ESS FINAL PREP: OCEANS

what percent of earth is ocean?

71% ocean and 29% land

E’s oceans

Pacific, Atlantic, Indian, Arctic, and Southern Oceans

E’s surface water

oceans contain 97% of E’s surface water w/ Pacific Ocean containing 52% of E’s surface water

Depth and Relief Comparison

avg ocean depth: ~3800 m while avg land elevation is ~840 m.

Challenger Deep

11,022 m and in the marianas trench. If E was smoothed out, E would be submerged under ~2700 m of water

Major Ocean Basins

Pacific is largest (166 Million km²) and deepest (4282 m)

Continental Shelf

def: shallow, submerged extension of the continent crust. Avg ~80km in width but can vary widely. Depth usually less than 150m.

shaped by wave action, glacial processes, + sea levells

Continental Margin

continental slope: steeper section that descends to 3000-5000m. may include submarine canyons like Monterey Canyon, formed by turbidity currents.

continental rise: the area where the slope levels off+transitions to the abyssal plain. composed of thick sediments deposited from the continent.

abyssal plain: found at depths of 4500-6000m and represents the flattest and most extensive regions of the seafloor. The remarkable flatness results from millions of years of sediment accumulation that gradually bury underlying features.

marine provinces: Pelagic Zone

def: the open water column, home to swimming and floating organisms.

horizontal divisons — Neritic province: from the low tide line to the continental shelf

ocean province: all ocean water beyond the shelf break

marine provinces: Benthic Zone

def: the ocean floor, inhabited by organisms that live on or in the seafloor.

vertical divisions of Oceanic provinces

epipelagic (0-200m): sunlit surface zone supporting photosynthesis (photic zone)

mesopelagic (200-1000m): dim light; the twilight or dysphotic zone.

bathypelagic (1000-4000m): completely dark; part of the aphotic zone

abyssopelagic (4000-6000m): near the ocean floor in most deep-sea regions

hadpelagic (>6000m): the deepest ocean waters in trenches, such as the Marina trench

depth-based divisions of Marine Provinces (zones)

supralittoral zone: above the high tide line; only submerged during storms

littoral zone: between high and low tide lines (intertidal zone)

sublittoral zone: extends from low tide to the continental shelf break

bathyal zone: from the shelf break to ~4000m; includes continental slope and rise

abyssal zone: 4000-6000m; includes most abyssal plains and ~80% of the benthic seafloor

hadal zone: >6000m; found in deep ocean trenches

where did all this water come from?

E formed through accretion followed by intense volcanic activity. Volcanic outgassing released water vapor and other gases into the atmosphere. As E cooled, water vapor condensed and fell as rain. oceans formed from accumulating surface water around 4 billion years ago

why are oceans salty?

salinity comes from weathering of rocks and river runoff. Ocean salinity remains relatively stable due to a steady state between ion input and removal.

source of salts

river runoff, volcanic and hydrothermal activity, groundwater, dissolution and decay within the ocean

ion removal process

biological uptake (shells), sedimentation, sea spray, crustal percolation, evaporation of isolated seawater bodies

residence time

def: average time an ion stays in seawater.

long residence time. → less reactive, slow removal

short residence time → actively cycled

reflects role in biological vs geological cycles

salinity

def: total amount of dissolved salts in seawater

expressed in parts per thousand (ppt) or practical salinity units (PSU).

ions that make up ~99.4% of total salinity

chloride (Cl^-)

sodium (Na⁺⁾

^{sulfate (SO₄ ²-)}

^{magnesium (Mg²+)}

^{Calcium (Ca²+)}

^{potassium (K^+)}

^{sodium and chloride alone make up over 85% of dissolved ions}

minor and trace elements

trace elements include: carbon, nitrogen, oxygen, silicon, phosphorus, iron, copper, etc

measured in parts per million (ppm) parts per billion (ppb) or parts per trillion (ppt)

though dilute, trace elements make up large total quantities due to ocean volumes.

many are essential nutrients that support marine life and primary productivity

salinity variations

high evaporation at subtropics → high salinity

equator has high rainfall → slightly lower salinity

polar regions have low evaporation + ice melt → low salinity

isolated seas often have higher salinity due to limit mixing

salinity variations cont

surface salinity influenced by evaporation, precipitation, and runoff

mixed layer: (0-200m): relatively uniform salinity due to wind and wave mixing

halocline

def: zone of rapid salinity change with depths

deep ocean: stable salinity, largely unaffected by surface processes - despite surface variability, deep salinity is fairly uniform across latitudes

dissolved gases in seawater

seawater contains dissolved gases: oxygen (O₂ ), Carbon Dioxide (CO₂ ), nitrogen (N₂ )

gases enter ocean from atmosphere, especially at surface.

