Which of the following elements has the longest oceanic residence time?

Correct option is D

Introduction
· Oceanic residence time is defined as the average time an atom of a particular element spends in the ocean before it is removed by sinking to the bottom or through chemical reactions.
· It is calculated by dividing the total amount of the element present in the ocean by its rate of addition (input) or removal (output).
· Elements with high solubility and low reactivity with organic matter or sediments tend to have longer residence times.
Sol. Sodium is the correct answer because it is a conservative element in seawater. Sodium ions ($$Na^+$$​) are highly soluble and do not readily react with other substances or get easily adsorbed onto sinking particles.

Consequently, sodium remains in the ocean for a very long period—approximately 68 to 100 million years—making its residence time significantly longer than that of transition metals like iron or lead, and even longer than alkaline earth metals like calcium.

Information Booster
· Sodium is the most abundant cation in seawater and its concentration remains remarkably constant (conservative) relative to salinity.
· The primary source of sodium in the ocean is the chemical weathering of continental rocks, delivered via river runoff.
· Because it has such a long residence time, the ocean is well-mixed with respect to sodium, meaning its concentration is nearly uniform globally.
· Elements with residence times greater than the ocean mixing time (approx. 1,000–1,500 years) are generally considered "conservative" or "unreactive."
· Long residence times indicate that the element is removed very slowly through processes like hydrothermal alterations or sea spray formation.

Additional Information
· Iron has a very short residence time (often less than 100–200 years) because it is a "scavenged" element; it is highly reactive and quickly adheres to sinking particles or is used by marine biota.
· Lead also has a short residence time (approx. 40–100 years) as it is rapidly removed from the water column through adsorption onto organic and inorganic particles (particle scavenging).
· Calcium has a significant residence time of about 1 million years, but it is much shorter than sodium because it is biologically "recycled" and removed to form calcium carbonate shells ($CaCO_3$) by marine organisms.