonate ooze is dominated by planktonic foraminifers and pteropods. Graded beds of carbonate sand ( ˆ calciturbidites) are dominated by aragonitic benthics

and bryozoans, with subordinate other calcite benthics (both low- and high-Mg-calcite; Fig. 6e–h). However, in periplatform oozes to chalks below this interval (at 52 and ⬎100 m bsf) planktonic foraminifers predominate, whereas only a trace quantity of pteropod shells were found. Internal moulds after pteropods were documented below 40 m bsf, but are volumetrically insignificant (⬍2%) as compared to pteropod shells within the uppermost part of the core. The loss of

T.C. Brachert, W.-C. Dullo / Sedimentary Geology 136 (2000) 169–187

Planktonic foraminifera [%] 0

20

40

60

Benthic foraminifera [%]

Pteropoda [%] 0

0 10 20 30 40 50 60 70 80

80 100

5

10

Carbonate [%] 15

20

Depth [mbsf]

0

0

0

0

100

100

100

100

200

200

200

200

300

300

300

300

400

400

400

400

500

500

500

500

600

600

600

600

700

700

700

700

A 0

5

B 10

15

20

0

0

0

100

100

200

200

300

300

Depth [mbsf]

400

High-Mg-calcite benthics (Echinodermata)

400

40

20

30

40

Calcareous algae (aragonite)

50

60

70

80

90

D 0

20

40

60

80

100

0

100

Bryozoa (low-Mg-calcite?)

200

300

Low-Mgcalcite benthics (sine foraminifera)

400

500

500

500

600

600

600

700

700

700

E

30

C 10

177

F

Low-Mg-Calcite fauna(tot al) Aragonite benthics

G

Fig. 7. Particle counts (fraction ⬎63 mm) and bulk carbonate data for Site 823. Light-grey background shading marks major calciturbidite interval. Dark-grey shading reflects concentration of high-Mg-calcite (E) and aragonite based (G) on X-ray diffraction. Carbonate and X-ray data from Davies et al. (1991).

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pteropods is interpreted as due to dissolution. Bivalve and codiacean algae remains (shallow-water components) were not documented deeper sub-bottom than pteropods (Fig. 6). Benthic calcite fauna other than foraminifers and bryozoans, such as echinoderms, brachiopods, ostracods, and calcareous worm tubes, are quantitatively minor within the sediments analyzed (⬍10%). The low abundance and little vertical persistence of these groups precludes tracking their distribution individually. However, their abundance as two groups of a high- and low-Mg-calcite fauna, respectively, is essentially constant downcore (Fig. 6e and f). The high-Mg-calcite grains (echinoderms, miliolid foraminifers, coralline algae) and bryozoans (calcite or high-Mg-calcite or both) have two maxima that correlate with sections of high calciturbidite density. The low-Mg-calcite fauna (brachiopods, calcareous worms, problematica) does not exhibit any significant correlation with lithofacies. Although lower in absolute numbers, there is a good agreement in the downcore trend and patterns of mineralogical variation documented by X-ray diffraction (bulk) and particle-counting (Figs. 3 and 6). This pattern is significant, because shipboard determination of carbonate mineralogy was performed with a different set of samples as the countings of particles. 2.1.2. Site 823: high TOC and non-carbonate content, high sedimentation rates Site 823 is characterized by greenish hemipelagic muds with high content of siliciclastics (⬍80%) and intervening graded beds interpreted to represent turbidites. Thick contorted units (⬍10 m) were interpreted as slump deposits. The turbidite beds consist either of pure quartz sands, pure carbonates or composites with a basal sand and an upper carbonate interval (cp. Watts et al., 1993). The concentration of organic matter is the highest of all cores but still low to very low (⬍0.55% TOC; Table 2; Davies et al., 1991). Sedimentation rates were uniform (16 cm/ky). At Site 823 sampling concentrated on the particle spectrum of the hemipelagic taphocoenosis with little sampling of discrete turbidite beds. This procedure, however, cannot fully preclude the incorporation of grains admixed by bioadvection from below or above and/or by time-averaging from pelagic sediments and