O₂ : used in respiration; CO₂ used in photosynthesis. N₂ : fixed by bacteria for nutrients

proportions in ocean differ from air due to solubility+biological use

surface: highest O₂ due to atmospheric exchange + photosynthesis

mid depths: (~200-1000m) oxygen minimum zone due to lack of light and active respiration

deep ocean: O₂ increases due to cold, high-pressure, oxygen-rich polar water sinking and circulating globally

atlantic deep water has more O₂ than Pacific due to newer water and less cumulative respiration

hypoxic and anoxic zones

H: O₂ < 2mg/L; stress or kill marine life

A: O₂ < 0.5 mg/L; no oxygen at all

may occur seasonally or persists longterm

often caused by excess organic matter, stratification, and poor mixing

CO₂ in seawater

opposite behavior to oxygen: surface CO₂ is low due to photosynthesis and shell-building.

increases with depth due to respiration, decomposition, and greater gas solubility

pacific deep water has more CO₂ than atlantic due to age and cumulative respiration

ocean pH

CO₂ helps regulate ocean pH by shifting between carbonic acid, bicarbonate, and carbonate.

ocean pH ~8.1 (slightly basic)

more H⁺ = lower pH (more acidic); less H⁺ = higher pH (more basic)

buffering: bicarbonate and carbonate neutralize pH fluctuations by absorbing/releasing H⁺

dissolved oxygen and ocean acidification

rising atmospheric CO₂ → more CO₂ dissolving in ocean → pH decline

ocean pH dropped from ~8.2 to 8.1 since Industrial Revolution (30% more acidic)

projected to reach ~7.8 by 2100 (120% increase in acidity)

still basic, but trend toward acidity disrupts marine ecosystems, especially calcifiers (corals, shellfish)

physical oceanography: presure

pressure in ocean = hydrostatic pressure (from weight of water above)

increases linearly with depth +1 atm every 10m)

at 1000m = 101 atm (100 atm from water + 1 atm from air)

deep ocean pressure can exceed 1000 atm in trenches

physical oceanography: temperature

oceans temp range from -2 to 30 (degrees Celius)

warmest at surface in low latitudes, coldest at poles

eastern sides of ocean basins are cooler than western sides (due to surface currents)

avg ocean temp: -4 (degrees Celsius)

tropical and polar oceans

T: warm surface, strong thermocline, little seasonal change.

warm low-density surface water; strong pycnocline. stratification limits nutrient mixing → lower productivity

P: cold surface and deep water; weak or no thermocline; stable temps

uniform cold temps at all depths, weak or no pycnocline → more vertical mixing.

nutrient-rich deep water can reach surface → higher productivity

physical oceanography: density

density= mass per unit volume (g/cm³ )

freshwater density: 1/cm³ at 4 C

sea water density: 1.02-1.03 g/cm³ (due to salts)

increased by: lower temps, higher salinity, and higher pressure

pressure has smallest effect; without compression, sea level would be ~50m higher

temperature has greatest impact on density

pyconcline

def: zone of rapid density increase with depths (mirrors thermocline)

deep water: cold, dense, and stable

stable stratification prevents mixing

primary production

def: the synthesis of organic matter from inorganic substances

carried out by autotrophs using CO_2, nutrients, and energy (light or chemicals)

two types: photosynthesis (algae) and chemosynthesis (vent bacteria)

forms of the base of marine food webs and drives energy flow

heterotrophs rely on consuming external organic matter

marine vs terrestial production

ocean NPP: 35-50 billion tons/years

land NPP: 50-70 billion tons/year

Ocean producers: 1-2 billion tons of biomass

land producers: 600-1000 billion tons → ocean is highly efficient despite low standing biomass

Marine primary producers: phytoplankton

def: microscopic, free-floating algae that drift with ocean currents and perform ~95% of marine photosynthesis.

major groups:

diatoms: silica shells, high efficiency, found in cold/coastal waters

dinoflagellates: flagella for movement, can be mixotrophic, no mineral shell

coccolithophores: calcium carbonate plates, found in water, open ocean

primary producer requirements

light: drives photosynthesis

nutrients: essential for growth and metabolism: structural elements: C,H, O — abundant, form backbone of organic matter. Primary nutrient: N, P, K - needed in largest amounts. Secondary nutrients: Ca, Mg, S — needed in moderate amounts. Micronutrients (trace metals): Fe, Zn, Cu, Mn, Mo,Co, Ni (needed in trace amounts.

temperature: affects metabolism rates; optimal range supports growth, extremes inhibit it

water: medium for nutrient uptake and biochemical reactions

marine primary production requirements and limitations

water availability is not a constraint

temperature is relatively stable and less limiting, though it is generally decreases with depth.

