Aligning Data and Predictions with Viewpoints

### Younger Dryas Debate

The Younger Dryas was a period of abrupt cooling that occurred approximately $12,900$ years ago. Two viewpoints discuss the potential cause of this event.

Meltwater Flood Model
A sudden influx of freshwater from the melting Laurentide Ice Sheet entered the North Atlantic Ocean. This freshwater cap was less dense than the saline ocean water, preventing the sinking of cold water and shutting down the Atlantic Meridional Overturning Circulation (AMOC). Without this ocean conveyor belt, heat transport to the high northern latitudes ceased, causing rapid cooling. This model predicts that cooling was localized to the North Atlantic region, and that North Atlantic sea surface salinity dropped abruptly.

Cosmic Impact Model
A fragmenting comet or asteroid collided with or exploded over the Laurentide Ice Sheet. The impact triggered widespread biomass burning and threw massive amounts of dust, soot, and sulfur aerosols into the upper atmosphere, blocking incoming sunlight. This model predicts that the cooling was global, occurring simultaneously in both the Northern and Southern Hemispheres, and that a distinct layer containing impact indicators (such as platinum anomalies) and soot was deposited globally at the start of the event.

Suppose researchers analyze marine sediment cores from the Southern Ocean, located in the Southern Hemisphere, and find that sea surface temperatures in this region did not decrease during the Younger Dryas, but instead continued a gradual warming trend. This finding is consistent with which model(s)?

Venusian Resurfacing Debate

Venus's surface has a low density of impact craters that are randomly distributed, suggesting a young average surface age of 300 to 600 million years. Two models explain this observation.

Model 1
Venus underwent a single, rapid, planet-wide volcanic resurfacing event approximately 500 million years ago. The entire crust melted and was replaced by lava, erasing all previous craters. Since then, volcanic activity has been virtually nonexistent, and craters have accumulated slowly and uniformly.

Model 2
Venus's surface is constantly and gradually renewed by ongoing, localized volcanic activity. Eruptions occur randomly in small patches across the surface, slowly erasing older craters while leaving others intact. This continuous equilibrium process maintains a young average surface age over time.

Based on these models, evaluate whether the following statement is true or false:

If planetary probes discover that volcanic plains across Venus have lava flows with ages ranging continuously from 50 million to 800 million years old, this data supports Model 1.

### Methane on Mars

Two scientists debate the origin of methane ($\text{CH}_4$) detected in the atmosphere of Mars.

Scientist 1
Mars's atmospheric methane is produced by subsurface methanogenic microbes. Because ultraviolet (UV) radiation rapidly destroys atmospheric methane, an active biological source must exist. Microbial metabolic activity fluctuates with temperature. Consequently, atmospheric methane concentrations will vary seasonally, peaking in the warmer summer months and dropping significantly during winter. Furthermore, this biological process does not generate substantial amounts of molecular hydrogen ($\text{H}_2$).

Scientist 2
Martian methane is produced geologically through serpentinization, an abiotic reaction between liquid water, carbon dioxide, and olivine minerals in the subsurface. This geological reaction is continuous and unaffected by surface temperature changes. Therefore, methane release rates remain constant year-round. Additionally, serpentinization produces molecular hydrogen ($\text{H}_2$) as a primary byproduct alongside methane in equal proportions. Thus, areas with atmospheric methane should also exhibit elevated $\text{H}_2$ levels.

A space probe monitored the atmospheric composition at a crater on Mars over one Martian year. The seasonal average concentrations of methane and hydrogen are shown in the table below:

Based on the provided information, do the data in the table better support the viewpoint of Scientist 1 or Scientist 2?

Green Sahara Transition

Approximately $5,000$ years ago, the Sahara region transitioned from a humid, vegetated grassland (the 'Green Sahara') into a hyper-arid desert. Two hypotheses discuss the cause of this rapid desertification:

Hypothesis 1 (Orbital-Monsoon Hypothesis)
The greening of the Sahara was sustained by high summer insolation due to Earth's orbital configuration. As Earth's precession cycle gradually changed over thousands of years, solar radiation decreased, weakening the monsoon. This gradual decline triggered a sudden vegetation-atmosphere feedback: less rain reduced vegetation, which increased albedo, further reducing rainfall and causing rapid desertification.

Hypothesis 2 (Human pastoralist Hypothesis)
Early human pastoralists introduced livestock to the region around $6,000$ years ago. Overgrazing removed vegetation, which increased surface albedo (reflectivity) and reduced evapotranspiration. This created a local cooling and drying effect that disrupted the monsoon cycle, driving the rapid shift to a desert state.

Match each new scientific finding with the hypothesis it supports and the reasoning behind it.

Two scientists debate the formation of the Grand Canyon.

Scientist 1
The Grand Canyon was carved slowly over approximately 6 million years. The Colorado River gradually eroded the uplifting Colorado Plateau. The erosion rate remained relatively constant, averaging about $0.15\text{ mm/yr}$. This slow process allowed the river to gradually cut through the rising rock strata, depositing fine, well-sorted sediments downstream at a steady rate.

Scientist 2
The Grand Canyon was formed rapidly, likely within a few weeks, by a catastrophic outburst flood. Approximately 5.5 million years ago, a massive prehistoric lake located on the Colorado Plateau breached its natural barrier. The sudden release of water created a massive, high-velocity flow that carved the canyon. This flood would have deposited a thick layer of unsorted gravel and large boulders downstream.

Based on the presented viewpoints, the discovery of a downstream sediment layer containing large, poorly sorted boulders dated to 5.5 million years ago is consistent with the predictions of Scientist 1.

### Hawaiian-Emperor Seamount Chain

The Hawaiian-Emperor seamount chain is a trail of volcanoes on the Pacific Plate. Two hypotheses discuss the origin of these volcanoes.

Hypothesis 1 (Stationary Plume)
A stationary mantle plume under the Pacific Plate supplies magma. As the plate moves northwestward at a constant rate, new volcanoes form directly over the plume. Volcanoes that have moved away from the plume become inactive. This hypothesis predicts that the age of the seamounts increases linearly with distance from the active hotspot ($0\text{ km}$ at Hawaii).

Hypothesis 2 (Lithospheric Stress)
Volcanism occurs along pre-existing fractures in the plate. The fractures open due to tectonic stress changes at plate boundaries. Volcanic activity can occur simultaneously along a fracture zone, and the age of volcanic centers is determined by when stress reactivation occurred, resulting in clustered or non-linear ages along the chain.

Researchers measured the age and distance from Hawaii of four seamounts in the chain, shown in the table below:

Which hypothesis is supported by this data, and why?