Sunlight, Color, Temperature, and Ocean Energy Dynamics

Course Logistics and Exam 1 Details

Exam 1 Schedule and Administration:
- Exam 1 covers all material up through this lecture.
- Review session: Wednesday evening at $6:30$ \,PM on Zoom. The session covers the practice exam in full, alternative question phrasings, writings, and drawings.
- Exam Opportunity 1: Wednesday night at $8:00$ \,PM on Zoom (immediately following the review session).
- Exam Opportunity 2: Thursday during regular class time (in-person in the classroom or remotely over Zoom).
- Exam Opportunity 3: Friday afternoon at $12:30$ \,PM (Zoom only).
- Exam Opportunity 4 (Makeup): A single scheduled makeup is available for students with pre-approved conflicts who notify instructors in advance. These represent the only four test-taking opportunities offered.
- Exams are administered entirely online through D2L; there are no paper versions. Students taking the exam in person must bring a laptop or device.
Proctoring and Rules:
- Students taking the exam outside the classroom must be logged into Zoom with a working camera for ID verification and proctoring by instructors and TAs (Joel and Paul).
- Students taking the exam in person in the classroom on Thursday do not need to log into Zoom.
- Identification: A valid CatCard or any official picture ID must be shown to the proctor over Zoom before starting.
- If a student's camera fails on exam day, they must type their student ID number into the Zoom chat.
- D2L Start Window: The "Start Exam" button on D2L is open for a $15$ -minute window. Late arrivals exceeding this $15$ -minute window are barred from starting and must negotiate taking a later exam opportunity.
- Exam Duration: Once started within the window, students have $70$ \,minutes to complete the exam.
- Each student is permitted only a single attempt.
- A password is provided in the Zoom breakout room or classroom to unlock the exam on D2L.
- Disability Resource Center (DRC) accommodations: Extra time is pre-configured in D2L. DRC students taking the test in person on Thursday cannot stay past standard class time due to a subsequent scheduled class; they must plan accordingly or take it over Zoom.
Assignments and Upcoming Course Schedule:
- Lecture 9 outlines are posted on the web.
- Homework 2 solutions and grades are posted.
- Homework 3 is currently being graded and will be returned by Friday.
- Project Proposals: Proposals are worth $20$ \,points, and the Final Project is worth $30$ \,points (total $50$ \,points). Students who have not submitted a proposal can still submit it late alongside the final project for retroactive full credit.
- Next Unit (Atmosphere): Begins next Tuesday. The unit covers atmospheric circulation, wind patterns, and water movement. Attendance is critical as it forms the foundational underpinning for the remainder of the course.

Review of Minerals, Seawater Chemistry, and Hydrologic Cycle

Body Minerals and Weathering:
- Calcium ( $Ca$ ) in the human body originates from the chemical breakdown of feldspar.
- Feldspar breaks down into calcium, sodium ( $Na$ ), and potassium ( $K$ ).
- Mica breaks down into clay (mud) and releases iron ( $Fe$ ) and magnesium ( $Mg$ ). These elements travel down rivers into soil, where crops (such as broccoli) absorb them for human consumption.
- Quartz chemically resists weathering and forms sand.
- Mica and feldspar are found exclusively in headwaters near source rocks (within granite boulders and gravel). By the time sediment reaches floodplains and continental shelves, chemical weathering has converted them entirely into quartz sand and clay mud.
- Calcite ( $CaCO_3$ ) on continental shelves originates from the skeletal remains of marine organisms.
Gas Solubility in Seawater:
- Gas solubility is inversely proportional to temperature: cold water holds significantly higher concentrations of dissolved gases (oxygen $O_2$ and carbon dioxide $CO_2$ ) than warm water.
- Polar seawater contains high oxygen concentrations specifically because it is cold.
- As seawater warms, thermal agitation causes dissolved gases to degas into the atmosphere.
Biological Gas Cycling:
- Coccolithophores (marine microplants) remove dissolved carbon dioxide ( $CO_2$ ) from seawater via photosynthesis to build organic tissue and calcite shells, releasing oxygen ( $O_2$ ) as a byproduct.
- Animals and bacteria perform respiration and remineralization/decomposition: they consume organic matter and $O_2$ , releasing $CO_2$ back into the water column.
Salinity and the Hydrologic Cycle:
- Average ocean surface salinity is $3.5\%$ (expressed by oceanographers as $35\,\text{per mil}$ or parts per thousand).
- Salinity across the global ocean varies by roughly $10\%$ relative to the baseline, ranging from $3.2\%$ ( $32\,\text{per mil}$ ) to $3.8\%$ ( $38\,\text{per mil}$ ).
- Seawater composition: $96.5\%$ pure water and $3.5\%$ dissolved salts.
- Sodium ( $Na$ ) and chlorine ( $Cl$ ) account for $85\%$ of all dissolved salts in seawater.
- Evaporation removes pure fresh water from the ocean surface, leaving salt behind and increasing surface salinity. High surface salinity regions (orange on global maps) correspond to dry sub-tropical desert latitudes where evaporation exceeds precipitation.
- Precipitation (rain) and ice melt add pure fresh water to the ocean, diluting surface salinity (blue on global maps).