nutrients and minerals: are absorbed as dissolved elements: supplied by river input, recycling of organic/inorganic matter ,and atmospheric dust.

light decreases with depth → productivity limited to the photic zone

light limitation

photosynthesis requires light, so marine primary production is restricted to the photo (or euphotic) zone, typically the upper ~200m

light intensity decreases exponentially with depth → less energy for photosynthesis

the compensation depth is where photosynthesis = respiration → net primary production is zero

below this depth, respiration exceeds photosynthesis, and no new organic matter is produced

nutrient limitation

phytoplankton requires nitrogen, phosphorus, and silica (for diatom shells)

nutrients occur in very small amounts in seawater, especially compared to soils

surface waters + deep waters

surface: nutrient-poor due to rapid phytoplankton uptake

deep: nutrient-rich due to decomposition of sinking organic matter

stratification

def: the layering of ocean water based on differences in temperature and salinity, which creates density gradients

these layers act as barriers that limit vertical mixing, preventing nutrient-rich deep water from reaching surface waters

upwelling

disrupts stratification by bringing cold, nutrient-rich water to the surface → supports high biological productivity

biological pump

phytoplankton fix CO₂ in the euphotic zone via photosynthesis, producing particulate organic carbon (POC)

particulate organic carbon (POC)

is grazed by herbivorous zooplankton

broken down by heterotrophic microbes

1-40% of surface production is exported below the euphotic zone

marine snow

fecal pellets, dead cells, organic aggregates.

sinking materials are consumed or remineralized en route; only ~1% reaches the seafloor

biological pump

transports carbon and nutrients from the surface to the deep ocean

helps regulate climate by lowering atmospheric CO₂ through long-term storage in the deep ocean.

in deep waters: organic matter is oxidized, releasing CO_2, nitrate, phosphate. Some material is buried in sediment, forming potential fossil fuel (oil) sources

upwelling

returns deep nutrients to the surface, closing the loop

coastal zones benefit from:

nutrient runoff: from land

shallow seafloors: that retain nutrients

open ocean has low productivity due to:

distance from nutrient sources

deep waters that trap nutrients below the photic zone

surface currents

driven by prevailing winds, but only ~2% of wind energy transfers to water

current affect upper 100-200m, ~10% of ocean volume

what is the coriolis effect?

def: phenomenon where moving objects (like air or water) appear to be deflected due to the Earth’s rotation

deflects currents ~45 degrees. Right in Northern Hemisphere. Left in Southern Hemisphere

creates bands of east-west and west-east currents by latitude

what is ekman transport?

def: wind blowing over the ocean sets surface water in motion

each layer below dragged by the one above, but deflected further and slowed —> forms Ekman spiral, typically to ~100m depth

the net transport of all layers is ~90 degrees to the wind: right to the Northern Hemisphere, left in the Southern

Formation of Ocean Gyres

trade winds and westerlies form consistent surface currents

continents and coriolis deflection cause turning of currents → form circular gyres

Western and Eastern boundary currents

W: fast, warm, poleward (Gulf Stream, Kuroshio)

E: slow, cold, equatorward (Canary, California)

result: clockwise gyres in Northern Hemi, counterclockwise in Southern Hemi

Gulf Stream

first mapped by Ben Franklin in 18th century

transports warm water north alone US east coast

narrow (50-100km) deep (to 1.5km) and fast (up to 9 km/hr)

transports more water than all water than all rivers on Earth combined

how was gulf stream formed?

by convergence of the North Atlantic Equatorial Current and Florida Current

Gulf Stream w North Atlantic Current

transport vast amounts of heat from the tropics to the North Atlantic

this heat is released to the atmosphere as the current moves east, significantly moderating Europe’s current

meanders, rings, and eddies

meanders can pinch off into large rings or eddies

rings redistribute heat, nutrients, and salinity, influencing regional ecosystem

similar to oxbow lakes, these isolated eddies form when loops detach from the main current

warm core rings (north side):

shallow, clockwise, ~100 km wide; carry warm water north

cold core rings (south side):

deep, counterclockwise, >500km wide; carry cold water north

upwelling

def: brings nutrient-rich deep water to the surface, fueling primary production

downwelling

def: sends surface water downward, reducing surface productivity but delivering oxygen to depth

what causes upwelling?

occurs when: ekman transport moves surface water away from coastlines at zones of diverge (equator, Antarctic)

seafloor features: deflect deep current upward

results of upwelling?

brings nutrient-rich deep water to the surface

stimulates primary production and supports rich ecosystems

equator divergence / upwelling

def: deep water rises to replace the diverging surface water

at the equator, trade winds blow surface water westward

this creates a divergence along the equator.

