What Ancient Sediments Reveal About Our Climate
Deep beneath the waves off Southwest Africa, a geological treasure chest lies buried, holding secrets to Earth's climate past that could illuminate our climate future.
Imagine possessing a diary that chronicles not just years or decades, but millions of years of Earth's climate history. This extraordinary record exists not between leather covers, but within layers of mud and rock beneath the ocean floor. In 1997, an international team of scientists aboard the research vessel JOIDES Resolution set out to read this diary during Ocean Drilling Program Leg 175, focusing on the Angola-Benguela upwelling system off Southwest Africa.
This region contains one of the most productive marine ecosystems on Earth, where nutrient-rich waters surge from the depths, supporting abundant marine life. The sediments that accumulate here hold an outstanding record of how this system has changed over time, preserving clues about past climate shifts, ocean currents, and global carbon cycles 1 9 . What they extracted from the depths would revolutionize our understanding of how climate change has shaped—and been shaped by—our oceans.
The sediments recovered during Leg 175 provide a climate archive with resolution nearing 1,000 years—exceptionally detailed by geological standards.
To understand the significance of this expedition, we must first grasp the fundamentals of lithostratigraphy—the science of classifying and interpreting rock layers based on their physical and chemical properties 3 . Think of it as reading a history book where each chapter is written in stone rather than ink.
In undisturbed layers, younger rocks lie above older rocks, creating a vertical timeline 3 .
Rock layers extend continuously in all directions until they thin out or encounter a barrier 3 .
Rock units are classified into a systematic hierarchy: beds, members, formations, and groups 4 .
The fundamental unit in this system is the formation—a mappable body of rock with distinctive lithological features that distinguish it from adjacent units 3 4 . By identifying and correlating these formations across different locations, scientists can reconstruct ancient environments and trace how they changed over geological time.
The Angola-Benguela upwelling system represents a natural laboratory for studying climate-ocean interactions. Here, the prevailing winds push surface waters away from the coast, allowing cold, nutrient-rich waters to rise from the deep ocean 1 9 . This upwelling process fertilizes the surface waters, supporting spectacular biological productivity that ultimately results in the deposition of organic-rich sediments on the seafloor.
What makes this region particularly valuable for climate research is that these sediments accumulate rapidly and continuously, preserving a high-resolution climate archive with a resolution nearing 1,000 years—exceptionally detailed by geological standards 9 . The sediments here are largely composed of diatomaceous and carbonate-rich clays with variable organic carbon content, creating distinctive layers that correspond to different climate conditions 9 .
During this period, the Leg 175 team drilled at thirteen sites along the southwest African margin, from 5°S to 32°S latitude, recovering sediment cores that captured the Late Neogene (approximately the last 10 million years) of Earth's history 5 9 .
At six of these sites, they deployed advanced downhole logging tools to continuously measure the physical and chemical properties of the rock formations without bringing actual cores to the surface 9 .
The team was particularly interested in how the Earth's orbital cycles—variations in our planet's tilt and orbit around the sun—might be recorded in these sediment layers, potentially revealing the mechanisms behind ice age cycles 9 .
When the scientists examined the sediment cores, they discovered a remarkably detailed climate archive. The lithostratigraphic summary for Leg 175, compiled by Peir Pufahl and colleagues, revealed a complex tapestry of sedimentary layers telling the story of how this marine environment evolved 5 .
The researchers identified distinct lithostratigraphic units across the various sites, allowing them to correlate layers and trace how environmental conditions changed both through time and across different locations.
One of the most striking discoveries was how clearly the sediment layers recorded the glacial-interglacial cycles. During ice ages, when the climate was colder and drier, clay-rich sediments dominated, characterized by high natural gamma radiation 9 .
| Sediment Type | Climate Significance |
|---|---|
| Diatomaceous Clays | Indicates intense biological productivity |
| Carbonate-Rich Clays | Balanced productivity and preservation |
| Organic-Rich Black Layers | Suggests expanded oxygen minimum zones |
| Dolomitic Layers | May indicate arid periods or seepage |
The team also identified prominent dolomitic layers—sediments rich in the mineral dolomite—that stood out in the physical measurements due to their high density, resistivity, and velocity 9 . These layers served as important marker horizons that helped correlate the stratigraphy across different sites.
The Leg 175 research utilized a sophisticated array of tools to extract information from both the recovered cores and the boreholes themselves. This multi-pronged approach allowed scientists to gather complementary datasets, building a more comprehensive picture of the region's climate history.
The downhole logging tools—instruments lowered into the boreholes after drilling—provided continuous, high-resolution data on the physical properties of the rock formations 9 .
| Tool/Technology | Primary Function | Key Data Provided |
|---|---|---|
| JOIDES Resolution | Deep-sea drilling vessel | Platform for retrieving sediment cores from ocean floor |
| Downhole Logging Tools | In-situ formation evaluation | Continuous measurements of physical properties in boreholes |
| Natural Gamma Ray Spectrometer | Measures natural radioactivity | Identifies clay-rich layers; correlates with glacial intervals |
| Formation MicroScanner (FMS) | Creates borehole wall images | Reveals sedimentary structures, bedding planes, fractures |
| Geomagnetic Tools | Measures magnetic properties | Provides magnetic polarity stratigraphy for dating |
One of the most fascinating discoveries from Leg 175 emerged when scientists performed spectral analysis on the natural gamma-ray data from Site 1077 in the Lower Congo Basin 9 . This mathematical technique identifies cyclic patterns in datasets, much like identifying the rhythmic structure in music.
The analysis revealed clear evidence of Milankovitch cycles—rhythmic climate changes caused by variations in Earth's orbit and rotation 9 . These orbital cycles include:
The presence of these cycles in the gamma-ray data demonstrated that global climate forcing mechanisms were faithfully recorded in the sediments of the Angola-Benguela system. The clay-rich glacial intervals appeared with a frequency that matched the timing of these orbital cycles, providing powerful evidence that orbital variations drive ice age cycles.
| Orbital Cycle | Time Period | Physical Cause | Expression in Sediments |
|---|---|---|---|
| Eccentricity | ∼100,000 years | Shape of Earth's orbit around sun | Strong clay-rich layers at 100,000-year intervals |
| Obliquity | ∼41,000 years | Tilt of Earth's axis relative to orbit | Moderate sediment variations matching tilt cycle |
| Precession | ∼23,000 years | Wobble of Earth's axis | Finer-scale layering patterns |
The correlation between the lithostratigraphy and these orbital cycles allowed scientists to refine the age models for the sediment sequences, creating a more precise timeline of climate events in the region. This detailed chronology is essential for understanding the rates and patterns of climate change, providing valuable context for current and future climate shifts.
The lithostratigraphic work from Leg 175 has provided fundamental insights into how marine sedimentation records climate change. By systematically classifying and correlating the rock layers across the southwest African margin, scientists have constructed a detailed history of how the Angola-Benguela upwelling system has evolved over millions of years 1 5 9 .
Perhaps most importantly, this work helps us understand how marine ecosystems respond to climate change over long timescales—information that is crucial as we face modern climate change.
As we continue to perturb the Earth's climate system through greenhouse gas emissions, the deep-time perspective provided by these lithostratigraphic studies becomes increasingly valuable. They remind us that oceans have undergone dramatic transformations in the past, and that the changes we're initiating today will leave their own distinctive signature in the sediment layers of the future.
The diary of Earth's climate past, carefully extracted from beneath the Angola-Benguela upwelling system, may hold essential clues for navigating our climate future. As scientists continue to study these remarkable sediments, we deepen our understanding of the intricate dance between climate, oceans, and life—knowledge that has never been more urgently needed.