thin turbidites. Within the samples studied, foraminifers predominate. The portion of planktonic foraminifers varies between 10 and 95%, and planktonics and benthics are distinctly negatively correlated (Fig. 7a and b). Planktonic foraminifer abundance is positively correlated with carbonate content (Fig. 7a and d). The abundance of benthic foraminifers, other low- and high-Mg-calcite benthic fauna, crudely correlates with non-carbonate content (Fig. 7d–f). Significant amounts of aragonite biota, both planktonic and benthic, were detected only above 300 m bsf. A maximum abundance of 50% aragonite grains (Fig. 7g) is significantly below the abundance documented at Site 824 (70%), but aragonite exhibits the same trend of downcore reduction and disappearance. The ratios of pteropods and planktonic foraminifers do not correlate. However, below 150 m bsf there appears to be a coincidence in the abundance of pteropods and other aragonite biota with a low carbonate content (Fig. 7a, c, d and g). Although different in absolute quantities, there is a good agreement in downcore variation documented by X-ray diffraction and particle abundance (Fig. 7g). 2.1.3. Sites 812 and 814: high carbonate content, low sedimentation rates Sites 812 and 814 are located on the south-western margin of the QP in 462 and 520 m of water. Sites 812 and 814 penetrated pure carbonate sections composed of nannofossil-foraminifer oozes grading downcore into micritic chalk with nannofossils and foraminifers and foraminifer micritic limestone (Davies et al., 1991). The skeletal composition of the cored section reflects gradual drowning of the QP carbonate platform during the middle Miocene. Lithified sections (minimum thickness of 0.95 and 0.85 m) topped by a hardground occur at 25 m bsf at Site 812 and 55 m bsf at Site 814. They consist of yellow-to-white foraminiferal bioclastic grain — to packstone with abundant biomoulds and a cap of reddish brown phosphate crusts and phosphate grains. The limestone intervals were formed during low rates of sedimentation that are interpreted to have been the result of strong bottom currents during the late Pliocene/early Pleistocene (Isern et al., 1993; Betzler et al., 1995). Sedimentation rates were in the order of 1.2–2.4 cm/ky and much lower in the lithified sections topped by hardgrounds (Gartner et al., 1993). At both sites,

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179

Fig. 8. Preservation of pteropods fragmented in the laboratory. (A) Diagenetically unaltered conical pteropod test from 2 m bsf. Note concentric grooves interpreted as growth rings. Sample 824B-1-3-62-64. (B) Intact shell microstructure composed of tightly packed aragonite fibres in a screw-like arrangement. 18 m bsf. Sample 823A-2-6-118-120. (C) Disintegration of pteropod shell into individual fibres producing a chalky preservation. Sample 824B-2H-2-120-122.

there is a continuous increase of wet-bulk densities with core depth and a corresponding decrease in porosity due to compaction (Table 2). The absence of any discontinuity in physical properties documents continuous sedimentation. For both sites (812 and 814) countings of skeletal particles were performed only within and immediately above and below the lithified core section (Figs. 4a

and b and 5a and b). Within the lithified interval, skeletal content was quantified by point-counting of thin sections (Figs. 4c and 5c). Planktonic foraminifers dominate the biota above the lithified section (Figs. 4a and b and 5a and b) The lithified section documents a relative decrease of planktonic foraminifers with a corresponding increase of various benthics (including foraminifers) and unspecified

Fig. 9. Surface corrosion of conical pteropods. (A) Incipient corrosion along growth rings and at the base of flat feather crystals. Apex is to the right. 53 m bsf. Sample 824B-4-6-120-122. (B) Dissolution predominantly affects feather crystals of surface layer, antapical end of shell. 53 m bsf. Sample 824B-4-6-120-122.

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Fig. 10. Internal moulds after pteropods formed by lithification of periplatform ooze. (A) Fragment of conical pteropod compacted at antapical end; 265 m bsf. Sample 823B-28-5-11-13. (B) and (C) Close-up of surface view. Large calcite crystals (⬎4 mm) oriented essentially normal to the surface of the mould formed as a cement. The crystals engulf small isolated aragonite fibres in a curved arrangement (B), interpreted as relicts of the former main body of the shell. Note diagenetically altered coccoliths (C). 122 m bsf. Sample 824A-8-5-120-122.