antarctic divergence

occurs where the West Wind Drift (eastward) and East Wind Drift (westward) move in opposite directions around antarctica

ekman transport

deflects surface water away from the convergence zone:

to the left in the southern hemi

creates divergence and results in strong upwelling of deep, nutrient-rich water

supports high biological productivity in southern ocean ecosystems

when does downwelling happen?

ekman tranport pushes surface water towards coastlines

at zones of surface current convergence, surface waters driven by prevailing winds accumulate ad are forced downward into the ocean interior, driving downwelling

what are the results of downwelling?

drives surface water to depth

reduces surface nutrients, limiting productivity at the surface

delivers oxygen to deep waters, which supports deep-sea life and enables aerobic decomposition of sinking organic matter

what is el nino- southern oscillation (enso) ?

def: a recurring climate pattern involving changes in ocean-atmosphere interactions across the equatorial pacific

comprises two phases: el nino (warm phase) and la nina (cool phase)

what does enso do?

alters trade winds, sea surface temperatures (SST), and atmospheric pressure patterns

has far-reaching effects on weather, ecosystems, and economics worldwide

what are the normal conditions of ENSO?

trade winds: blow westward, piling warm surface water near Southeast Asia

upwelling: occurs near South America, bring cold, nutrient-rich water to the surface

results in a shallow thermocline and high productivity off Peru

what is walker cell?

low pressure and rainfall in the west, high pressure and dry air in the east

what are the conditions of el nino (ENSO warm phase) ?

trade winds weaken or reverse → warm water moves eastward towards South America, causing ocean stratification

upwelling shuts down, thermocline deepens → reduced nutrients and productivity

warm sea surface temps lead to rain and flooding in eastern Pacific, drought in western Pacific

what are the global impacts of el nino?

altered rainfall

storm patterns

jet stream shifts

what are the normal conditions of el nina (ENSO cool phase) ?

trade winds strengthen → enhanced westward flow of surface water

increased upwelling near south america → cooler sea surface temps and higher productivity

wetter monsoons in Asia; cooler, wetter, NW US, drier SE US

la nina often follow El Nino but not always

what are the enso cycles?

enso cycles occur every 2- 7 years, lasting months to a year

tracked using the multivariate enso index

strong historical el nino events: 1983, 1997-1998, 2015

what does enso affect?

fisheries

agriculture

water resources

global climate

what drives ocean circulation?

wind-driven surface currents affect only ~10% of the ocean. the remaining ~90% is moved by thermohaline circulation

driven by differences in seawater density, which depends on temperature and salinity

cold salty is denser and sinks → warmer fresher water stays near the surface

what is bottom water?

forms primarily near the poles, where seawater becomes very cold by loss of heat to the atmosphere and salty due to sea ice formation and brine rejection, increasing its density enough to sink

what are the controls of seawater density?

density increases by: cooling, evaporation, sea ice formation (removes freshwater)

density decreases by: heating, precipitation, ice melt, and river runoff

these changes occur at the surface and drive vertical water movement that forms deep currents

what are water masses?

def: has distinct temperature-salinity (T-S) characteristics

once formed at the surface, water masses sink and retian their properties

what are the major water masses?

antarctic bottom water (AABW) - densest, formed in Weddell Sea

north atlantic deep water (nadw) - formed in greenland sea

antarctic intermediate water (aaiw) and mediterranean intermediate water (MIW)

what is thermohaline circulation?

thermohaline combines “thermo” (heat) and “haline” (salt) referring to the temperature and salinity-driven density differences that power deep ocean circulation

aabw and nadw masses sink and spread through the atlantic, indian, and pacific ocens → global density-driven flow of deep water → with age, they mix and become deep water

what is the global conveyor belt?

def: continuous cycle of sinking, spreading, mixing, and upwelling and takes ~1000-2000 years to complete

what are the environmental roles of thermohaline circulation?

heat transport: moves warm water poleward, cold water equatorward

oxygen delivery: deep water originates from cold, oxygen-rich surface water

nutrient distribution: accumulates as water ages — pacific deep water is nutrient-rich but oxygen-poor

what are water age indicators?

high nutrients + low O2 = older

high O2 + low nutrients = younger

collapse of the amoc?

warming and ice melt in the arctic add freshwater → lower salinity → reduced sinking

disrupts deep water formation and thermohaline circulation, particularly the atlantic meridional overturning circulation (amoc)

what is the amoc?

def: part of thermohaline circulation in the atlantic that transports warm water northward at the surface and returns cold, dense water southward at depth

weakens the gulf stream, possibly cooling europe despite global warming

may reduce oxygen and nutrient delivery to deep-sea ecosystems

observations suggest this circulation is already weakening