WHAT HAPPENED IN THE EARLIER MIOCENE? THE BRASSO AND TAMANA FORMATIONS

REVISITED

Or

How can I convince you thatquantitative micropalaeontology really is worthwhile?

Dr Brent Wilson FGSPetroleum Geoscience ProgrammeThe University of the West Indies

St. Augustine

How I looked in 1965, when Batjes presented his model

Already a lecturer?

What are Foraminifera?For those who got their degree a geological age ago

• Single celled bugs <1 mm

• Planktonic (float near sea surface)

• Benthonic (live on seafloor)

• Shelled

• Narrow ecological niches

• Abundant in marine environments

• Beautiful

Bolivina jiattongi Wilson, 2006

Why the Brasso and Tamana Formations?

• A point of attack towards basin analysis on Trinidad

• Good documentation of Brasso forams (Renz, 1948) simplifies systematics

• Easily accessible type locality (Upper Caparo River)

• A chance conversation with Laurent de Verteuil• Even uneconomic clays deserve to be

(((((loved)))))

Localities examined, shown on map courtesy of GSTT’s website

Brasso Formation1. Guaico-Tamana Road2. Brasso Village3. St. Fabien Quarry

Tamana Formation4. Gasparillo Quarry West5. Mayo Quarry

1

32

45

The Brasso Formation I

• ~3800’ (~1170 m) thick (Kugler, 2001)• Inky blue, massive clay (not very photogenic)• Some limestone members• Lateral equivalent of lowest part of conglomeratic Miocene-

Recent Cunapo Formation• Shallow-water equivalent of part of Cipero Formation

(a deep water Globigerina Ooooooooooze)• Deposited in piggy-back basin on advancing thrust sheet• Partly equivalent to early Middle Miocene Tamana

Formation

The Brasso Formation II

Early to Middle Miocene (Globigerinatella insueta through Globorotalia fohsi robusta planktonic foraminiferal zones

[N7-N12 of Blow, 1969])

~12.4-18 million years old

Depositional rate ~213 m per million years

(~0.2 mm yr-1)

Possibly older locally (Catapsydrax dissimilisZone (N5—20.5 million years old)

Pla

nkto

nic

Fora

min

ifera

l Zon

es

App

rox.

N Z

one

equi

vale

nts

Age

at b

ase

(milli

ons

of y

ears

, ap

prox

imat

e)

Globorotalia fohsi robusta N12 13.9Globorotalia fohsi lobata N11 14.7Globorotalia fohsi fohsi N10 15.3Globorotalia fohsi peripheroronda N9 16Praeorbulina glomerosa N8 17.2Globigerinatella insueta N7 18Catapsydrax stainforthi N6 18.6Catapsydrax dissimilis N5 20.5

Age

Early

Middle

Miocene

Rich Benthonic and Planktonic Foram Fauna

Renz (1948)—159 species: mostly benthonics

Wilson (2003)—28 species of planktonics in Globigerinatella insuetathrough Globorotalia fohsi fohsi Zones (N8-N10) alone (Guaico-Tamana Road, 24 samples)

Wilson (2004)—182 species of benthonics from Guaico-Tamana Road samples

Textularia carrbrowni Wilson, 2006

Little known palaeo-environment

• Renz (1948): 50-600 m water depth

• Stainforth (1948): peripheral neritic rim to deep-water Cipero Formation (i.e., <200 m)

• Kugler (1953): Neritic

http://www.glossary.oilfield.slb.com/files/OGL98002.gif

Materials and Picking Methods• 24 samples from Guaico-Tamana Road taken every

5 m (a BIG outcrop)

• From Brasso Village, St. Fabien and Gasparillo West Quarries every 1 m (TINY outcrops)—20 samples at Brasso, 6 at St. Fabien Quarry, 24 at Gasparillo West

• All foraminifera (planktonics+benthonics) picked to 200 benthonics, then a further 200 benthonics

• Statistical analyses limited to samples with >100 benthonics

Calculating palaeodepths using the percentage of planktonic forams

• As water depth increases, percentage of foram assemblage as planktonics (%P) increases, but rate of increase differs from area to area

• Off the Nile, D = e(81.9+%P)/24 —de Rijk et al. (1999)

• On modern day Trinidad shelf D = 19.7 + 1.34*%P (Wilson 2007)—only valid down to ~100 m