Vertical Ocean Profiles and Deep Sea Tracers

Standard Vertical Water Column Profiles:
- Light: Absorbed rapidly near the surface; decays exponentially with depth.
- Temperature: Warm and well-mixed by wind in the uppermost layer; drops rapidly across the thermocline to cold baseline temperatures at depth.
- Pressure: Increases linearly and continuously with depth due to the hydrostatic weight of the overlying water column.
- Carbon Dioxide ( $CO_2$ ): Concentration is low at the surface due to plant uptake during photosynthesis; concentration is high at depth due to bacterial decomposition and remineralization of sinking organic material.
- Oxygen ( $O_2$ ): Concentration is high at the surface due to direct atmospheric exchange and photosynthetic production by plants; concentration is low at depth due to oxygen consumption by bacterial respiration during decay.
Man-Made Tracers and Deep Water Formation:
- Chlorofluorocarbons (CFCs) are synthetic, man-made ozone-depleting chemicals with no natural oceanic sources.
- Detection of CFCs on the deep sea floor near Antarctica provides absolute proof that surface water recently in contact with the atmosphere became cold and salty (dense) enough to sink directly to the ocean bottom.
- Oceanographers utilize chemical tracers (CFCs, salinity, dissolved gas profiles) to track the origin, movement, and circulation history of deep ocean water masses.

Light Zonation and Deep Ocean Exploration

Oceanic Light Zones:
- Photic Zone:
  - Depth Range: $0\,\text{m}$ to $100\,\text{m}$ .
  - Characteristics: High sunlight intensity, supporting all marine plant growth and photosynthesis.
  - Boundary Alignment: The $100\,\text{m}$ photic depth conveniently matches the average depth of the continental shelf.
  - Light Attenuation: Only $1\%$ of surface solar energy penetrates to a depth of $100\,\text{m}$ .
- Twilight Zone:
  - Depth Range: $100\,\text{m}$ to $1000\,\text{m}$ .
  - Characteristics: Contains extremely faint, residual sunlight. Organisms living here possess oversized eyes to capture low light levels.
  - Biological Limit: Light levels are insufficient for photosynthesis; no living plants exist in this zone.
- Dark Zone:
  - Depth Range: Below $1000\,\text{m}$ .
  - Characteristics: Absolute absence of sunlight. The only illumination present originates from organism bioluminescence or artificial sources on submersibles.
Global Volumetric Context:
- The average depth of the deep sea floor is $4000\,\text{m}$ .
- At least $75\%$ of the ocean's total volume exists in total darkness (below $1000\,\text{m}$ ), and over $90\%$ contains little to no light.
- Oceanic plant life and primary productivity are restricted to the paper-thin photic layer at the surface.
Deep-Sea Observations and Video Case Studies:
- Sperm Whale Biologging (CritterCam):
  - Suction-cup cameras deployed on sperm whales record diving behavior down to giant squid hunting grounds.
  - Sperm whales surface for only a few minutes per hour and can hold their breath for $20\,\text{minutes}$ to over $1\,\text{hour}$ .
  - Water temperature drops noticeably by $200\,\text{m}$ ( $600\,\text{ft}$ ).
  - Whales descend past $300\,\text{m}$ ( $900\,\text{ft}$ ) into the twilight zone to hunt.
- Continental Slope and Deep Sea Floor (Submersible Alvin):
  - Photosynthesis is impossible below $150\,\text{m}$ .
  - Animals below the photic zone survive by consuming "marine snow"—falling detritus, dead organic material, and fecal matter from surface waters.
  - Only $3\%$ of total organic food produced in surface waters reaches the continental slope.
  - The continental slope extends roughly $150\,\text{miles}$ from the shelf edge down to a baseline depth of $4000\,\text{m}$ .
  - Deep sea environment: Water temperature drops below $4^\circ\text{C}$ ( $39^\circ\text{F}$ ), and hydrostatic pressure reaches up to $400$ \times surface atmospheric pressure.
  - Dominant sea floor benthic organisms: Echinoderms (sea cucumbers, brittle stars, sea urchins) which sweep the sediment for organic particles, and Crinoids (sea lilies)—sessile animals related to urchins and sponges anchored to rocky outcrops with feather-like tentacle umbrellas that filter passing currents.

