Oceanography Lecture 6: Sea Ice Formation

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Questions and Answers

What factors influence the ice formation process in seawater?

The ice formation process in seawater depends mainly on its salinity and temperature.

Define 'ice floe' and explain its significance in oceanography.

An 'ice floe' is a large, flat piece of sea ice that forms and grows in thickness and lateral extent, playing a significant role in influencing ocean currents and ecosystems.

Explain the difference between 'fast ice' and 'ice floes'.

'Fast ice' refers to sea ice that is attached to the coast, whereas 'ice floes' are free-floating pieces of ice in the open water.

What are 'leads' and 'polynyas' in the context of sea ice?

'Leads' are small breaks between ice floes, while 'polynyas' are larger recurrent open water areas within the sea ice. Signup and view all the answers

What is the approximate density range of sea ice compared to freshwater ice?

The density of sea ice ranges from approximately 857 to 924 kg/m³, which is lower than the density of freshwater ice, approximately 916.8 kg/m³. Signup and view all the answers

What is indicated by the presence of supported 210Pb in sediments?

It is derived from the decay of 226Ra in eroded rocks. Signup and view all the answers

Which isotopes of lead are considered stable?

207Pb and 208Pb Signup and view all the answers

How is excess or unsupported 210Pb generated in marine sediments?

By the decay of 238U in continental rocks leading to atmospheric escape. Signup and view all the answers

What does the activity (A) of a radionuclide represent?

The number of disintegrations per unit time per unit mass. Signup and view all the answers

Which condition best describes the role of primorial nuclides in geochronology?

They allow reconstruction of historical sedimentation patterns. Signup and view all the answers

What does the detection coefficient, C, represent in terms of radioactivity?

The fraction of disintegrations that are detected Signup and view all the answers

When integrating the equation relating activity to the radioactive decay constant, what are typically set as limits of integration?

Two distinct points in time with measured activity Signup and view all the answers

How is the sedimentation rate expressed in relation to radioactive decay?

As a function of depth and sediment age Signup and view all the answers

In sediment analysis, what role does Pb-210 play?

It reflects activity levels influenced by bioturbation Signup and view all the answers

Which expression correctly represents the relation between the activities in different sediment layers?

Pb-210 activity denotes the decay rate between ages Signup and view all the answers

Study Notes

Sea Ice and Its Formation

Sea ice is primarily frozen seawater, with a formation temperature of around -1.91°C.
The process of ice formation in seawater is influenced by salinity, which affects freezing points.
Sea ice formed in shallow coastal waters is referred to as "fast ice."

Types of Sea Ice

The term "ice floe" or "ice sheet" describes pancakes of sea ice that grow in both thickness and lateral extent.
The density of sea ice ranges from approximately 857 to 924 kg/m³, contrasting with freshwater ice density around 916.8 kg/m³.

Ice Structure and Features

"Leads" are small breaks that occur between ice floes, allowing for openings in the ice cover.
Larger areas of recurring open water are identified as "polynyas," which can form due to wind or ocean currents.

Marine Sediment Geochronology

Pristine marine sediment records provide insights into historical ocean conditions.
Key dimensions studied: sedimentation rates, oceanic/atmospheric circulation, seawater chemistry, sea levels, and climatic variables.

Sediment Core Analysis

Sediment core sampling and repository techniques help uncover past sedimentation patterns.
Sediment composition includes fossils, minerals, and bulk materials.

Isotopes and Dating Techniques

Primordial nuclides relevant to studies: 206Pb, 207Pb, 208Pb (stable), and 204Pb (stable).
210Pb serves as a critical radionuclide for dating young sediments, with supported and unsupported distinctions.

Supported vs. Unsupported 210Pb

Supported 210Pb originates from the decay of 226Ra in eroded rocks.
Unsupported 210Pb results from the decay of atmospheric 222Rn, which integrates into sediments but is not replenished as it decays.

Understanding Radioactivity

Activity (A) measures disintegration rate per unit time per mass, typically expressed in decays per minute per gram (dpm/g).
The relationship between activity, concentration, and detection coefficient crucial for determining sedimentation rates.

Mathematical Models

Two main equations derive age and sedimentation rates from 210Pb activity through decay constants and depth measurements.
Integrating equations yields insights into the geological timeline of sediments.

Pb-210 Activity in Sediments

Profiles of Pb-210 activity versus sediment depth assist in evaluating age and sedimentation rates.
Background levels are influenced by atmospheric decay of radon, establishing a baseline for comparison.

Plotting and Analysis

Visual representation involves plotting natural logarithm of Pb-210 activity against depth, considering data accuracy within specific frameworks.
Activity decay constant λ is approximately 0.0311 yr⁻¹.

Limitations of the Techniques

Assumes uniform sedimentation rates, which may not apply in dynamic environments (e.g., turbidites).
Requires stable initial and background concentrations of Pb-210 for reliable results.
Other anthropogenic radioisotope tracers (e.g., 137Cs, 90Sr) complement Pb-210 for heightened accuracy in sedimentation rate assessments.

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Description

This quiz explores the formation of sea ice in oceanography, focusing on the processes influenced by seawater salinity and temperature. Participants will learn about different types of sea ice, including fast ice and ice floes, and the factors that affect their growth. Perfect for students studying earth and environmental sciences.