The Guaico-Tamana Road Outcrop

Grid Reference [Trinidad Government Cadastral Coordinates] N1161709 E0701400 links

Planorbulinella trinitatensis

The Guaico-Tamana Road Outcrop

Age determined using planktonic foraminiferal index fossils

Praeobulina glomerosa to Globorotalia fohsi fohsiZones (N8 – N10)

Globorotalia praemenardii

Guaico-Tamana Road OutcropInferred Palaeodepths using de Rijk et al. (1999)

Sam

ple

Dis

tanc

e ab

ove

JBW

-1 (m

)

Per

cent

age

Plan

kton

ics

(%P)

JBW -24 158 60.8JBW -23 139 71JBW -22 134 64.9JBW -21 129 78.6JBW -20 124 11.6JBW -19 117 33.1JBW -18 112 27.5JBW -17 107 n/aJBW -16 101 29.1JBW -15 94 18.1JBW -14 89 52.6JBW -13 84 n/aJBW -12 77 n/aJBW -11 55 n/aJBW -10 50 50.9JBW -9 45 64.3JBW -8 40 66JBW -7 35 62.4JBW -6 30 54.8JBW -5 25 46.5JBW -4 20 n/aJBW -3 15 31.5JBW -2 10 n/aJBW -1 0 n/a

0

100

200

300

400

500

600

700

800

900

0 50 100 150 200Distance above base of outcrop (metres)

Palaeodepth (computed from de Rijk et al.’s 1999 expression)

metres

Outlined section anomalous—inner to middle neritic Probably <50 m—shown by Pseudononion atlanticum + Elphidium cf. poeyanum

MeasuringBenthonic Foram Diversity I

• Species Richness (S) supposedly increases with water depth

• S is of limited use– Gives equal weight to

dominant and rare species, and is dependent on number of specimens found (N) so that SαN

Textularia framptoni Wilson, 2006

MeasuringBenthonic Foram Diversity II

Information Function (H)—a measure independent of N

– To find H for a sample, first calculate pi=ni/N for each species in it– Then calculate pi*ln(pi) for each species– Add the results and multiply by -1– So, H = -Σ pi*ln(pi)

H typically positively correlated with depth (Murray and Alve, 2000). Should be correlated with %P if change in water depth is real

Benthonic foram diversity patterns in the Guaico-Tamana Road outcrop

Rise in diversity (H) in lower part of section, decreasing in higher (transgression followed by regression)

H high around sample JBW-7 through 9 (flooding surface)

H significantly correlated with %P (excluding uppermost samples)—r = 0.627, p<.05

So, both benthonic H and %P suggest transgressive-regressive cycle

H low on flooding surface (JBW-8) due to stagnation at maximum flood

0

20

40

60

80

100

120

140

160

180

0 0.5 1 1.5 2 2.5 3 3.5 4

H'

Dis

tanc

e ab

ove

JBW

-1 (m

)JBW-7

JBW-8

JBW-20

JBW-3

Benthonic Foraminiferal AssemblagesGuaico-Tamana Road outcrop

Four benthonic foraminiferal assemblages:

•Assemblage 1 ( Brizalina alazanensis), outer neritic, transgressive phase

• Assemblage 2, Anomalinoides mecatepecensis, Uvigerina carapitana, U. subperegrina. upper bathyal (height of transgression) to middle neritic. Regression.

• Assemblages 3 and 4, Amphistegina gibbosa and Textularia framptoni -Pseudononion atlanticum - Elphidium cf. poeyanum respectively, middle neritic.

Sample Assemblage Environment AgeJBW-24JBW-23JBW-22JBW-21JBW-20JBW-19 Assemblage 3JBW-18JBW-17 n/aJBW-16 Outer NeriticJBW-15 Middle NeriticJBW-14 Upper Bathyal (shallow)JBW-13JBW-12JBW-11JBW-10 Upper Bathyal (shallow)JBW-9JBW-8JBW-7JBW-6JBW-5JBW-4 n/a n/aJBW-3 Assemblage I Outer NeriticJBW-2JBW-1 n/a n/a

Middle Neritic

Indet

Late

N8

- Ear

ly N

9

N9

Late

N9

- Ea

rly N

10

Assemblage 2

Assemblage 4

Upper Bathyal (shallow)

