Pacific Transect

SEDIMENT REGIMES OF THE PACIFIC BASIN

Recent sediments flooring the Pacific Ocean basin fall into 5 major categories; calcareous, biosiliceous, deep-sea clay, glaciogenic and terrestrial. Smear slide photographs of representative sediments of a central Pacific mid-ocean transect (Fig. 1) are presented below. Click on the photographs to view larger images.

Fig. 1. Distribution of major sediment types in the Pacific basin. Numbers indicate approximate location of the representative samples shown in the photographs below.
(Map after R. G. Rothwell, 1989, Minerals and Mineraloids in Marine Sediments, p.16.)

 

Northern biosiliceous (diatom) zone

NORTH PACIFIC OCEAN
Water depth: 4194m
PPL

53ºN, 163ºW
Sample Ref: LFGS-16G, Top
Courtesy: Paula Worstell, SIO

(1) Diatom clay

Description:
Diatom clays and oozes form a band of sediments across the floor of the northern North Pacific basin. This example comprises whole and broken centric diatom frustules, sponge spicules, siliceous debris and clay-grade material. Diatoms range in size from 2-2000 µm (0.002-2 mm), although most fall within the size range 10-100 µm (0.01-0.1 mm). High power (x100) view.


NORTH PACIFIC OCEAN
Water depth: 5323m
PPL

47ºN, 164ºW
Sample Ref: CHIN-6G, Top
Courtesy: Paula Worstell, SIO

(2) Diatom clay with some radiolarian fragments

Description:
Diatom clays and oozes of the northern biosiliceous zone tend to contain more radiolarians toward the south. Arrow points to radiolarian fragment. High power (x100) view.

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North Pacific deep sea 'red clay' zone

 

NORTH PACIFIC OCEAN
Water depth: 5772m
PPL

 

25ºN, 162ºW
Sample Ref:NOVA-A-3G, Top
Courtesy: Paula Worstell, SIO

(3) Deep-sea red clay

Description:
Fine-grained sediments in the North Pacific red clay zone are often nearly barren of fossils. The clay-grade sediment shown contains wind-blown dust, a few tiny zeolites, and fragments of siliceous microfossils. Iron oxide grains and staining gives the sediment its red colouring. High power (x100) view.


NORTH PACIFIC OCEAN
Water depth: 4940m
PPL

25ºN, 162ºW
Sample Ref: JYN2-1G, Top
Courtesy: Paula Worstell, SIO

(4) Deep sea red clay with sponge spicules

Description:
This deep-sea clay contains a few siliceous sponge spicules. High power (x100) view.


SUBEQUATORIAL NORTH PACIFIC OCEAN
Water depth: 5556m
PPL

16ºN, 165ºW
Sample Ref: PROA-171G, Top
Courtesy: Paula Worstell, SIO

(5) Zeolitic red clay with radiolarian fragments

Description:
The zeolite, phillipsite, occurs as colourless, euhedral to subhedral doubly-terminated prismatic crystals, crystal fragments, and broken cruciform twins. The authigenic phillipsite forms in place; that is, the larger grain size does not reflect a more high-energy depositional environment. High power (x100) view.

 

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Subequatorial biosiliceous (radiolarian) zone

SUBEQUATORIAL PACIFIC OCEAN
Water depth: 5142m
PPL
8ºN, 167ºW
Sample Ref: MSN-9G, Top
Courtesy: Paula Worstell, SIO

(6) Radiolarian clay with diatoms

Description:
In a band of nutrient-rich waters north of the equator, radiolarian tests and test debris comprise most of the sediment. This sample also includes centric diatoms and sponge spicules. High power (x100) view.

SUBEQUATORIAL PACIFIC OCEAN
Water depth 5399m
PPL/XPL (right)
3ºN, 165ºW
Sample Ref: PROA-131G-2, Top
Courtesy: Paula Worstell, SIO

(7) Radiolarian ooze with some diatoms

Description:
Radiolarians, as seen here, are abundant in sediments along an equatorial band in the Pacific Ocean. Diatoms are also present in this sample. High power (x100) view.

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South Pacific calcareous zone

EQUATORIAL PACIFIC OCEAN
Water depth 4308m
PPL
5ºS, 165ºW
Sample Ref: PROA-124G, Top
Courtesy: Paula Worstell, SIO

(8) Foraminifer fragment/nannofossil ooze with some siliceous microfossil fragments

Description:
Calcareous ooze, comprised primarily of foraminifer fragments and nannofossils, floors the equatorial, southeastern and southwestern Pacific. Some diatom and radiolarian fragments are also present.

EQUATORIAL PACIFIC OCEAN
Water depth: 4165m
PPL
5ºS, 165ºW
Sample Ref: PROA-124G, Top
Courtesy: Paula Worstell, SIO

(9) Foraminifer fragment/nannofossil ooze

Description:
This sample from a calcareous "island" in the deep-sea red clay zone is also representative of the calcareous zone. Some diatom and radiolarian fragments also present. High power (x100) view.