bioclasts. This pattern is particularly obvious at Site 814 (Fig. 5c), but is more complex at Site 812 due to marked sedimentary cycles (Fig. 4c; Betzler et al., 1995). The cycles at Site 812 begin with partly washed packstone composed dominantly of large planktonic foraminifers. Interparticle porosity is partially reduced by fibrous cement. These lithologies grade upward into skeletal packstone dominated by bioclasts, essentially small planktonic foraminifers, serpulids, bivalves and brachiopods, bryozoans and benthic foraminifers. This facies is capped by crusts overgrown by serpulids and solitary corals; some of the caps are phosphatic. Below the lithified section benthic fauna, particularly benthic foraminifers, are more common. Taxonomic composition of benthic foraminifers and an increasing abundance of nannofossils clearly indicate a deepening trend upcore at both sites that had already started below the lithified section (Katz and Miller, 1993; Betzler et al., 1995). Aragonite grains in general are not present within the sediments immediately above and below the lithified section. However, within the lithified section, biomouldic porosity is common, and therefore documents the original presence of aragonite fauna like solitary corals and molluscs. Preservation of these grains implies stability of aragonite within the sedi-

ment close to the sea floor, and early marine cementation prior to dissolution of aragonite (Brachert et al., 1993). 2.2. Fossil preservation 2.2.1. Aragonite biota Pelagic aragonite fauna is dominated by trochosiral and conical gastropod shells (Figs. 8–10). The state of preservation of these shells may be influenced by dissolution or corrosion at the sea floor, which is described by the degree of fragmentation (Haddad et al., 1993). We selected only intact or nearly intact shells to document patterns of corrosion and dissolution produced within the sediment. Shells fragmented in the laboratory do not exhibit differences in preservation of skeletal microstructures or individual crystallites over core depth (Fig. 8). However, the presence of a soft, chalky preservation is common in shells near the downcore zone that is barren of aragonite shells (Fig. 8b). Although there is no variation within the state of preservation of freshly broken surfaces, the outer and inner surfaces of the shells exhibit significant variation. The outer surface of well-preserved pteropods exhibits distinct channels vertical to the growth axis (Fig. 9a). In addition,

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Fig. 11. Shell microstructures of miliolid foraminifers do not exhibit any change within the depth window investigated. The porosity, morphology and orientation of the crystallites remains unchanged. (A) 15 m bsf. Sample 823A-2-6-118-120. (B) 380 m bsf. Sample 823B40-6-147-149.

there is a clear variation of microstructures away from the apex. At the antapical end, there are tufts of large (⬎50 mm) feather crystals oriented away from the apex. Towards the apical end, the feather crystals continuously change their habit into small (1 mm) complex interfingering plates or fans. Corrosion of shells causes successive exposure of deeper crystal layers (Fig. 9b). Below the first crystal layer there are vermiform interfingering crystals oriented parallel to the surface of the shell that cover the principal layer of pteropod shells. The latter is composed of fibrous crystallites in a screw-like arrangement (Fig. 8b). Attack of this main body of the shell causes increasing separation of individual crystals, which is equivalent with the chalky preservation (Fig. 8c). Thus, total destruction of thick shells by dissolution and their removal from the record must be delayed as compared to thin shells. In carbonate-rich sections (Sites 812, 814 and 824) steinkerns (internal moulds) consist of limestone composed of skeletal grains (coccolithophorids) and idiomorphic to hypidiomorphic calcite crystals of variable size (⬍5 mm). The crystals are prominent at the surface of the moulds and essentially arranged radially to the surface of the former shell. The calcite crystals have engulfed fibrous crystals arranged subparallel to each other and to the surface of the mould (Fig. 10). These fibrous crystallites are interpreted as relicts of the original aragonite shell because of the

slightly curved screw-like orientation. A similar scenario for the lithification of aragonite needle mud and for the formation of microspar has been described by Lasemi and Sandberg (1984) and Munnecke et al. (1997). In addition to calcareous preservation, internal moulds composed of framboidal pyrite are common.

2.2.2. High-Mg-calcite biota Miliolid foraminifers are present in many samples, but they are quantitatively unimportant. In all examined specimens, no modification of the typical miliolid wall structure could be observed (Fig. 11), and the pore network of the wall structure is retained. Echinoderm skeletal elements do not exhibit any visible alteration of the stereome nor cementation of the stereome porosity.

2.2.3. Low-Mg-calcite biota Planktonic and benthic foraminifers do not exhibit any significant evidence for recrystallization of shell microstructures with burial depth (⬍600 m bsf). Microstructures observed in fresh specimens are retained in deeply buried shells. However, carbonate cements formed on planktonic foraminifer tests are common, i.e. bladed to fibrous crystals oriented normally to the surface of the walls.