Upper Bathyal (deep)

n/a

Guaico-Tamana Road Near Top of Section:

Hypersaline, Lagoonal Interlude

Gypsum in some samples near top of section barren of forams

Some nearby samples dominated by Discorbis tholus

or miliolids

U

iron concretions

molluscs

c coquina

U UU burrowed horizon

claystone

fine-grainedsandstone

siltstone

euhedral gypsum

lignite partings

gMinor Residue Components

Log Symbols

molluscs (wholeand fragments)m

p iron pyrites

h hematite cementedlithic fragmentsechinoid spinese

c c

JBW-117

JBW-107

JBW-108

JBW-109

JBW-110

JBW-111

JBW-112

JBW-113

JBW-114

JBW-115

JBW-116

JBW-101

JBW-102

JBW-103

JBW-104

JBW-105

JBW-106

JBW-100

JBW-99

JBW-97

JBW-98

c ccccccc

U UUUU

ccccccc

0 m

10 m

5 m

20 m

15 m

g

0 5 10 15 20 25 30

m

m

m

m

me

m

h

m

m

m

m

h

h

p

p

p

p

p

p

p

h

p

Mass of residue (grams)

UU

0 100 200Total benthonicforaminifera

species JBW‐97

JBW‐98

JBW‐99

JBW‐100

JBW‐101

JBW‐102

JBW‐103

JBW‐104

JBW‐105

JBW‐106

JBW‐107

JBW‐108

JBW‐109

JBW‐110

JBW‐111

JBW‐112

JBW‐113

JBW‐114

JBW‐115

JBW‐116

JBW‐117

Ammonia  cf. catesbyana 24 0 4 6 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0Amphistegina gibbosa 5 4 0 6 0 22 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0Cibicides floridanus 18 89 66 127 6 1 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0Discorbis tholus 78 9 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0Eponides parantillarum 0 0 3 1 0 4 5 0 1 1 0 5 0 0 3 143 0 1 0 0 1Hanzawaia carstensi 0 0 0 0 0 0 1 0 2 34 0 0 0 0 0 0 0 0 0 0 0Lenticulina acutistriata carolina 0 0 0 0 0 0 3 6 11 8 0 0 0 0 0 4 0 0 0 0 0Lenticulina  spp. 2 1 1 3 1 3 0 0 109 125 1 0 0 0 0 5 0 1 0 0 0Pseudononion atlanticum 0 0 1 1 0 2 0 0 28 1 0 0 0 0 1 1 0 0 0 0 0Quinqueloculina seminulangulata 0 0 0 0 0 0 0 0 0 0 0 0 1 1 38 43 0 0 0 0 2Textularia framptoni 0 0 0 0 0 58 5 29 1 1 5 1 0 0 0 2 0 0 0 0 0

The Brasso Village Section

• Age: Middle Miocene

• Globorotalia fohsi lobataand Globorotalia fohsi robusta [N11-N12] Zones

• Section younging to NW

• Patterns not as obvious as in Guaico-Tamana Road (outcrop in meander cut banks only)

MeasuringPalaeo-oxygenation Levels

• Kaiho’s (1994) Benthonic Foraminiferal Oxygen Index (BFOI) assesses dissolved oxygen levels in bottom waters in bathyal and abyssal environments

• BFOI = [O/(O + D)] × 100 where O and D are respectively numbers of oxic and dysoxic indicators

• Uniformitarianism (“The key to the past lies in the present”) allows application of BFOI to later Cenozoic

Globorotalia mayeri

What is anOxygen Minimum Zone (OMZ)?

http://www.galapagosonline.com/Galapagos_Natural_History/Oceanography/Upwelling.jpg http://earthguide.ucsd.edu/virtualmuseum/images/OceanicOxygenProfile.jpg

• At Brasso Village, complicated pattern in %P (slight reduction over time?)

