The following graph shows the average pH of the ocean over the last 20 million years of Earth’s history.

Question

A line graph showing changes in the pH of the ocean due to an uptake of carbon dioxide from the atmosphere over the last 25 million years. Time is plotted on the x-axis in millions of past years from -25 to 5.The pH is plotted on the y-axis from 7.9 pH to 8.4 pH. The curve starts in the point of 8.05 pH and 25 million year ago and increases until 22 million years ago. Then it remains flat on the level of 8.2 pH until 18 million years ago where it decreases for a short time and then shoots up to more than 8.3 pH. After it the curve fluctuates between 8.1 pH and 8.2 pH to the present time. Nowadays the ocean pH is shown at 8.1 pH.
Source: Acidification Model Source CO2SYS (Pierrot, Lewis and Wallace. 2006. Carbon Dioxide Information Analysis Center, ORNL, US DOE, Oak Ridge, TN, USA)

What can be concluded from the graph? Select all that apply.

Ocean acidity has been steadily increasing from 20 million years ago to the present time.
In the last 20 million years of Earth’s history, the average pH has never gone below 8.0.
Ocean acidity has been steadily decreasing from 20 million years ago to the present time.
Average ocean pH has fluctuated over the millennia from just over 8.0 to about 8.3.

Answer 1

Based on the description of the graph, the following conclusions can be drawn:

1. Ocean acidity has been steadily increasing from 20 million years ago to the present time. - False. The graph indicates fluctuations in ocean pH rather than a steady increase.

2. In the last 20 million years of Earth’s history, the average pH has never gone below 8.0. - True. The description states that the pH has remained above 8.0, even mentioning that the present ocean pH is at 8.1.

3. Ocean acidity has been steadily decreasing from 20 million years ago to the present time. - False. The description indicates fluctuations in pH levels rather than a steady decline.

4. Average ocean pH has fluctuated over the millennia from just over 8.0 to about 8.3. - True. The description mentions fluctuating pH levels between approximately 8.1 and 8.3 throughout the time frame discussed.

In summary, the correct conclusions are:

• In the last 20 million years of Earth’s history, the average pH has never gone below 8.0.
• Average ocean pH has fluctuated over the millennia from just over 8.0 to about 8.3.