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Aragonite

100

0

Aragonite

100

>21C

>21C

0

812 0

100

Aragonite

Aragonite

100

Lith.Sect.

100

Shallow Platform

upper

200

Shallow Platform

Moldic Porosity after Primary Aragonite Shells Internal Molds after Primary Aragonite Shells

400

0

Aragonite Saturation (Morse & Mackenzie,90)

lower

500

(Haddad et al.,93)

600

700

823 823

A

B

824 824

25

TregrosseR. Tregrosse R.

Present-day Aragonite SaturationDepth

812 812 814 814 811/

HolmesR. Holmes R.

Pliocene

Depth [mbsf]

300

Shallow Platform

1000

Present Water Presen ate Depth pth

Pleistocene

0

814

824

Middle to Late >21C Miocene

823

>21C

182

2000

Fig. 12. Transect of ODP Sites 823, 824, 814, and 812 according to present-day water depth and aragonite saturation. (A) Aragonite distribution is diachronous over the transect. Biomouldic porosity in limestone/dolostone and internal moulds in unlithified sediments prove former existence of aragonite biota over the whole sections. Bulk X-ray diffraction data from Davies et al. (1991). Water temperature estimates (vertical bar) from Isern (1994). (B) Present-day water depth of Sites 812, 814, 823, 824, and aragonite saturation of sea water.

3. Discussion 3.1. Corrosion and dissolution of aragonite at sea floor Pteropods have rapid sinking rates and therefore suffer minor corrosion during sinking through the water column (Morse and Mackenzie, 1990). Mineralogical variations of the sediment composition may therefore result from intermittent shallow-water input and pteropod production, fluctuating rates of dissolution/preservation at the sea floor and diagenetic stabilization during burial, or a combination of these factors (see Haddad et al., 1993 for a discussion). The present-day aragonite saturation depth within the study area is located between ⬃500 and

⬃1000 m bsl (Haddad et al., 1993; Morse and Mackenzie, 1990). Site 812 is located above the 500 m line, Site 814 is located above the 1000 m bsl line and Sites 824, 823 below 1000 m bsl (Fig. 12). In any case, however, the shallow sites have a limited record of metastable carbonate, whereas aragonite content is high and extends deepest sub-bottom at the deepest and most distal site (Fig. 12). We therefore consider a factor other than input or sea floor preservation/dissolution rates to be most relevant for producing the distributional and preservational trends of carbonate minerals and fossils in the cores studied. Indeed, aragonite biota in intervening beds of calciturbidites derived from the nearby reefs do not exhibit different depth distributional record patterns from those of slowly and continuously accumulating sediments.

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Instead, both fit the general trend of a gradual reduction of aragonite with core depth (Fig. 6e and h). These observations clearly suggest that although particles and sedimentation rates were significantly different within adjacent beds, the diagenetic environment is entirely governed by post-depositional processes within the sediments, and independent from shortterm fluctuations of import, pelagic production, or dissolution/preservation at the sea floor. 3.1.1. Distribution of Halimeda: preservation of input signal vs. diagenetic overprinting At Sites 824 and 811/825 there are abundant intercalations of calciturbidites which were derived from the predecessor of present-day Holmes Reef (Fig. 1). At Site 824 the tropical green alga Halimeda is documented downcore up to 125 m bsf (Fig. 6h). At Site 811/825 calciturbidites extend downcore to 230 m bsf, however, Halimeda is documented only within the uppermost core sections (35 m bsf) in much younger deposits (Betzler et al., 1995). Thus, the absence of Halimeda within time equivalent calciturbidites at Site 811/825 cannot reflect surface water temperatures too low for chlorophycean algal growth. In fact, at Site 811/825 stable isotopic temperature estimates reflect tropical conditions (T ⬎ 21⬚C as a conservative estimate; Isern, 1994) from the early Pliocene onward (Fig. 12a; 75 m bsf). The last downcore occurrence of aragonite (X-ray diffraction) at all sites studied is located at different core depths and is clearly diachronous (Fig. 12a). Internal moulds, both pyritic and calcareous, within unlithified sediments and chalks, and biomouldic porosity within early lithified hardgrounds (Sites 812 and 814) clearly document the former existence of aragonite biota continuous over the cores. Therefore, the downcore decrease in the abundance of the green alga Halimeda and aragonite content does not simply reflect the NE Australian warming trend during the Neogene, but is interpreted as a diagenetic signal (Fig. 12a). 3.2. Record of the metastable carbonate biota: effects of sedimentation rate, non-carbonate residue, and organic matter 3.2.1. Sedimentation rates (pure carbonate sections, Sites 812, 814 and 824) The diagenetic scenario for pure carbonate sedi-

