OCEANOGRAPHY — Primary reason sea water is saline Question: Why is ocean water saline on Earth? Select the best scientific explanation.

Introduction / Context:
Sea water salinity reflects the balance of inputs, outputs, and concentration processes acting over geologic time. Understanding the main cause clarifies why oceans are salty while lakes and rivers are comparatively fresh.

Given Data / Assumptions:

• Rivers deliver dissolved ions eroded from continental rocks.
• Hydrothermal vents and submarine volcanism add ions from the crust.
• Precipitation and evaporation redistribute water but do not create salt.
• Salinity is a long-term accumulation phenomenon moderated by mineral precipitation and biological uptake.

Concept / Approach:
The ocean is the end point of the hydrologic cycle. Weathering and runoff bring dissolved ions (chloride, sodium, sulfate, magnesium, etc.) to the sea. Over millions of years, these inputs accumulate because most ions are not removed as fast as they arrive. Evaporation only removes water, increasing local concentration, but the fundamental reason for salinity is the prior presence and continual supply of dissolved salts.

Step-by-Step Solution:
Identify primary sources: continental weathering and submarine inputs deliver ions.Acknowledge sinks: mineral precipitation, burial, and limited removal do not fully balance inputs.Recognize that evaporation intensifies salinity regionally but does not generate salt.Conclude: oceans are saline because salts are present and accumulate over geologic time.

Verification / Alternative check:
Chemical budgets of the oceans show residence times of ions like sodium and chloride are very long, allowing accumulation to stable average salinity (~35 PSU) despite local variability.

Why Other Options Are Wrong:
A) Evaporation concentrates but does not create salt; it is not the root cause.B) Rainfall dilutes surface layers; it does not raise salinity.D) Combination of A and B fails because B is incorrect and A is secondary.E) Oceanic crust is basalt, not halite; salt beds are localized deposits.

Common Pitfalls:
Assuming evaporation “makes” salt. It only increases concentration by removing water; the salts must already exist in solution.

Final Answer:
Salts are already present in sea water due to long-term inputs and accumulation