NutrientSeawater in lava-affected area, 5–45 meters below surfaceSeawater in lava-affected area, 75–125 meters below surfaceSeawater outside of lava-a...
GMAT Information and Ideas : (Ideas) Questions
| Nutrient | Seawater in lava-affected area, 5–45 meters below surface | Seawater in lava-affected area, 75–125 meters below surface | Seawater outside of lava-affected area, 5–45 meters below surface | Seawater outside of lava-affected area, 75–125 meters below surface |
|---|---|---|---|---|
| Nitrate (micromoles per liter) | 3.1 | 0.4 | \(\leq 0.03\) | \(\leq 0.01\) |
| Phosphate (micromoles per liter) | 0.17 | 0.09 | 0.14 | 0.06 |
After a volcanic eruption spilled lava into North Pacific Ocean waters, a dramatic increase of diatoms (a kind of phytoplankton) near the surface occurred. Scientists assumed the diatoms were thriving on nutrients such as phosphate from the lava, but analysis showed these nutrients weren't present near the surface in forms diatoms can consume. However, there was an abundance of usable nitrate, a nutrient usually found in much deeper water and almost never found in lava. Microbial oceanographer Sonya Dyhrman and colleagues believe that as the lava plunged nearly 300 meters below the surface it dislodged pockets of this nutrient, releasing it to float upward, given that ______
Which choice most effectively uses data from the table to complete the statement?
at 5–45 meters below the surface, the average concentration of phosphate was about the same in the seawater in the lava-affected area as in the seawater outside of the lava-affected area.
for both depth ranges measured, the average concentrations of nitrate were substantially higher in the seawater in the lava-affected area than in the seawater outside of the lava-affected area.
for both depth ranges measured in the seawater in the lava-affected area, the average concentrations of nitrate were substantially higher than the average concentrations of phosphate.
in the seawater outside of the lava-affected area, there was little change in the average concentration of nitrate from 75–125 meters below the surface to 5–45 meters below the surface.
Step 1: Decode and Map the Passage
Part A: Passage Analysis Table
| Text from Passage | Analysis |
|---|---|
| "After a volcanic eruption spilled lava into North Pacific Ocean waters, a dramatic increase of diatoms (a kind of phytoplankton) near the surface occurred." |
|
| "Scientists assumed the diatoms were thriving on nutrients such as phosphate from the lava, but analysis showed these nutrients weren't present near the surface in forms diatoms can consume." |
|
| "However, there was an abundance of usable nitrate, a nutrient usually found in much deeper water and almost never found in lava." |
|
| "Microbial oceanographer Sonya Dyhrman and colleagues believe that as the lava plunged nearly 300 meters below the surface it dislodged pockets of this nutrient, releasing it to float upward, given that ______" |
|
Part B: Passage Architecture & Core Elements
Main Point: Scientists discovered that a diatom bloom after a volcanic eruption was caused by nitrate from deep water that was dislodged by the plunging lava, not phosphate from the lava itself.
Argument Flow: The passage moves from observation to failed explanation to correct explanation. It starts with the puzzling diatom bloom, eliminates the obvious phosphate explanation, identifies nitrate as the true cause, and presents a theory about how deep-water nitrate got to the surface.
Step 2: Interpret the Question Precisely
This is a fill-in-the-blank question asking us to choose the best logical connector. The answer must create the right relationship between what comes before and after the blank.
Step 3: Prethink the Answer
- The theory is that lava plunged deep and dislodged nitrate, causing it to float upward
- For this to be true, we'd expect to see higher nitrate levels in the lava-affected area compared to areas without lava influence
- This pattern should hold at multiple depths to show the upward movement effect
- The data should specifically support nitrate (not phosphate) as the key nutrient
- Looking at the table, nitrate levels are dramatically higher in lava-affected areas at both depth ranges compared to non-lava areas
- This suggests the lava did indeed disturb and redistribute nitrate throughout the water column
at 5–45 meters below the surface, the average concentration of phosphate was about the same in the seawater in the lava-affected area as in the seawater outside of the lava-affected area.
✗ Incorrect
- This focuses on phosphate levels being similar between areas, but the passage already established that phosphate wasn't the cause
- This doesn't support Dyhrman's theory about nitrate
for both depth ranges measured, the average concentrations of nitrate were substantially higher in the seawater in the lava-affected area than in the seawater outside of the lava-affected area.
✓ Correct
- Points to substantially higher nitrate concentrations in lava-affected seawater at both depth ranges
- This perfectly supports the theory that lava dislodged nitrate from deep pockets and shows the effect occurred throughout the water column
for both depth ranges measured in the seawater in the lava-affected area, the average concentrations of nitrate were substantially higher than the average concentrations of phosphate.
✗ Incorrect
- Compares nitrate to phosphate within the lava-affected area only
- Doesn't provide the crucial comparison between lava-affected and non-lava-affected areas needed to prove the lava caused the nitrate increase
in the seawater outside of the lava-affected area, there was little change in the average concentration of nitrate from 75–125 meters below the surface to 5–45 meters below the surface.
✗ Incorrect
- Focuses on nitrate levels outside the lava-affected area only
- Shows little change with depth in unaffected areas, but doesn't compare to lava-affected areas to support the theory that lava caused nitrate disturbance