• Overall decrease in S over time

• H shows general decrease over time

• No significant correlation between %P, H and BFOI

• Regression brought seafloor in contact with OMZ

• BFOI constant below OMZ, low at OMZ core, then increases above OMZ

• General overall regression

• Succession of foraminiferal assemblages (oldest first) reflects changes in palaeo- oxygenation:

• 1 (Uvigerina quesqueyana) upper bathyal, moderately-oxygenated water beneath OMZ;

• 2 (Siphonina pulchra, Cassidulina laevigata, lesser Globocassidulina subglobosa) outer neritic, moderately-oxygenated water below OMZ;

• 3 (Uvigerina subperegrina) occupied outer neritic, lower margin of OMZ;

• 4 (Brizalina subaenariensis) lived at core of OMZ. Rates lowest on the Benthonic Foraminiferal Oxygen Index (BFOI);

• 5 (middle-neritic with few Uvigerina spp. and Brizalina spp.) well-oxygenated water above OMZ.

• Note Low H at core of OMZ, marked by lowest BFOI

Foram Microhabitats in theBrasso Village Section

• Microhabitats–depths at which foraminifera live in sediment (Barmawidjaja et al., 1992)

• Epifauna on top of sediment• Shallow infauna in top 2 cm of sediment• Deep infauna at >2 cm• Infaunal taxa dominate where dissolved oxygen

reduced• 28 species forming >1% of total recovery from

Zones N11-N12 were assigned to microhabitat groups

Fauna changes from mostly infaunal below OMZ to mostly epifaunal above(youngest sample at left)

Oxygen depletion (diagrammatic only)

Upper OMZ. Dominant Uvigerina subperegrina, Uvigerina carapitana rare.

Core OMZ. Dominant Brizalina alazanensis. Uvigerinids rare.

Shallow

Deep

Lower OMZ. Dominant Uvigerina carapitana, common Cassidulina carapitana. Uvigerina subperegrina rare.

The OMZ and palaeo-environmental preferences for some Brasso benthonic foraminifera

Guaico-Tamana Road section

At Brasso Village:

•B. alazanensis replaced by B. subaenariensis

•U. carapitana replaced by U. quesqueyana

St. Fabien Quarry

• Planktonic forams Indicate an early Middle Miocene age (Globorotalia fohsi fohsi Zone, N10)

• %P suggestive of outer neritic to upper bathyal palaeo-depths

• Trend towards deeper water in the upper part of the 6 m section

Globigerina bulloides

Trends at St. Fabien Quarry

• Planktonic foram H positively correlated with %P

• Implies planktonic foram diversity increased with palaeo-depth or distance from shore

• Globigerina praebulloides, rare in the tropics after the earliest Early Miocene, is abundant at St. Fabien Quarry. Indicates a tropical refuge due to upwelling of cool, nutrient-rich water

Globigerina praebulloides

Overview of Tamana Formation• Four members: Lower Concord Calcareous Silt; Guaracara

Limestone (a series of bioherms); Upper Concord Calcareous Silt; Los Atajos Conglomerate

• Kugler (2010) – Globorotalia mayeri Zone (N14) age, overlies Brasso Formation

• Deposited on pop-up structure along southern edge of Northern Basin

• Kugler (2001): Brasso and Tamana separated on account of faunal differences

• Biofacies, not formation!

• Guaracara Limestone Member – belongs to Brasso Formation?

Gasparillo West Quarry• 24 samples from Upper Concord

Calcareous Silt, 1 m apart

• Early Middle Miocene age (N9-N10, not N14)

• Same age as Brasso Formation at Guaico-Tamana Road

• Single transgressive-regressive cycle

• Oxygen minimum zone

• Maximum depth ~225 m (cf. 475 m at Guaico-Tamana Road)

Upper Concord Calcareous Silt Member

Lower Concord Calcareous Silt Member

Gasparillo limestone (Guaracara Limestone Member)

Sample Site

10o20'23"N

61o25'20"W 250 m

N

Palaeoenvironmental Model, Gasparillo West Quarry

Amphistegina gibbosa+ Cibicides spp.

Cibicidoidescrebbsi

Uvigerinasubperegrina gr.

Brizalina alazanensisvenezuelana

Uvigerinasubperegrina gr.

Cibicides spp.