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ments is comparatively simple as the effects of organic matter and clay minerals are minor. At Site 824 (expanded section) alkalinity increases rapidly within the uppermost sediment layer from the sea water value (2.7 mM), and then successively decreases downcore (Fig. 3a). Correspondingly, Ca 2⫹ concentrations are depleted as compared to sea water values within the uppermost sediment, and then increase downcore (Fig. 3a). A minimum in dissolved Ca 2⫹ corresponds to a maximum in Sr 2⫹ concentrations (Fig. 3b). Within the sediment, Sr 2⫹ is enriched as compared to sea water. According to Davies et al. (1991), this pattern is a consequence of near-surface precipitation of calcite cements (no Sr-incorporation), of dissolution of aragonite which continues deeper sub-bottom, and of pore-water flow (Elderfield et al., 1993; Swart et al., 1993). These data fit the observation of an increased corrosion of outer surfaces of aragonite biota with depth (Fig. 9). Some of the dissolved carbonate may have precipitated as internal moulds within the upper metres of the section, although some shells collapsed due to compaction prior to cementation of the moulds (Fig. 10a), according to the rapid decrease of Ca 2⫹ concentrations within the uppermost 10 m of sediment (Fig. 3b). In any event, the abundance of internal moulds is much too low to compensate for the amount of shells probably originally present. Mould formation requires lithified sediment. Leaching of biota in unlithified sediments results in a complete loss in the record. Preservation of aragonite fauna, although commonly neomorphosed or transformed into cryptocrystalline casts, has been described from deep-sea ooze subject to early marine cementation in a context of low rates of sedimentation (Milliman, 1974; Schlager and James, 1978; Allouc, 1990). The sections at Sites 812 and 814 are significantly reduced in thickness as compared to Site 824. Seismic stratal geometries, biostratigraphical data, and the presence of hardgrounds indicate low to very low rates of sedimentation during the late Pliocene/early Pleistocene. Continuous wet-bulk density trends provide no evidence for an erosive reduction of section (Davies et al., 1991). Significant amounts of aragonite are not recorded below 22 and 17 m bsf, respectively. However, hardgrounds with abundant biomoulds occur at 25 and 55 m bsf, which proves

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the common former existence of aragonite biota significantly deeper downcore. The pattern of the alkalinity profiles at Sites 812 and 814 with low rates of sedimentation is identical to Site 824 with a high sedimentation rate regime. Within the uppermost meters of Site 814, alkalinity is higher (3.9 mM) than ambient sea water (2.5– 2.7 mM). Below, alkalinity decreases over the first 100 m bsf and then remains essentially constant around sea water values (Fig. 5e; the profile is highly irregular in Hole 812C but appears also to decrease downcore, Fig. 4e). At Site 812 Sr 2⫹ concentrations increase while Ca 2⫹ stays constant. However, below the lithified section Ca 2⫹ is increasingly enriched compared to sea water, while Sr 2⫹ remains constant. According to Davies et al. (1991) this pattern is due to dissolution of aragonite and precipitation of calcite. The very low concentration of Sr 2⫹ below the lithified section and total disappearance of aragonite from the sediment is due to advective fluid transport (Elderfield et al., 1993). Because of sediment starvation the sediments were exposed for a longer time to the alkalinity maximum and associated cementation processes to become the hardgrounds. Thus, the maximum aragonite distributions below sub-bottom depths cannot reflect a primary signal, and in terms of preservation of palaeontological information within the burial environment, aragonite biota suffer quantitatively incomplete fossilization and depletion. 3.2.2. Non-carbonate material and organic matter At Site 823 aragonite exhibits the widest distribution sub-bottom. This is interpreted as a consequence of high sedimentation rates and high clay content causing low permeability, and thus little interstitial water exchange. The high content of organic matter results in more extensive sulphate reduction with a corresponding alkalinity increase (maximum of 12.55 mM at 55.8 m bsf; Fig. 2a). Below 55.8 m bsf, alkalinity drops continuously back to seawater concentration towards the bottom of the studied section. Concentrations of Ca 2⫹ decrease from seawater within the top 30 m bsf to minimum values of 3.13 mM. Below this depth values increase towards the bottom of the hole to nearly 20 mM. In contrast, Sr 2⫹ values steadily increase from seawater concentrations to 1338 mM at the lower end of the studied section. According to Davies et al. (1991), Ca 2⫹ and