Sunlight, Solar Radiation, and Equatorial Heating

Four Primary Roles of Sunlight in the Ocean:
1. Warms the ocean surface.
2. Reflects out from the surface, providing visible light.
3. Drives plant photosynthesis.
4. Drives liquid water evaporation (fueling the hydrologic cycle).
Mechanism of Equatorial vs. Polar Heating:
- The Equator is significantly warmer than the poles due to direct solar heating, NOT because of minor differences in distance to the sun.
- At the Equator, the sun is directly overhead, concentrating incoming solar energy over a small, localized surface area.
- At higher latitudes and poles, solar rays strike the curved Earth at an angle (indirect heating), spreading the exact same amount of solar energy over a much larger surface area.
- Seasonal variation is driven by Earth's axial tilt relative to the sun (pointing toward the sun in summer, away in winter).
- Fall Equinox (occurring today, approximately day $265$ of the year): Earth's axis is parallel to solar radiation; every location on Earth receives exactly $12\,\text{hours}$ of daylight and $12\,\text{hours}$ of night.
Surface Water Temperature Extremes:
- Equatorial Western Pacific (around Indonesia): Average surface ocean temperature is $84^\circ\text{F}$ ( $28\text{--}29^\circ\text{C}$ ) year-round.
- Southern Ocean: Surface temperatures drop below the freezing point of seawater; human survival time without protective thermal gear is approximately $3\,\text{minutes}$ .

Ocean Color, Light Wavelengths, and Photosynthesis

Physics of Ocean Color:
- Objects appear a specific color because they reflect those specific wavelengths of light while absorbing all other wavelengths.
- The ocean appears blue because water molecules absorb longer, lower-energy wavelengths (reds, oranges, yellows) rapidly, while blue wavelengths are reflected back or scattered to human eyes.
- The sky appears blue due to a similar mechanism: atmospheric water vapor scattering blue solar photons.
Wavelength Penetration and Attenuation Depths:
- At $1\,\text{m}$ depth: $55\%$ of all incoming solar heat/energy is absorbed. Virtually all red photons are completely absorbed within the first meter.
- At $10\,\text{m}$ depth (~ $30\,\text{ft}$ ): $84\text{--}85\%$ of solar energy is absorbed. Orange, yellow, and violet wavelengths fade out.
- At $100\,\text{m}$ depth: Only $1\%$ of surface light energy remains, consisting almost entirely of blue and green photons.
Visual Color Shifts Underwater:
- At depths between $3\,\text{m}$ and $6\,\text{m}$ ( $10\text{--}20\,\text{ft}$ ), red objects (such as a red soda can or red fish) appear completely gray because no red light photons exist to reflect off the object.
- Blue fish retain their blue color at depths of $50\,\text{m}$ to $100\,\text{m}$ because blue photons remain available to reflect.
- Vivid colors in underwater photography at depth require artificial strobe lighting.
Satellite Ocean Color and Chlorophyll Mapping:
- Satellites measure reflected light to determine surface chlorophyll concentrations, providing maps of global primary productivity.
- Chlorophyll concentrations are highest along the Equator, in the Southern Ocean, across the North Pacific and North Atlantic, and along coastal upwelling margins.
- Satellite sensors capture surface skin processes only (the upper $1\text{--}2\,\text{m}$ ) and cannot measure temperature or color at depth.

Latent Heat, Evaporation, and Atmospheric Energy Transport

Thermodynamics of Evaporation:
- Evaporation converts liquid water into water vapor by supplying thermal energy to break intermolecular hydrogen bonds, allowing high-energy molecules to escape into the gas phase.
- Wind passing over the water replaces saturated air with drier air, accelerating evaporation rates.
Quantitative Energy Requirements for Water State Changes:
- Sensible Heat (Water/Ice): Adding $1\,\text{cal}$ of heat raises the temperature of $1\,\text{g}$ of liquid water by $1^\circ\text{C}$ .
- Latent Heat of Fusion: Melting $1\,\text{g}$ of ice at $0^\circ\text{C}$ into liquid water at $0^\circ\text{C}$ requires $80\,\text{cal}$ of heat energy (stored as latent heat without a change in temperature).
- Sensible Heating (Freezing to Boiling): Raising $1\,\text{g}$ of liquid water from $0^\circ\text{C}$ to $100^\circ\text{C}$ requires $100\,\text{cal}$ .
- Latent Heat of Vaporization: Evaporating $1\,\text{g}$ of liquid water into water vapor requires $540\,\text{cal}$ (or $543\,\text{cal}$ ) of heat energy.
Global Latent Heat Transport:
- When water evaporates from warm ocean surfaces, $540\,\text{cal/g}$ of latent thermal energy is absorbed and stored in water vapor molecules.
- Atmospheric winds carry this water vapor toward higher latitudes.
- When water vapor condenses to form clouds and rain, it releases the stored $540\,\text{cal/g}$ directly into the surrounding atmosphere as sensible heat.
- This condensation process heats local climates (explaining moderate temperatures in rainy regions like Seattle and London).
- Global Heat Redistribution Balance: Approximately $80\%$ of total heat transport from the Equator to the poles is carried by the atmosphere (primarily via water vapor latent heat exchange), while ocean currents directly move only $20\%$ of global heat.