Time

Time

MRA1MRA23

Guaracara Limestone at Mayo Quarry

• Limestone-Marl Alternations in Bioherm

• Yielded abundant forams• Planktonic Forams

indicate an N10 age • ?Same age as or

succeeding Gasparillo West Quarry outcrop

?Upper ConcordSilt Member

Lower ConcordSilt Member

lower yellowlimestone

upper yellowlimestone

Guaracaraconglomerate

OC1

OC2

OC3

OC4

OC1-1

OC1-2

OC1-3

OC1-4

OC1-5

OC1-6

MayoVillage

200 m

N

*OC1

*OC2OC3**OC4

= Mayo limestone

Palaeodepths and palaeoenvironment in Mayo Limestone

0

10

20

30

OC1 OC2 OC3 OC4base top

AB1 AB2 AB3 AB4 AB5 AB6 AB7

Planktonic forams show a series of small (~30 m) T-R cycles (Milankovitch control?)

Benthonic forams indicative of photic zone (Amphistegina spp., Elphidium spp.)

B C

Amphistegina n. sp., drawing by Annalize McLean

OC1 OC2 OC3 OC4

OC1 OC2 OC3 OC4

OC1 OC2 OC3 OC4

OC1 OC2 OC3 OC4

AB1 AB2 AB3 AB4

AB5 AB6 AB7

AB1 AB2 AB3 AB4

AB5 AB6 AB7

AB1 AB2 AB3 AB4 AB5 AB6 AB7

AB1 AB2

AB3

AB4

AB5 AB6

AB7

GroupA Group B

Group C Group D

Asterigerinata dominicanaElphidium dominicense

Hanzawaia carstensiRosalina subaraucana

15

10

5

0base top

10

5

0base top

base top

base top

40

30

20

10

0

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40

20

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A

B

C

D

Benthonic forams indicative of photic zone (Amphistegina spp., Elphidium spp.)

Group A, (Outcrops 1,3,4) Elphidium poeyanum, Nonionella basiloba, Asterigerinata dominicana, Bolivina plicatella mera: Group B, (Outcrop 2) Amphistegina sp., Rosalina subaraucana

Amphistegina – intolerant of turbidity Elphidium – tolerant of nutrient enrichment associated with river outflow.

Variations in fauna: control by depth or some factor associated with variation nutrient supply. Control by

changing distance from shore ?

Assemblage A

Assemblage D

Assemblage C

Assemblage B

Several Hundred Kilometers

Assemblage A Assemblage B Assemblage C

•In Mayo Quarry, fluctuations in fauna uncorrelated with change in palaeodepth shown by planktonic forams

•Model from Whitsuntide Islands, •Western Australia.

•Upper figure, depth control

•Lower figure, distance from shore

A Warning!“There is something

fascinating about science. One gets such wholesale returns of conjecture from out of a trifling investment of fact.” (Mark Twain)

“We all know that we do not need a complete data set to write an acceptable (hi)story. A nice story can equally well be written on the basis of a very few data and a fair amount of imagination.” (C. W Drooger, 1993, Radial Foraminifera; Morphometrics and Evolution, p. 19)

Tectonic or eustatic control on transgressive-regressive cycles in Brasso and Tamana

Formations? I

• Catuneanu (2006) suggests eustatically controlled transgressive-regressive cycles have tabular sedimentary sequences (equal creation of accommodation throughout basin) but basin rim lacks proximal conglomerate

• In tectonically controlled cycles, sedimentary sequences wedge-shaped, basin has proximal rim of conglomerate from uplift of source areas

Tectonic or eustatic control on transgressive-regressive cycles in Brasso Formation? II

• Geometry of Brasso sequences unknown

• Rim of conglomerate (Cunapo Formation)

• Transgressions in Brasso Formation at least partly tectonically driven

Diapir of Brasso Formation within limestone of Tamana Formation, Mayo Quarry

7m

Tectonic or eustatic control on transgressive-regressive cycles in Brasso Formation? III

• Pulses of loading-induced subsidence can explain transgressions

• Difficult to reconcile regressions with erosional unloading of the hinterland

• Eroded sediment would continue to load and depress proximal foredeep within piggy-back basin (cf. Varban and Plint, 2008)

• More research at the basin analytical level required

Conclusions

• At least 2 T-R cycles in the Brasso Formation:

1. N8-N10 (Guaico-Tamana Road, St. Fabien Quarry)2. N11-N12 (Brasso Village)

• Transgressions tectonically induced at least in part

• Earlier cycle found in Upper Concord Silt (not a true member?)