Sr 2⫹ profiles are due to recrystallization of biogenic carbonates into low-Mg-calcite (Fig. 2). Little increase of Sr 2⫹ and decrease of Ca 2⫹ concentrations are interpreted as reflecting stability or little dissolution of aragonite and contemporaneous precipitation of calcite forming internal moulds within the upper 50 m of the core. The absence of any compaction features within internal moulds is evidence for formation under little burial depths. The prominent sulphate reduction zone explains the dominance of moulds composed of pyrite over calcite (Fig. 2a). Corrosion of biota is significantly retarded as compared to Sites 812, 814 and 824 for the same reason. Pteropods are still very common at a burial depth of 170 m bsf and exhibit few signs of dissolution. Below, however, corrosion becomes increasingly significant. Deeper in the core, below the last occurrence of aragonite biota, corrosion of calcite grains such as bivalve prisms, is obvious. Common dissolution of carbonate at grain contacts was observed below 570 m bsf. 3.3. Pathway of diagenetic stabilization A complete dataset including relative concentrations of aragonite, high-Mg-calcite and low-Mgcalcite in bulk sediment samples (X-ray diffraction data) is available only for Site 823 (Davies et al., 1991). These sediments have low-Mg-calcite concentrations which remain below a maximum of 44% (Fig. 2d). If differentiated for the upper (0–200 m bsf), intermediate (200–300 m bsf), and lower section (⬎300 m bsf), there is a clear separation of sediments characterized by the presence of all carbonate phases in the upper section, and the absence of Mg-calcite from the intermediate section onward. Within the lower section calcite dominates, whereas aragonite is absent and dolomite is unimportant (Fig. 2c and d). This evolutionary trend in the mineralogical composition is interpreted as a diagenetic sequence characterized by destruction of high-Mg-calcite (200 m bsf) prior to aragonite (300 m bsf). An analogous pathway has been described for periplatform sediments of the Maldives (Malone et al., 1993) and for meteoric diagenetic alteration (Gavish and Friedman, 1969; Tucker, 1981). However, when particle counts (size-fraction ⬎ 63 mm) are considered with respect to their original mineralogical composition,

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the trend of diagenetic alteration differs (Fig. 7g). In this case, only aragonite particles are almost absent from the intermediate and lower sections (⬎200 m bsf), whereas grains that were originally composed of high-Mg-calcite persist within the lower section in a rather continuous abundance (Fig. 7b). This particular pattern is interpreted in the following way: (1) Aragonite grains get dissolved as a function of core depth. This interpretation is supported by increasing corrosion of grains, beginning with aragonite particles (surficial corrosion grading into chalky preservation) and ending up with grain-contact corrosion of calcite grains. (2) In the lower section, high-Mg-calcite is not detected by X-ray diffraction although the relative abundance of biota originally composed of highMg-calcite remains constant with core depth, which suggests continuous transformation of high-Mg-calcite into low-Mg-calcite (Stoffers and Ross, 1979; Richter, 1979, 1984; Dullo, 1983, 1990; Budd and Hiatt, 1993) taking place without any visible ultrastructural change in miliolid foraminifers and echinoderm ossicles. For these reasons, we conclude that there was no significant physical loss of grains or palaeontological information during the stabilization process. (3) Sedimentation rates control the degree of exposure of sediments to the zone supersaturated with respect to carbonate minerals (i.e. the alkalinity maximum). Low net accretion therefore favours early cementation and formation of hardgrounds. Although subsequently dissolved, aragonite biota from lithified sections will be recorded by biomouldic porosities. On the other hand, missing or limited cementation in a context of average periplatform sedimentation therefore results in a quantitatively highly biased fossil record (cp. Hood and Nelson, 1996). In order to quantify the losses during shallow burial diagenesis, Holocene periplatform sediments may represent good analogues. They typically have concentrations of ⬃50% of aragonite (aragonite ⬎50% and high-Mg-calcite ⬍50% in tropical periplatform settings, and ⬍50%/⬎50% in cool-water systems; Rao, 1997), which, according to this study, is not being preserved and which

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is insufficient to produce lithifications exceeding the early chalk stage.

