Instructions
Activity 1: Gulf of Alaska Sediment Match
Your oceanography detective work involves matching sections of sediment core (1- 7)
with their description (A – G). These cores came from a JOIDES expedition to the Gulf of Alaska.
There is no time limit and you have 2 attempts. Each letter matches only one number.
For example, this is the answer for D: D. 5.
Description of Core Segments: (scroll through the multiple choice questions and decide which image matches the description)
A. Description: Volcanic Ash. Layers of sediment of a different color and texture within the core, varying
in color to light gray to brown.
Identifying characteristics: Layers are generally well–defined and volcanic origins are positively
identified by microscopic analysis of grains in smear slides and volcanic texture is identified with the
scanning electron microscope (SEM). Volcanic ash has sharp edges.
Clues to the past: Provides evidence for volcanic eruptions on land in the past. Radioactive elements
within volcanic ash, such as potassium–40, are useful for determining exact ages of the ash layers and
surrounding sediment.
B. Description: Mud is representative of typical deep water sediments in the Gulf of Alaska.
Identifying characteristics: Mud is gray in color and consists of fine–grained particles (smaller than a
grain of sand).
Clues to the past: Because of the very small grain size of mud particles, mud is usually deposited in calm
water environments where it can settle out of the water column. When mud is uniform in color and
doesn’t contain other sedimentary features, scientists can infer that the source of the sediment
remained relatively constant.
C. Description: Diatom Ooze. Sediments containing more than 50% diatoms is classified as “diatom
ooze.�
Identifying characteristics: Mud with a large percentage of diatoms (i.e. diatom ooze) often has a green–
ish color and a gritty texture. Diatoms are identified in sediments using a microscope. Diatoms come in
many shapes and are often symmetrical; the arrows point at radially symmetrical diatoms.
Clues to the past: Diatoms are among the most common type of photosynthetic plankton
(phytoplankton) living in the world’s oceans and freshwater today. Because their shells are composed of
silica (a very strong mineral), diatoms living millions of years ago become preserved in sediments when
they die, making them useful for determining ages of sediments. Diatoms also provide clues about the
environmental conditions in the earth’s past because they thrive when nutrients and sunlight in the
upper layers of the ocean are high and are absent when conditions for life are not favorable.
D. (5). Description: Sediment grains larger than 2 mm in size intermixed with mud.
Identifying characteristics: Sediment grains are poorly sorted, ranging in size from 2 mm to several
centimeters, and can appear as well defined layers in mud, or randomly distributed throughout
intervals of mud.
Clues to the past: Because the coarse sediment grains are poorly sorted and randomly distributed
within a mud matrix, they were likely transported to the deep sea in ice bergs as ice rafted debris (IRD).
Therefore, this texture in sediment cores provides evidence for the presence of ice in S. Alaska.
E. Description: Mud, Sand, Silt. Layering of sediments (mud–sand–mud) or (mud–silt–mud) of different
particle sizes.
Identifying characteristics: Layers of fine–grained gray mud alternating with layers of sediment with
larger grain sizes, such as sand or silt. The layers are different colors and textures. To quickly determine
grain size, sedimentologists rub a little bit of sediment between their fingers. If the particles feel gritty
like sand paper, this means more sand is present. If sediment grains slide past each other smoothly,
smaller grains such as silt and mud are present. Microscopes are used for a more quantitative
determination of grain size.
Clues to the past: The layering of sediments represents a change in energy of the depositional
environment. Because more energy is required to transport larger particles (sand) than smaller particles
(mud), this change in sediment grain size could indicate that the intensity of fast moving currents
(turbidity currents) on the ocean floor increased, depositing coarser sediments (sand and silt) in layers
with finer sediment (mud).
F. Description: Bioturbation. Sediments are disturbed, or reworked, due to the activity of living
organisms (plants or animals) moving through the sediments. (Bio = life, turbation = disturbed)
Identifying characteristics: Commonly resulting from animals burrowing through sediments. In cores
bioturbation appears as a change in texture or color, often present as an overlapping network of
lines or wiggles.
Clues to the past: Burrowing, ingesting and defecating sediment grains, and filling in abandoned
dwellings are all examples of bioturbation caused by organisms that live in sediments on the seafloor.
Bioturbation can provide evidence for organisms like annelids (ringed worms) and bivalves (mussels,
clams) at the seafloor.
G. Description: Dropstones. Fragments of rock within the sediment core.
Identifying characteristics: Varying in composition and generally several centimeters in size, these rock fragments stand out from the surrounding muddy matrix.
Clues to the past: Originating on land in S. Alaska, rock fragments travel to the deep sea in ice bergs
(rafted). Therefore, dropstones provide evidence for glaciers and sea ice in S. Alaska.
Take the QuizQuestion 12 pts
Group of answer choicesB
A
F
E
Flag question: Question 2Question 22 pts
Group of answer choicesG
A
E
F
Flag question: Question 3Question 32 pts
Note: 3C is a microscope image of sediment shown in 3B. Arrows point to phytoplankton.
Group of answer choicesC
D
A
G
Flag question: Question 4Question 42 pts
Group of answer choicesC
A
G
F
Flag question: Question 5Question 52 pts
Group of answer choicesA
D
B
F
Flag question: Question 6Question 62 pts
Group of answer choicesG
F
B
C
Flag question: Question 7Question 72 pts
7E is Microscope image of 7B. Note sharp edged grains of sediment.
Group of answer choicesG
F
C
A
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