• Both cycles show presence of oxygen minimum zone

– Environmental preferences of some foraminifera elucidated, especially as palaeo-oxygen indicators

• Trinidad was a refuge for Globigerina praebulloides—due to upwelling

A Suggestion

“Knowing is not enough. We must apply.”Johann Wolfgang von Goethe

“If basin analysis on Trinidad is to attain its full potential, then it must make abundant use of fully quantitative and statistical micropalaeontology.” – B. Wilson

Convinced? ☺

Acknowledgements (in no order of preference or importance)

• Professor Richard Dawe of UWI for invaluable mentorship• Mr Barry Carr-Brown and Dr John Frampton

(BioStratigraphic Associates) for discussions• Ms Ann Ramsook (Petrotrin) for encouragement• Dr Laurent de Verteuil (Latinum) for the location of the

Guaico-Tamana Road outcrop• Mrs Jacqueline Attong-Wilson of UWI for fieldwork

assistance • The UWI Research and Publications Fund—for the cash to

photograph the bugs

Selected References I

• Barmawidjaja, D. M., Jorissen, F. J., Puskaric, S., & van der Zwaan, G. J. (1992). Microhabitat selection by benthic foraminifera in the northern Adriatic Sea. Journal of Foraminiferal Research, 22, 297-317.

• de Rijk, S., Troelstra, S. R., & Rohling, E. J. (1999). Benthic foraminiferal distribution in the Mediterranean Sea. Journal of Foraminiferal Research, 29, 93-103.

• Kaiho, K. (1994). Benthic foraminiferal dissolved-oxygen index and dissolved-oxygen levels in the modern ocean. Geology, 22, 719-722.

• Keller, G. (1985). Depth stratification of planktonic foraminifers in the Miocene ocean. In J. P. Kennett (Ed.), Geological Society of America Memoir 163. The Miocene Ocean: Paleoceanography and Biogeography, 177-196.

• Kugler, H. G. (1953). Jurassic to recent sedimentary environments in Trinidad. Bulletin de l'Association Suisse des Géologiques et l’Ingéneurs du Pétrole, 20(59), 27-60.

• Kugler, H. G. (2001). Treatise on the Geology of Trinidad. Part 4: Paleocene to Holocene Formations. Basel, Switzerland: Museum of Natural History, 309 p.

• Renz, H. H. (1948). Stratigraphy and fauna of the Agua Salada Group, State of Falcón, Venezuela. Mem. Geol. Soc. America No. 32, 219 p.

• Stainforth, R. M. (1948). Description, correlation, and paleoecology of Tertiary Cipero marl formation, Trinidad, B. W. I. AAPG Bulletin; 32, 1292-1330

• Varban, B. L.. & PLint, A. G., Sequence stacking patters in the Western Canada foredeep: influence of tectonics, sediment loading and eustacy on deposition of the Upper Cretaceous Kaskapau and Cardium Formations. Sedimentology, 55, 395-421.

Selected References II

• Wilson, B. (2003). Foraminifera and Paleodepths in a Section of the Early to Middle Miocene Brasso Formation, Central Trinidad. Caribbean Journal of Science, 39, 209-214.

• Wilson, B. (2004). Benthonic Foraminiferal Paleoecology Across a Transgressive-Regressive Cycle in the Brasso Formation (Early-Middle Miocene) of Central Trinidad. Caribbean Journal of Science, 40, 126-138.

• Wilson, B. (2005). Planktonic Foraminiferal Biostratigraphy and Paleo-Ecology of the Brasso Formation (Middle Miocene) at St. Fabien Quarry, Trinidad, West Indies. Caribbean Journal of Science, 41, 797-803.

• Wilson, B. (2006). Depths, paleodepths and the percentage of foraminiferal assemblages comprising planktonics in Trinidad. Forams 2006 Anuario do instituo de Geosciencias UFJR, 29, 373-374.

• Wilson, B. (2006). Four new species of benthonic foraminfiera from the Miocene of Trinidad, West Indies, and their paleobiogeographic importance. Revue de Paleobiologie, 25, 519-524.

• Wilson, B. (2007). Benthonic foraminiferal paleoecology of the Brasso Formation (Globorotalia fohsi lobata and Globorotalia fohsi robusta [N11-N12] Zones), Trinidad, West Indies: A transect through an oxygen minimum zone. Journal of South American Earth Sciences, 23, 91-98.

Any questions?