4. Conclusions and implications Pelagic and hemipelagic sediments drilled off NE Australia suffered significant taphonomic modification of the original sediment composition during burial (0–600 m below sea floor), as documented through quantitative analysis of skeletal compositions, and previous results from X-ray diffraction and pore-water geochemistry. Destruction of metastable carbonate biota and particles at the sea floor is not significant as a modifier of the preservation potential. Metastable carbonate continuously disappears from the sediments downcore. Aragonite biota are affected by dissolution, and in unlithified sediments they get entirely removed from the record. Formation of internal moulds (steinkerns), replacement of shells within unlithified sediments, and mouldic porosity within intervening limestone sections (hardgrounds formed during sediment starvation) prove the former existence of aragonite biota over the section, however, significantly lower in abundance as compared to Holocene sediments. Steinkerns formed within the upper tens of meters of section, depending on external factors (sedimentation rates, non-carbonate residue, organic matter content), before mechanical compaction. According to its lower stability, high-Mg-calcite vanishes prior to aragonite from the record. However, high-Mg-calcite biota physically remain quantitatively unchanged within the sediment because their skeletons are transformed into low-Mg-calcite without modifying the microstructures. The sediments remain at early chalk stage although primary aragonite content is estimated to have amounted to ⬎50%. The implications of this study apply to shallowwater carbonate sediments that did not undergo cementation and stabilization of the depositional fabric prior to dissolution of aragonite biota. This is of particular significance with respect to the coolwater carbonate sediments, because micritic envelopes preserving the outline of aragonite biota after dissolution do not exist (Betzler et al., 1997). Thus, the original presence of significant amounts of

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aragonite biota may be obscured due to dissolution beginning during shallow burial and before cementation. Aragonite biota account for a significant portion in all marine communities (Hood and Nelson, 1996; Brachert et al., 1998). Therefore the diagenetically inactive cool-water calcite sediments (cf. James and Bone, 1989; 1991) originally may have contained more aragonite than is commonly assumed. Care should be taken, when using fossil assemblages as paleoenvironmental indicators, and when using calcite/aragonite ratios in ancient sediments to reconstruct fluctuations of climate or sea level. Acknowledgements Stimulating discussions on taphonomy and on an early draft of this paper with J.C. Braga (Granada) helped to bring into focus the questions raised during this study. We thank R.G. Maliva (Fort Myers), B. Sellwood (Reading), and an anonymous reviewer for their constructive comments and suggestions. W. Hofmeister (Mainz) identified calcite growth habits and crystal faces. G. Bohrmann (Kiel) kindly produced some of the X-ray diffraction analyses. Technical support during sample preparation by K. Schuchmann, G. Ritschel (Mainz) is gratefully acknowledged. G. Fo¨rsterling, F. Fuhlert, M. Grimm, W. Gru¨ninger, U. Krautworst, P. Maerz, and K. Schindler (all Mainz) patiently helped to wash and to count the microscopic samples. C. Betzler (Frankfurt) provided some samples. This work was funded by the Deutsche Forschungsgemeinschaft (Br 1153/2-1), which is gratefully acknowledged. References Allison, P.A., Briggs, D.E.G., 1991. Taphonomy: releasing the data locked in the fossil record, Topics in Geobiology, 9, Plenum Press, New York. 560 pp. Allouc, J., 1990. Quaternary crusts on slopes of the Mediterranean Sea: a tentative explanation. Mar. Geol. 94, 205–238. Bathurst, R.G.C., 1976. Carbonate sediments and their diagenesis, Elsevier, Amsterdam, pp. 1–658. Betzler, C., Brachert, T.C., Kroon, D., 1995. Role of climate for partial drowning of the Queensland Plateau carbonate platform (north-eastern Australia). Mar. Geol. 123, 11–32. Betzler, C., Brachert, T.C., Nebelsick, J., 1997. The warm-temperate carbonate province — a review of the facies, zonations and delaminations. Courier Forsch. Inst. Senckenberg 201, 83–99.

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