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South Pacific deep sea red clay zone

SOUTH PACIFIC OCEAN
Water depth: 4165m
PPL
13ºS, 165ºW
Sample Ref: AMP4-78PG, Top
Courtesy: Paula Worstell, SIO

(10) Zeolitic red clay

Description:
As in the central North Pacific, deposition of deep-sea red clay dominates in the central South Pacific. The relatively high relief colourless prismatic crystals are zeolites (variety phillipsite). Flakes and grains of fine-grained iron oxides give the sediment its red color. High power (x100) view left, (x200) view right.


SOUTH PACIFIC OCEAN
Water depth: 5283m
PPL/XPL
30ºS, 165ºW
Sample Ref: SOTW-41G, Top
Courtesy: Paula Worstell, SIO

(11) Deep sea red clay with volcanic glass

Description:
Deep-sea red clay containing volcanic glass fragments. Fractured bubble walls can be seen in the large grain top left (arrow) and pipe vesicles in the grain bottom right (arrow). Cross-polarised view, right, shows glass is isotropic and rather fresh as devitrification commonly results in low order interference colours.

SOUTH PACIFIC OCEAN
Water depth: 5230m
PPL/XPL
41ºS, 164ºW
Sample Ref: MSN-111G, Top
Courtesy: Paula Worstell, SIO

(12) Deep sea red clay with volcanic glass

Description:
Deep-sea red clay containing silt-sized volcanic glass fragments. Cross-polarised view, right, shows glass is isotropic and most likely rather fresh as devitrification commonly results in low order interference colours. Southern biosiliceous diatom zone.

Southern biosiliceous (diatom) zone

SOUTHERN PACIFIC OCEAN
Water depth: 2932m
PPL
64ºS, 166ºW
Sample Ref: MSN-91G, Top
Courtesy: Paula Worstell, SIO

(13) Diatom ooze with radiolarians and silicoflagellates

Description:
Siliceous microfossils, mostly diatoms, are deposited in a band encircling Antactica. A silicoflagellate is arrowed. Many of the diatoms are spindle- and rod-shaped planktonic pinnate forms, typical of Antarctic waters. Magnifications are x100 (left) and x200 (right).

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FLUCTUATING SEDIMENTATION PATTERNS

Distribution of Recent sediments in the Pacific Basin is shown in Figure 2. A sequence of samples taken from core PROA-131G-2, however, demonstrates that patterns of sedimentation have fluctuated during the geologic past. (Click on photographs below to view larger images).
Fig. 2. Recent sediment distribution on the floor of the Pacific Ocean. Down-core samples from site PROA 131G-2 (X) demonstrate changing depositional environments at a single site. (Map after Rothwell, "Minerals and Mineraloids in Marine Sediments," p.16).

Down-hole depositional changes

SUBEQUATORIAL PACIFIC OCEAN
Water depth: 5399m
PPL/PPL
3ºN, 165ºW
Sample Ref: PROA-131G-2, Top
Courtesy: Paula Worstell, SIO

Radioloarian ooze with some diatoms

Description:
Today only siliceous microfossils are being deposited at site PROA 131G-2. Radiolarian tests and test debris comprise most of the sediment in this view. Some centric diatoms are also present. High power (x100) magnification.

SUBEQUATORIAL PACIFIC OCEAN
Water depth: 5399m
PPL/XPL
3ºN, 165ºW
Sample Ref: PROA-131G-2, 41cm
Courtesy: Paula Worstell, SIO

Nannofossil ooze with radiolarians

Description:
A distinctive nannofossil ooze, sampled between 40 and 103cm below the water/sediment interface, tells of a significant change in depositional environment during pre-Quaternary times. The cross-polarised (XPL) view, right, shows the high birefringence calcite. Note the discoasters (arrowed), showing a Tertiary, but pre-Quaternary age. Radiolarians and a large monaxon sponge spicule are also present in this view. High power (x100) magnification.

SUBEQUATORIAL PACIFIC OCEAN
Water depth: 5399m
PPL/XPL
3ºN, 165ºW
Sample Ref: PROA-131G-2, 105cm
Courtesy: Paula Worstell, SIO

Radiolarian clay with diatoms

Description:
The cored interval between 103-108cm sub-bottom is a radiolarian clay with highly fragmented radiolarian tests and some diatoms. These sediments were deposited in an environment more similar to that of today. The cross-polarised (XPL) view, right, shows the lack of calcareous material. High power (x100) magnification.

Site PROA 131G-2 is near the present boundary between calcareous and biosilicesous ooze deposition. Slightly changing current or tectonic patterns could produce fluctuating sedimentation.

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